How to Figure Out What is Wrong With Your Furnace

Updated on August 1, 2015
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Dan has been in the HVAC industry for 22 years with experience ranging from installation and service to sales and distribution.

How a Furnace Works; How It Turns Itself On and Off

Often the best way to figure out what is wrong with something is to know how it is supposed to work. If you know the sequence of operations, you can pinpoint where the sequence is being disrupted. Your furnace is no exception to the rule.

When your furnace is called into action by the thermostat, there is a rhyme and reason to the procedure it follows in safely turning itself on. Next time your furnace doesn't respond to the call, you will be able to see where the problem lies, and you can either decide that it is within your skills and resources to repair it, or that you need a professional HVAC technician to handle the issue for you.

The following discussion can save you time and money, whether you fix the problem yourself, or end up showing the problem to your service technician.

NOTE: Furnaces burn natural gas, and use electricity, normally 120 volts; both of these present hazards. As always, be sure the power is shut off to your unit before you do any work on it. Also, never assume anything. If what you see in your home differs from what I describe here, do not guess. You could make the problem worse and ultimately more costly to repair.

A fairly standard 80% efficient furnace, easily identified by its metal chimney. Higher efficiency furnaces usually have PVC (plastic) chimneys.
A fairly standard 80% efficient furnace, easily identified by its metal chimney. Higher efficiency furnaces usually have PVC (plastic) chimneys.

Firing Order: Steps in the Operation of a Modern Forced-Air Furnace

When the thermostat asks for heat, the furnace jumps into action, going through the steps and components below. You can see many of these components in the video.

  1. The thermostat tells the furnace to come on.
  2. The inducer motor starts up.
  3. The pressure switch confirms proper venting of the chimney.
  4. The hot-surface ignition module (if you have one, and not a spark ignitor or a pilot) begins to glow.
  5. The gas valve opens and the gas is ignited by the ignition source.
  6. The flame sensor verifies that the gas has been lit.
  7. The high-limit switch reaches its set temperature.
  8. The blower motor comes on.
  9. The furnace runs until the thermostat is satisfied and tells it to shut down.
  10. The gas valve shuts.
  11. The high-limit switch reaches its low temperature setting.
  12. The fan shuts down.

A newer furnace may have even more bells and whistles; an old furnace may have just the minimum, a gas valve and a thermocouple (like an old-fashioned flame sensor). The thermocouple or flame sensor tests whether the pilot is lit, and stops the main gas valve from opening if it is not. Of course a broken thermocouple can also stop the gas valve from opening.

The Thermostat

The thermostat is where it all begins. The thermostat is really just a set of switches that open and close depending on the temperature, allowing power to flow to certain circuits in the heating and cooling system. Think of them like drawbridges that swing up and down. They are normally drawn up, but can be lowered to close the bridge and allow power to pass. When the temperature in your home drops, the thermostat drops its bridge, sending power to the furnace to let it know that heat is needed. As the room temperature rises again, the thermostat raises its bridge and shuts off the power. For air conditioning, the thermostat works the same way but in a mirror image, closing its bridge when the house gets too warm.

If the temperature in the house is lower than the temperature to which you set your thermostat, your thermostat may not be functioning as it should. You can test this by jumping (touching together) the red and white wires to your thermostat. If the furnace comes on, your thermostat is quite likely the problem.

What Do the Wires in the Thermostat Do?

Wire Color
What it Controls
Red
Red is for power. This carries 24-volt power, supplied by the furnace. This power waits at the “bridge” until it is told where to go.
White
White is for heat. When the bridge (switch) between the red and white wires closes, the thermostat is calling for heat.
Yellow
Yellow is for cooling. When the bridge (switch) between the red and yellow wires closes, the thermostat is calling for cooling.
Green
Green is for the fan. When the bridge between red and green closes, only the fan runs. No heat or cooling is called for.
Blue
Blue is a rogue or wild card. It can be used to power a display or for advanced features. Usually, though, it is wrapped back into the wall and not used at all.
Other colors of wires in a home system may be for advanced functions or future use.

The Inducer Motor and Fan

The inducer motor and fan. The fan draws the "bad" exhaust gases out of the heat exchanger and pushes them up into the chimney.
The inducer motor and fan. The fan draws the "bad" exhaust gases out of the heat exchanger and pushes them up into the chimney.

The inducer motor and fan push exhaust up the chimney, getting rid of carbon monoxide (CO). Older systems relied solely on "natural draft" to vent these gases, but inducers have been a great addition to the furnace system. They extend the life of chimneys, and when used with pressure switches they help prevent CO poisoning.

The Pressure Switches

This unit has two pressure switches because it is a two-stage furnace. Inside the casing is a diaphragm, that when pressurized, completes the connection to the next component.
This unit has two pressure switches because it is a two-stage furnace. Inside the casing is a diaphragm, that when pressurized, completes the connection to the next component.

The function of the pressure switch is to verify that the inducer motor is actually pushing air up the chimney. If the chimney is blocked, for example by a bird's nest, the pressure switch shuts down the system in order to eliminate the risk of CO entering the home. Of course the pressure switch may also shut down if the motor is old and running too slow to satisfy the switch, or the hose from the fan to the switch is pinched or broken, or the switch itself is bad.

Again, a rather simple device in a complex system. The hot surface ignitor uses electrical power to heat a very fragile ceramic element, charcoal-like in appearance, to such a high temperature that natural gas or propane bursts into flame as it flows by. The hot surface ignitor is located at the orifice at the first burner port, in order to light the gas immediately as it's introduced to the port. This prevents a buildup of unburned gas in the unit.

Some furnaces employ what is called spark ignition. Instead of a red-hot surface, a spark ignitor creates a series of sparks to ignite the gas.

It's not important whether you have hot-surface or electronic ignition in your furnace; the important thing is that you are avoiding using a pilot light. Pilot lights waste gas; they burn gas constantly during the heating season, and some people let them burn year round. Want to save a few dollars? Make sure you shut off the pilot on your furnace when it's not being used for the season. Just be sure you know how to relight it next year before you do.

The hot surface ignitor is very fragile, and when working, very hot.
The hot surface ignitor is very fragile, and when working, very hot.

What type of ignition source do you have?

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The Gas Valve

The gas valve is basically an electronically controlled gateway. When the unit calls for heat, and the circuit board confirms that the right conditions have been met, the unit passes energy to the valve, causing it to open and release gas to be ignited by the ignitor or pilot. When the heat cycle has run its course, the energy to the valve is cut, and the valve once again shuts, cutting off the gas supply to the burners.

The Flame Sensor

The flame sensor is very simple, yet causes a lot of problems for homeowners. Its only job is to verify that the gas has been lit, by sensing the heat. If there's no heat, it shuts off the gas, to avoid any dangerous buildup of gas in the unit. The flame sensor is simple to fix when it goes bad or just needs some TLC.

Video: Cleaning a Flame Sensor

The High-Limit Switch

This high-limit switch is not adjustable. Older switches had a dial that could be adjusted, which in reality was not a good idea.
This high-limit switch is not adjustable. Older switches had a dial that could be adjusted, which in reality was not a good idea.

The high-limit switch serves several purposes.

First, it keeps the furnace fan from turning on until it detects that a set temperature has been reached. Otherwise, the fan would come on prematurely, and blow cold air into the house each time the thermostat asks for heat.

Second, it also tells the blower how long it should continue to run before shutting off, once the thermostat has been satisfied. Otherwise, hot air remaining in the duct work and heat exchanger, after the furnace has cut off, would just go to waste.

Lastly, the limit switch will turn the unit off if the heat exchanger reaches a certain high temperature, to prevent dangerous overheating.

The Furnace Fan and Blower Motor

Here's a good exposed shot of what your blower assembly looks like outside of the unit.
Here's a good exposed shot of what your blower assembly looks like outside of the unit.

Ah, finally. Heat for us all. Now that all the sensors have been satisfied, and the limit switch tells the blower that the air is warm enough to send into the living space, it will start the air flowing.

Our furnace's blower will typically use 120 volts and push anywhere from 800 to 2000 cubic feet per minute (cfm). Most blowers require the use of a capacitor, and have a half-inch drive that hooks directly to the fan, unless you have a belt-driven motor.

Tid-Bit: In the case of a malfunction, or a cut in power to the unit, it is common for the blower to turn on and run for a period of time. This may be because there is too much heat, and a cool down is called for, or there may be a build up of "bad gases" which need to be spead out or dispersed by the fan.

The Roll-Out Switch

One other little safety gadget is the roll-out switch. This little guy can sense if flames "roll out" of their proper domain, and shut down the system as well.
One other little safety gadget is the roll-out switch. This little guy can sense if flames "roll out" of their proper domain, and shut down the system as well.

Save Time and Money By Knowing Your Furnace

I hope this helped you to understand your heating system a bit better. Here are some troubleshooting ideas involving the parts you just read about, and here are a few more parts tips.

  • In case of a malfunction, your furnace will usually try to restart three times. If on the third try it cannot reach successful ignition, it will go into a lockout mode for a specified amount of time, usually an hour. Resetting the power (turning it off and on) will usually reset the board and allow the unit to show you what was happening in case you missed it.
  • The little blinking light in the unit isn't a Christmas decoration or little disco in your furnace. This light blinks a sort of Morse Code that can lead you to the problem, or say "Hey, all is well here." Look inside and outside your unit. There is likely a code chart to tell you what the dot, dot, dash, dash sequence you're seeing means.
  • Some annoying noises your home furnace makes may be simple and cheap to quiet down.
  • Take care of your furnace and it will likely take care of you. But if you don't clean the unit and change the filter like you're supposed to, your furnace may break down, or break your bank.

Questions & Answers

  • My furnace is working fine. It is vented out the front of the house. Why does it have such a strange odor outside only when the furnace turns on?

    The only foul smells (rotten egg-like) I'm aware of have been a result of a bad heat exchanger. I'm not saying for sure that's you because I don't know what you're smelling but usually, there is no bad odor from these vents.

  • Furnace kicks on. Home temp reaches desired temp. Then the burners turn off but the blower keeps blowing. Normally the blower turns off after approx 100 seconds but occasionally will continue to blow and the only way to turn off is the fuse box. Can it be the switch above the burners or the switch that is by the blower?

    The blower usually only does that when there is a fault. If no fault and no changes have been made recently then perhaps the board is going bad and there is a short causing the blower to run continuously. Can't say that for sure but this is what I'd be looking at.

  • The furnace runs and heats the house fine, but the house is dry. The furnace humidifier is working, but the house is still dry? What can I try?

    Have you turned up the percentage of humidity created by the humidifier? Adding humidity is the only way to fix the issue. Perhaps your humidifier isn't sized properly. I wish I could add something more, but there's really no "fix" if everything is working.

  • Is 80 degrees too hot to run your gas furnace?

    In my opinion yes. It can do it but will likely cost a small fortune to do so.

  • My gas furnace model nugm100ehb1 takes combustion air from inside the home, is this correct or proper?

    Yes as long as there is enough free air space. There is a chart in the install manual that will tell you how many sq. ft. You should have. Also, I'm not sure if it applies to your model, but some say it will shorten the warranty on the unit. You may want to look at your manual and warranty details to find out.

Comments

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    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      3 months ago from Ohio

      Craigh S - I can't be sure without testing. Maybe the board is bad? Are you sure it's the rollout switch and not the temperature sensor on the blower housing?

    • profile image

      Craigh S 

      3 months ago

      Why would rollout switch error show when it a/c is used but not when heat is used?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      3 months ago from Ohio

      Derek - I really can't think off why these two would be running simultaneously because of your cutting concrete unless you got water on the board and shorted something out. I could maybe understand just the blower but not along with the inducer. And are you sure this is a result of your work or that you just happened to notice because you spent so much time down there?

    • profile image

      dgrandstaff 

      3 months ago

      I was cutting concrete close to the furnace and there was a lot of dust, now the exhaust fan and the blower run at the same time nonstop.

    • profile image

      Dee Kae 

      4 months ago

      Thank you for responding.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      4 months ago from Ohio

      Well Dee, Like other mechanical working things, they sometimes just do something weird and turning off and on the stat can trigger a new call and voila, it works. I wish I could offer a better explanation but it does happen. My furnace will sometimes shutdown on a real cold night because I believe the "heavy" cold air along with a right wind direction overcomes the pressure switch/inducer and can trigger a lock out. Wind stops, I reset the unit, voila...it works. It is very difficult when a cause can't be pinpointed. Next time the furnace shuts down see if you can catch the blinking red light code and maybe that would help.

    • profile image

      Dee Kae 

      4 months ago

      I have a Tud-B series older Trane furnace. It is cleaned professionally every fall and last fall HVAC said it's still in great condition. On Feb. 22, March 3, March 6 and March 25, the furnace stopped working but started again when the thermostat was turned off and back on. I haven't called HVAC yet because it's happened late at night and on weekends so it has seemed to be better to try turning it off and on first. Once last summer the newer air conditioner stopped and they couldn't duplicate the problem because it started working when they got here and turned the thermostat back on. I have a newer electronic programmable thermostat. I've changed the batteries twice (display was still working), cleaned it (no visible dust) and tightened a loose anchor screw. What else could cause my problem besides a faulty thermostat? Turning the thermostat off and right back on has turned the furnace back on all four times. Thank you!

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      5 months ago from Ohio

      I would say perhaps you have a slow close gas valve however if it keeps running or goes into lock out then I don't think that's it. It sounds like the valve itself maybe going bad. It's a "normally closed" valve and if the power is being lost in the valve then it will not hold it open and thus creating this intermittent issue. I can't swear to it without running tests but I would have it checked out because it will cause other components to wear out if it's constantly starting and stopping.

    • profile image

      RandysPCs 

      5 months ago

      I have a Coleman furnace that acts like it's running out of gas sometimes. I have watched it and what happens is, the induction fan runs, ignitor glows, gas lights but then gradually the flame gets smaller and goes out. Then the gas valve clicks off and the cycle starts over, sometimes it will stay running sometimes it goes into lock out. Any ideas?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      5 months ago from Ohio

      Lowski - I'm nearly positive it's the flame sensor and is probably just a bit in need of a cleaning. You likely won't even be able to see dirt on it but it doesn't take much at all. I've written on how to do this so perhaps if you need some guidance you could check that out. Thanks for reading.

    • profile image

      lowski 

      5 months ago

      Hi Dan,

      I recently replaced the circuit board on my Ruud 80%. Apparently there was a bad relay, causing my inducer motor to constantly switch between hi and low, delaying the furnace from ever igniting. Since I switched out the board, it has worked fine, with the caveat that every once in awhile, when the inducer motor turns on and the pressure switch makes, I hear the gas ignite but then turn off right away; but the blower motor still comes on and blows cold air. Once the blower motor turns off, the inducer comes immediately on and the whole sequence starts again, but this time with the flame staying lit and working properly. Thoughts?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      5 months ago from Ohio

      Roger, There are a lot of possibilities based on the info you've provided. I would need more to offer any sound advice. Maybe igniter is bad? Maybe inducer/pressure switch is preventing it due to obstruction or part failure? Maybe board? I'm sorry but I can't help much at this time.

    • profile image

      Roger 

      5 months ago

      furnace turns on but the igniters don't light up.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      6 months ago from Ohio

      I don't think it's either to be honest. Sounds more like an intermittent pressure switch issue. See my reply to John.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      6 months ago from Ohio

      Sounds similar to how a pressure switch will act sometimes. It's cheap and easy to fix so maybe give it a try if it's in the budget.

    • profile image

      Jakobsdad 

      6 months ago

      My furnace works perfectly for one or two cycles then it goes through the start up process but the igniter doesn't come on. The furnace continues through the whole start up process then blows regular air through the system. if I turn the thermostat to the off position and then back to heat it works perfectly for one or two cycles then repeats. do you think this is a thermostat issue or an igniter issue? the igniter works as it should after the manual power cycle from the thermostat.

    • profile image

      john 

      6 months ago

      My furnace works perfectly for one or two cycles then when the thermostat calls for heat the igniter doesn't light and it blows cold air only. I can turn the unit off then right back on and it works perfectly again until one or two more cycles then I have to repeat a powering down to get it to work

    • profile image

      Kathleen8 

      6 months ago

      You are probably right! hahahhaha I figure it there was a problem, it would have shown itself by now. Thanks!

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      6 months ago from Ohio

      And though I hate to say it, that may be exactly why this is an issue for you. Furnaces aren't as quite when you have to sleep next to one as they are in a basement and is likely why the evenings seem worse...it's quieter, colder, and you're right there for it all.

    • profile image

      Kathleen8 

      6 months ago

      Honestly, I don't know about furnaces. It's a Carrier 59SC5A I believe.. I don't believe the manual says it's 2 stage. I thought the filter was dirty, so I changed it with a nice Filtrete filter and no sluggish noises upon the start of cycling. Then 3 weeks later, it started doing it again. I looked at the filter and it was dirty again. So, I jut put in a filter from ACE Hardware and the noise came back. It will only usually happen in the evening when it actually does happen.. You hear the switch and then it's this sluggish noise like it's having a bit of trouble getting going) from what I assume is the air draft inducer and then it starts blowing just fine. My brother thinks maybe it's just working harder? I can't believe I'm the only person with this issue..then again, I live in a tiny house and the furnace is right there next to my bedroom so I hear EVERYTHING.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      6 months ago from Ohio

      Kathleen8 - I can't really say much based on the information here but since it's condensing...is there a chance it's also a 2 stage unit??? If so, the gas output and blower speeds may vary by cycle and whether it's calling for the first or second stage.

    • profile image

      Kathleen8 

      6 months ago

      Hi, I have a Carrier Condensing gas furnace. Sometimes, when the furnace kicks on and starts it's cycle, it sounds sluggish before the blowing begins. Most times, you hear the click, the cycling starts and it starts blowing but it seems like since we are in the midst of winter and it's on all the time, it happens for a few cycles during the evening, but not all the time... Any ideas? Could there be dust stuck somewhere? Thanks!

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      7 months ago from Ohio

      Unfortunately there are a too many possibilities for me to say anything certain. Maybe a fan limit? Intermittent pressure switch? Gas valve? Circuit board? Unless you just want to start throwing parts at it, you'd need a tech to troubleshoot the circuit.

    • profile image

      Phyrfitr 

      7 months ago

      i have a york 90+ 100,000 btu single stage unit, the thermostat will call , the inducer motor will spin up the solid igniter will glow the gas valve will open and the furnace will cycle, for five minutes or so then the flame will go out but the inducer and blower motor will continue to run, the igniter will glow and the gas valve will start clicking constantly and the flame will ignite multiple times causing multiple fuel ignitions sometimes making a Poof sound, I sit and watch it thru the peep hole, this has been going on now for about A week, Any thoughts ?

    • profile image

      Bill Fish 

      7 months ago

      Thanks for responding to my question. You said that my problem might be related to the flame sensor, with the limited info I gave ya. So I cleaned the flame sensor, which was very simple, and my furnace is working great again. I probably saved a couple hundred dollars by doing that myself. Thanks again.

    • Steve Stark profile image

      Steve Stark 

      7 months ago

      I have a 14 yo Tempstar gas furnace. During a long heating cycle, the inducer motor will cut off for a second and then come back on. This happens maybe 3 or 4 times during the heating cycle. Does the IDM need replacement? Or, could the thermostat be faulty? (round manual Honeywell thermostat) Thanks

    • profile image

      Dale 

      7 months ago

      Have a 10 year old furnace, have it cleaned and checked twice a year. A month ago the fella took the thing apart like never before, put it back together and left. Now the filter is filthy after two weeks. That never happened before. Have any ideas what happened? Thank you. 1/3/18. Dadrottar@aol.com

    • profile image

      Earl 

      7 months ago

      My furnace will not stay running unless the door is off, why?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      7 months ago from Ohio

      Angelo - It seems maybe the thermostat is the issue since the furnace will only turn on when the stat says to so perhaps it's not reading properly???

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      7 months ago from Ohio

      Jeff - I'm sorry but there isn't enough for me to offer much. Check your filter. Could be a high limit switch??? Not sure though.

    • profile image

      Angelo 

      7 months ago

      I have Lennox Meritt series that's 13 years old. It started to act weird yesterday. It will only turn the furnace on and maintain the house temperature if the thermostat is set at 73 and above. If I set the thermostat below 73, it will will not turn on the furnace.

    • profile image

      Jeff 

      7 months ago

      I have a Coleman I replaced the pressure switch and the thermostat it is running a cycle at two set to 70 but any higher it will run as well then shut off and not fire in less I turn it back down to 70 and cut power and reset it

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      7 months ago from Ohio

      RK - It sounds like you either need a new motor and/or motor capacitor. I don't recommend using your hand. I suppose it may be getting bad voltage from the board as another possibility but you may want to call a tech.

    • profile image

      RK 

      7 months ago

      Hello, I have Carrier weathermaker 8000 gas furnance. Very often furnace would not start and I have to rotate the motor by hand and press the cover switch to start again.. If i leave it, it may start randomly after few hours on its own Any suggestions

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      8 months ago from Ohio

      Craig, Though I can't say for sure from here, I would first check the hi limit and is the coil for your a/c new too? If not, perhaps it is clogged and blocking air flow??? Is the new furnace fan perhaps more powerful? If so, your ducts may not be big enough on the return side...especially if you added an air cleaner. These are my suspects. Thanks for reading.

    • profile image

      Craig 

      8 months ago

      Have a Goodman furnace installed on 2016. I set the thermostat to 72 the furnace starts up. Runs for a while tries to make temperature but then cuts off and there is red light blinking 4 times. After a while when the temperature has dropped it kicks back in. I replaced the filter. The thermostat was recently replaced. I have checked the airflow outlet and inlet they don't have any objects. Would a leaking window sill or older windows result in this kind of behavior where the furnace cuts off? This is again a brand new furnace.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      8 months ago from Ohio

      Greg, I'm not sure what advice to offer based on this info. You experienced some sort of failure that is apparently intermittent so when you reset the breaker it started over and apparently didn't have the same problem at that time. In short, something is happening to shut down the unit and you'd need to know at what point in the cycle the unit shut down and caused the unit to "lockout". Any failure that happens 3 times will put the furnace into "lockout" for a set period of time before it tries again. Sorry I can't be of much help based on this info.

    • profile image

      Greg Staples 

      8 months ago

      Furnace stopped working the other day, wiggled wires, tripped breaker back and forth, after a bit it came on and worked through the night fine. Same thing happened again the next night. Replaced the breaker because there didn't seem to be any power going to the furnace. Furnace is working so far today but when it is running it seems to ossolate power now and then. Any ideas?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      8 months ago from Ohio

      Thomas - It sounds like you're not getting enough air flow when the filter is in. Perhaps it's restricting the airflow just enough to make the unit overheat and shut down prior to satisfying the thermostat. Often when adding a high end air cleaner, the ducts coming to it need increased in size to allow for more air to keep the unit temperature within it's confines and not overheat the motor or exchanger. Hope this helps at least to understand what the issue MIGHT be. Thanks for reading.

    • profile image

      Thomas 

      8 months ago

      If I remove the air filter on my electric furnace then I heat until it reaches the Nest thermostat selected target temperature. If I place the air filter in, it doesn’t reach the selected temperature but is about 4 degrees colder. What is wrong and what is the fix?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      8 months ago from Ohio

      Dave - They can vary. The best way to know your range is to check the High Limit/Fan Switch. There is usually a temp range on there for your unit.

    • profile image

      Dave 

      8 months ago

      What should the heated air coming out of heat exchange Be is there a minimum temp ??? Like 120 deg ?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      9 months ago from Ohio

      John - That's an odd one but likely an issue with communication between the stat and furnace. I'm sorry but can't offer much there other than rechecking your programming and running electrical tests to see what's making the unit shut down in that first cycle.

    • profile image

      John Beattie 

      9 months ago

      My gas furnace comes on for the 1st program cycle of the morning but doesn't complete the cycle and will stop with out reaching the programed setting. If I turn the furnace on/off it then acts like it completes a heat cycle without the burners on. Then in a minute or so later the furnace comes on and then regularly runs the rest of the day. what might be wrong?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      9 months ago from Ohio

      Ozzie - A high efficient furnace has an intake and exhaust pipe and connection usually. Sometimes guys will let the unit pull air from in the home on the intake and not run it all the way outside. In order to do that, they are supposed to make sure there is enough free square footage to provide the needed intake. Taking that door off may be giving you just enough to do the job but you shouldn't have to leave any covers off your unit to make it run. Hope that helps.

    • profile image

      OzzieSch 

      9 months ago

      What exactly brings in the outside air for an efficient furnace? I don't get enough outside air to combust the gas. When I take off a little side cover on the furnace to let in inside air, the furnace works properly. What is my problem?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      9 months ago from Ohio

      Lee - If that's the case then they may need a good cleaning. I'm a little stunned though since a flame sensor should detect that the burners aren't lit since one needs to fire to fire the next and the next. ???

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      9 months ago from Ohio

      Marco - I'm sorry but for me to help I would have to perform electrical circuit testing to pinpoint your problem here. I would start with the thermostat. Sounds like it may be suspect.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      9 months ago from Ohio

      Thank you Jonas!

    • profile image

      Jonas Porter 

      9 months ago

      This was just an awesome explanation. Thank you. Thank you.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      10 months ago from Ohio

      Kevin - There is obviously a short somewhere. Likely culprits are the transformer and/or entire board. There could be other causes but this is what I'd suspect. Thanks for reading.

    • profile image

      Trina Bowers 

      10 months ago

      Ok i checked the fuse, i clean the heating sensor and the blower is working, glow ignitor comes on, slow flashing green light. I checked the thermostat ....the problem is i cant seem to get a flame ignited i dint know what else to do...i want to call for help to be my last resort.

    • profile image

      Jw clink 

      10 months ago

      I replaced my ignition last year and furnace work fine.

      This year it will not even get hot.

      Fan works fine.

      Any idea?

    • profile image

      Kevin Zoch 

      10 months ago

      I have a Trane Furnace that I determined has a blown fuse on the control panel. When I replace the fuse and push the black switch that the metal panel touches it instantly blows again.

    • profile image

      Aaron 

      10 months ago

      I have a Colman Evcon. It is flashing 4 flashes meaning bad limit switch. Tested all switches and checked out. Have a green flash when unit not heating. Bad board?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      10 months ago from Ohio

      Duane - It sounds like it. If the inducer fan/motor is shot then nothing will work as it's the first step in the heating cycle and if it is bad, the whole assembly is usually sold as a piece and is typically specific to it's brand so universal parts aren't likely going to be an option for repair. You'll need to find the distributor for your brand of furnace to find the part most likely. Thanks for reading.

    • profile image

      Duane 

      10 months ago

      The induction fan does not start it just hums. It does not spin. Is the fan bad or is there something else I should look for?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      11 months ago from Ohio

      Amacblue - The coil sits on top of the furnace. It's not part of the furnace. It is not always possible for a homeowner to do this since sometimes there is not way to access the underneath of the coil where dirt and such collects in which case it would need to be removed and that DEFINITELY requires a service technician. If you can however access the underside, congrats and it should be cleaned but carefully and not to bend the fins over so that air can't pass through. Thanks for reading.

    • profile image

      Amacblue 

      11 months ago

      I would like to clean my coil on my furnace however o can't seem to locate it. It is a carrier just like the one pictured thank you

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      13 months ago from Ohio

      Luis - You may have a blown fuse on the board. It's usually 3 amp fuse that is like that in a car. Can't say for sure but that's where I would start. Thank you for reading.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      17 months ago from Ohio

      Robby =- There is a laundry list of things to check from here however it doesn't seem coincidental that this is happening after the stat was changed. Sounds like perhaps your white wire isn't making good contact or in the right spot. Double check your stat wiring. Beyond that, there are too many things to check to list here. Fuse on board? Gas valve? Ignitor bad? Limt? Pressure Switch? I'm sorry I can't be more helpful from here. Thanks for reading.

    • profile image

      robby 

      17 months ago

      My heater stopped working yesterday nigh (it had stopped once few months ago, we changed the thermostat battery and it worked fine). After it stopped working, I put a new thermostat and still no luck. These are the few observations, please help in letting me know what is wrong

      1) the AC works fine with both thermostat. The AC turns on (outside the house) and the blower blows air

      2)On heat, the fan ON position gets the blower working (and you can hear the blower unit below the furnace making noise).

      2)On heat, auto, nothing is working. The blower does not work, the flame is not lit (however the pilot flame is lit fine)

      While changing to new thermostat, twice on Heat/AUTO, the blower came on but it kept pushing cold air for minutes. Now even that is not happening. Manually pushing the switch in the furnace gets the blower going, but there is no furnace flame(only pilot flame).

      Please let me know what to do next

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      17 months ago from Ohio

      Arash - It sounds like you had/have a short circuit somewhere. If you don't have a voltmeter you will have a very hard time finding it. You may want to call a tech or you could find yourself spending a lot of money on parts you don't need or buying a part that will short again because you didn't identify the source. Thank you for reading.

    • profile image

      Arash 

      17 months ago

      Hi Daniel, I have a Lennox hvac with two zone. My heater stopped working last month an no power on thermostats. I notice the fuse on zone controller board is blown. I changed the fuse with a 5 amp. The power return to thermostats but heater didn't turned on again. I ordere a new zone controller but again the fuse blown in new board. So I thought the problem is from main controller board. I ordered and replaced the main controller plus adaptor. Still heater doesn't work. The egniter glows but the gas valve doesn't release gas to flame. I checked the voltage on gas valve and it's not 24v. It keeps trying ignite and doesn't go to watch guard mode. Would you please help me to figure it up? Thank you

    • profile image

      Ccole456 

      18 months ago

      Thank you. I looks up the difference and it appears it is an ignition board. Thanks for confirming. We have a number of things to try and hoping to keep the unit going for at least a few more months!

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      18 months ago from Ohio

      If you have a spark ignition board then you perhaps will not have what I was thinking of. My apologies. There are so many boards out there and this was not what I was envisioning. It would be in plain site if you could change it and I don't believe you can open those without causing further damage.

    • profile image

      Ccole456 

      18 months ago

      Daniel- Curious if you can assist in locating the fuse on the board. The white Rogers control seems to be sealed. The fuse would be inside the case correct? The main fuse at the switch is fine we did check that one.

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      18 months ago from Ohio

      Perhaps you've blown the fuse on the board. Typically a 3 or 5 amp automotive type fuse and/or perhaps the 120v/24v transformer took a hit. That could be the humming you hear. All of these are cheap places to start. I'm sorry but troubleshooting over the net is very tough so I try to recommend some of the simplest, cheap repairs to folks.

    • profile image

      Ccole456 

      18 months ago

      Daniel Robbins thanks. Nothing we have read or viewed yet has led us to the capacitor. Haven't been able to make any of the initial trouble shooting techniques give us a clear yes or no. We had a number of power failures from wind the day before it stopped working. If I understand you the the old White Rodgers control panel should illuminate to indicate at least something.

      We will look into the capacitor then probably need to call technician if that doesn't work?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      18 months ago from Ohio

      Ccole456 - Sounds like the main blower isn't turning and that humming may be the motor trying to start. There is a capacitor usually mounted to the blower housing that may be bad and would be a cheap easy place to start and if that doesn't work, perhaps the blower is shot. That said, if the light on the board isn't lit, then you may have taken a hit electrically and then the potential possibilities get to be too many to cover here. That said, I would start at that capacitor. Thanks for reading.

    • profile image

      Ccole456 

      18 months ago

      York gas furnace clicks on and then you hear a low continuous buzzing noise (seems to come from near the control panel) and nothing else happens. Replaced inducer motor thinking that was the problem, same issues.

      Stat working and fan doesn't turn on when switched "on" at stat. Getting power where stat wires connect to pressure switch. Tubing at pressure switch appears unblocked and in good shape.

      Could it be the pressure switch? Could it be the control panel? There is one light on control panel but it doesn't illuminate, no blink code, nothing. Could it be something else? Thoughts?

    • profile image

      Jill 

      18 months ago

      Just replaced an oversized furnace with the right sized one as i was having issues with the furnance constantly coming on. I lowered the thermostat several degrees during the day so it would not come on as much. With the new furnace I thought this would all be resolved but it has not. It comes on more frequently and even with the temp set lower it comes on every 10 mins. Any advice? I've been dealing with this for 2 years and I just want an answer!

    • profile image

      Wayne Hogan 

      18 months ago

      Have a 2-stage furnace. furnace runs great on high, runs for about 2 minutes on low then gas shuts the gas valve and the cycle repeats many times till thermostat is met. I have replaced the roll out switch and flame rod and it still occurs. Any suggestions?

    • profile image

      Chris taft 

      19 months ago

      Carrier weathermaker has been having flame out problems, had a tech come out and tell me that the intake/exhaust wad too small and needed to be 3", so all that is changed and now it seems like it won't cycle back to back without having an issue. Turn power off to furnace and back on will work for 1 maybe 2 times. Any advice?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Michael Riemer - Not real sure. Could be too large of BTU. Could be the LP kit to make it LP wasn't adjusted properly. Could be a new stat is more accurate and the old one wasn't keeping the temp as consistent. Sorry but this is a tough one that would require further investigation onsite than I can provide here.

    • profile image

      Michael Riemer 

      19 months ago

      Put in a new weather king LP furnace in fall and it uses what I think is to much fuel. It will kick on up to four times in a 15 minute period even when it is in the 30 and 40 outside. This furnace uses way more fuel than the 25 year old one taken out. What is wrong

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Mickey - If you have air conditioning then perhaps the coil needs cleaned. If plugged, air cannot get out and thus trips the limiter you mentioned. It sounds like it is doing what it's supposed to by shutting down if too hot. Pulling the filter is helping because it's allowing better air flow but obviously, not good enough and you can't run it without a filter. Can't be positive but I would have that checked. Probably would be very difficult to do yourself since it's under the coil that likely needs cleaned the most and is usually very tough without removal which requires many tools.

    • profile image

      Esmeralda Medina 

      19 months ago

      So last night at wanna say 9:00 pm it was getting super hot so I put the thermostat on 70 and I had fell a sleep woke up at 6:00am n usually I turn the thermostat on 73 and it takes a couple of seconds to turn on so I notice didn't kick on well my son said It snowed n he check and it was coverd in snow so then it didn't work and still hasn't work I really like to fix it because Haven a professional would charge an arm n a leg thank u

    • profile image

      Elmer Garcia (Houston, Texas) 

      19 months ago

      thank you so much, I got problems with my furnace. During the day is working pretty well if the outside temp. is over 40 but instead that, means below 40's the system don't starts up...I changed the ignite and after I read your troubleshooting chart I will proceed with the pressure switch.

    • profile image

      Jeanine 

      19 months ago

      How do I keep ants out of my pressure switch?

    • profile image

      Mikey 

      19 months ago

      Hello.

      I have a 90+ Achiever furnace for about 14 years. Nice furnace until this last month. I'm receiving a 3 blink error code which suggests a limiter problem. The furnace lites properly and runs for about 3-5 minutes and then the torches are shut down and the 3 blink code appears. This happens over and over again and never achieves the targeted temperature. If I remove the air filter then the furnace will run until it reaches it's selected room temperature. The filter is new and of the same type that I have been using for years. I also use to shut off rooms in the house by using a butterfly door for the feed line of a couple rooms. That technique saved me a lot on heating, but I think that I am wearing something out as the system can no longer run in this fashion. After opening all of the room feeds the system worked ok for another few weeks. Something is changing and I am without a clue.

      Any advice is appreciated,

      Mikey

    • profile image

      Kevin 

      19 months ago

      I got at house that i been working on. I got furnace to run normally for 1/2hr, ran to the store. When i got back it was blowing cold air out. Went down to find there was no flame. What could be the cause?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Jesse Adkins - I wouldn't wait too long. Sooner or later it will stop completely. Thanks for reading.

    • profile image

      Jesse Adkins 

      19 months ago

      I have experienced a problem the Tech deemed the gas control valve. I had intermittent lighting, but since they left it has ran trouble free for two hard days. He said it was sticking or going bad. Should I just leave it?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Frank D - You're probably right. The motor or the capacitor for that motor. If the wheel is hard to spin though, I lean toward the motor. Could try cleaning up the motor and fan blade/housing and oiling the shaft just a hair...see if it frees up. May need done either way so it can't hurt. Make sure the power is off. Thanks for reading.

    • profile image

      Frank D. 

      19 months ago

      Everything works except the blower motor does not come on. I tried it on fan and still nothing. It ignites lites and produces good flame for a minute then it goes out. The blower will not come on. I think the squirrel cage used to spin easier, however, I am guessing it is the motor. Do you have any suggestions?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Bandit - If you do, turn the power off first. Beyond that, gaskets are hard to predict. It's always a gamble. You could try and disconnect the pipe to it first and see if you can see anything from up top first and then proceed from there. Just go slow. If you take the screws loose and give it a little tug, you should be able to tell if the gasket is gonna hold or disintegrate.

      Thor - That's weird but perhaps it's not getting proper voltage from the board and it may be your culprit. Can't say for sure though and may need some testing I can't explain here to confirm or deny that.

      Calihome and Ed Poole - I'm sorry but there are a lot of things it "could" be based on the information provided. Start with the easy stuff...filter, trying to document conditions when it's acting up to see if there is some tie to it all, when it acts up...try to capture the code the service light is blinking and refer to the code chart.

      Thank you all for reading!

    • profile image

      Bandit 

      19 months ago

      Great Site. Lots of useful information. I noticed a ticking sounds coming from my furnace the other day. After digging into it a little bit, it appears the ticking is coming from the inducer fan/motor. It sounds as if the fan blades are rubbing on something but I haven't opened anything up yet. I was wondering if its safe to remove the assembly since it appears to have a sort of gasket where it mounts and I wouldn't want any gases leaking into the house if it doesn't make a good seal when I reassemble everything.

    • profile image

      thor 

      19 months ago

      I have a gas furnace and the fan won't spin. After I give it a spin manually it starts spinning and will stop and spin again as the heater turns on and off but at some point it will not spin again as the heater goes from off to on. I thought it was the capacitor and bought a new one and it worked the first time I flipped the switch on. However, if I flip the switch off and back on again the fan won't spin. Moreover, if I unplug the capacitor wires and plug them back the fan will start when i first turn the switch on but won't start again after turning the switch off then back on.

    • profile image

      Calihome 

      19 months ago

      My furnace is running intermitently. The thermostat clicks on the flame lights but nothing blows out, would I start with checking the high limit switch or would it directly be the blower motor? And why would this only happen occasionally? Thanks!

    • profile image

      Ed Poole 

      19 months ago

      My furnace will work properly at times, and sometimes not. It will heat up to what the thermostat says, and sometimes it won't heat up all the way to the setting that we have it set on, and will shut off. What could the problem be?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      19 months ago from Ohio

      Tommy and Wayne - Given the information, you may just want to call a tech. There is not enough here to determine what of many possibilities you may have. Thank you for reading.

    • profile image

      Tommy 

      19 months ago

      I have a Goodman 90% gas furnace that won't stay running if the inducer exhaust pipe is connected to it. No clogged pipes.

    • profile image

      Wayne 

      20 months ago

      I have a bryant furnace and when it comes on the fan blower comes on and the ignitor lights up but my furnance will not ignite ?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      20 months ago from Ohio

      Newbietohomeowning - I think if it's running normal now, you're fine. Lucky it didn't take out the blower but if it was removed and all seems well again, leave well enough alone...so long as he didn't condemn the heat exchanger. Your cycles seem pretty normal for those temps too.

      AngieFreezing - Sounds like a flame sensor to which I've written how you can check that easily.

      Ken - If you're blowing ignitors that quick, there has to be bad power coming from the board or your ignitors are the right voltage. I can't think of anything else.

      James Wellman - It sounds like you have the door off and are holding the door switch which is like your refrigerator light button. It's there for safety so when working on it the power is cut from that button forward as a protection. When you push it, you're overriding that. If you put the door back on, it will work as it should.

      Thank you all for reading!

    • profile image

      Newbie2homeowning 

      20 months ago

      This is a fantastic article! Thank you for keeping up with it!! I have a Bryant. A few weeks ago, through error, I learned furnaces have filters! Never knew this. So long story short: it was cold out, I bumped the heat from 64 to 66 and a puff of smoke came out of my vent. Only one vent...

      I panicked and called a technician. The filter had been old and was sucked into the 'monkey cage' as the tech called it. Turns out the smoke may have only been dust blowing out of the vent since it's the closest to the intake. His immediate response was to try selling me a new furnace since this one is approx. 15-20 yro. He didn't service it. Just tried to sell me a new one. However...now I'm paranoid about it. It sounds no different than ever...new filter in there...house maintains average temp as set. On really cold days of about 10 to 15 degrees with wind (drafty house, 125 yro, 3 story) it runs five times an hour. Is this normal? 30-40 degree days 2 to 3 times an hour. It meets the thermostat and shuts off as expected but then temp drops quickly due to chill. So it re-runs. I keep it set between 64/64 as that's quite comfy.

      Help my paranoia!!

    • profile image

      AngieFreezing24/7 

      20 months ago

      Ok my furnace goes through all the steps but after it ignitor glows and it flames up with blue flames then blower kicks on and gas cuts off.

    • profile image

      Ken R 

      20 months ago

      Having intermittent issues with my Rheem gas furnace. Call for heat and fan comes on. Gas valve cycles and most times the igniter won't heat up to light gas. It runs through two cycles and stops. Blower still runs for a few minutes as it should. I have two green lights at control board. No flashing lights and no red lights. I have had technician replace igniter twice in one week and he checked all of the electrical continuity. What are we missing in trouble shooting?

    • profile image

      James Wellman 

      20 months ago

      Here is a problem. Maybe you can help? The heater only works when i press and hold ( what im assuming is) a manual override switch. When i press and hold the switch the hot surface igniter heats up , the exhaust blower comes on, the burners ignite, then the blower motor kicks in. When i let go of the switch, it all shuts down. What is a likely cause of this?

    • Cre8tor profile imageAUTHOR

      Dan Robbins 

      20 months ago from Ohio

      Steven - There could be a lot but I would start with the stat and it's wiring.

      Addison - The gas valve may be the culprit. Do you hear it click when the ignitor glows?

      Regan - Most furnaces within the last 25-30 years have a 3 amp automotive type fuse on the circuit board. This could be it.

      Thank you all for reading.

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