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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 in aspects 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?

See results

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.

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    • Cre8tor profile image
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      Dan Robbins 2 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.

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      robby 2 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 image
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      Dan Robbins 2 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.

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      Arash 2 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

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      Ccole456 3 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!

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      Dan Robbins 3 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.

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      Ccole456 3 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 image
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      Dan Robbins 3 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.

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      Ccole456 3 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 image
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      Dan Robbins 3 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.

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      Ccole456 3 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?

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      Jill 3 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!

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      Wayne Hogan 3 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?

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      Chris taft 4 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?

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      Dan Robbins 4 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.

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      Michael Riemer 4 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

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      Dan Robbins 4 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.

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      Esmeralda Medina 4 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

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      Elmer Garcia (Houston, Texas) 4 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.

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      Jeanine 4 months ago

      How do I keep ants out of my pressure switch?

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      Mikey 4 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

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      Kevin 4 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 image
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      Dan Robbins 4 months ago from Ohio

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

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      Jesse Adkins 4 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?

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      Dan Robbins 4 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.

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      Frank D. 4 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 image
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      Dan Robbins 4 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!

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      Bandit 4 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.

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      thor 4 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.

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      Calihome 4 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!

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      Ed Poole 4 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 image
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      Dan Robbins 4 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.

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      Tommy 5 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.

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      Wayne 5 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 image
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      Dan Robbins 5 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!

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      Newbie2homeowning 5 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!!

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      AngieFreezing24/7 5 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.

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      Ken R 5 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?

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      James Wellman 5 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?

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      Dan Robbins 5 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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      Steven 5 months ago

      When I turn power switch on all that happens is control transformer buzzes an nothing happens.I have two green lights on no blinking lights.

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      Regan B 5 months ago

      Hi.

      This site is awesome, but I'm a newbie to this furnace stuff. To make a long story short, our power went out and 2 circuit breakers went, in an area where my thermostat and furnace are located. I switched them back on and everything worked but our old Rheem Highboy-Lowboy- Counterflow gas furnace. I've tried all the cursory inspections and resets, but still it's not working, although the pilot light remains on. Is there a possible directly related solution that would be related to a power surge or the power outage ( i.e. the thermostat versus other links in the step by step chain mentioned above) that might help me fix this problem?

      Thanks in advance for your help.

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      Addison 5 months ago

      Hello, This article was very helpful in helping me understand ow my furnace works. Thanks! I am having an issue with my furnace. The ignitor seems to be working. It glows red when it cycles but there is not ignition of the gas. I am not sure how to check if the gas flow is working properly.

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      Dan Robbins 5 months ago from Ohio

      Eric - You may have something blocking the first orifice where the gas enters and hits the ignitor. Check that out.

      strechy - I'm thinking your board is the issue. Anytime something is so erratic, that's my first thought.

      Thank you both for reading.

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      stretchy61 6 months ago

      I'm having a problem with my Rheem gas furnace that is wacky: the symptoms changed almost every other day. first it was the ignitor not working intermittently. I changed the ignitor and still had the same problem, but them it worked for a few days, then it started cycling on and off before the temperature was satisfied. then the next day it was working fine again, but two days later when I called for heat, the vacuum switch on the draft inducer started clicking and the motor was going on and off, so I shut it down, but the next morning it ran flawlessly. last night when I called for heat the sequence started as normal but when the blower fan started it was running slower than I've ever heard, so I shut it off and tried to restart it, but it was acting the same way-- running slower, so I shut it off. this morning however, it ran all morning perfectly, and when I got home from work I just fired it up and it's working fine! I'm thinking it must be a malfunctioning circuit board? Oh, I might add that it's almost 26 years old and I'm sure I'm better off having it replaced, but I can't resist the challenge of trying to repair it even though I've already called for an estamate for a new one next week. My new circuit board is hopefully going to be here tomorrow so I can find out if that's actually the problem? Have you ever heard of intermittent/multiple problems like this? FYI:My Furnace Info: Rheem, Model # RGLG-07NAMGR/ Serial #CE1D307 F1092 2736/ Gas Valve: Honeywell #VR8205H-8016

      Board #1012-83-800B

      board #62-22694-01

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      Eric 6 months ago

      My furnace has been acting up the last month. I thought it was the gas valve because the furnace would run through its cycle all the way to the glow plug heating up, I'd here it call for gas but then nothing. So I changed the valve. Now it will do the same thing except it will light about every 15 to 20 tries I don't know what's going on. It's a 90% any ideas?

    • Cre8tor profile image
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      Dan Robbins 6 months ago from Ohio

      Jason - I'm glad you got the issue resolved and thank you for reading and commenting. Your experience will hopefully help others.

      Christine - It sounds like there may be an issue in the low voltage wiring from the stat to the furnace. Has any work been done to that recently? Did they check that? This is often overlooked because it can seem too obvious. Thank you for reading.

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      Jason 6 months ago

      Update: I have resolved my issue. Just for future reference in case someone else encounters this issue, the flame sensor wire was connected to the six pin connector on the original board with the ignitor wires. On the new board, the flame sensor connects to the #7 hole in the nine pin connector. The furnace is now working properly.

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      Christine Novack 6 months ago

      Hi. I have a 2p year old Trane 90 furnace. This is the problem. When I turn it up more than four degrees the fan continues to run after the desired temperature is reached. I can hear the thermostat telling the furnace to to send heat after the room temperature drops. The furnace will not blow heat until I manually turn the furnace off and on. Two service calls, hot surface ignitor and thermostat both replaced with no fix for the problem. Help please!

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      Jason 6 months ago

      I have a comfort-aire conquest 80 furnace with a rheem 62-22737-08 control board. The furnace will come on, flame will light however the blower will not come on when set to the Auto position at the thermostat. The fan will come on with the thermostat set to the ON position.

      I have replaced the control board with a new one however I am still having the same issue when the thermostat is set to Heat and the fan is set to Auto.

      Auto does work for A/C however. Auto did not work for A/C prior to changing the board. Any thoughts on what could be causing this?

    • Cre8tor profile image
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      Dan Robbins 6 months ago from Ohio

      Eric,

      I would check or replace since they're cheap, the roll out switch and hi limit switch. The roll out is mounted near the burners and about the size of a dime with 2 wires. The hi limit is usually mounted above the burner area to the exchanger wall. Rectangular plastic with 2 wires. Pull it out and it has a disc like the roll out on it about the size of a dime.

      Hope this helps. Sorry for delay. Thanks for reading.

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      Eric 6 months ago

      Hi, I nave Rheem RGDG-15EARJR.

      Once power on, the inducer is working, ingitor on and then flame on but 1 or 2 seconds later gas shut off and keep 3 times repeating then shut off. But still even no flame, fan motor is working.

      I replaced new flame sensor but still same problem.

      What is the next step to take?

      Is it control board problem ? Or any other do I need to check before replacing control board?

      Thanks in advance. Eric.

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      Dan Robbins 7 months ago from Ohio

      dahoop - Thank you. I'm glad I could help.

      Dee and Franc75 - When the heat is off or set to cool, nothing on the furnace should be trying to do anything except turn on the blower when the A/C asks it to. The wiring from the stat to the furnace could either be wrong or shorted out so that it's miscommunicating. This is the first thing I would check.

      sixcto - If everything is turned off, the issue may just be that the sun beats on that side of the house or that is the weather side of the house. I can see how plumbing would have anything to do with it.

      Thank you all for reading and my apologies for the extremely late response to some. Hopefully it will be helpful to others who read the comments.

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      Dahoopd 7 months ago

      Awesome video! I could not get the pilot to light so I followed the video and cleaned the igniter and booya, we have heat. Thanks again for the great video.

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      Dee 8 months ago

      I am completely ignorant with this, please be patient. When I do not have my thermoatat set the furnace sounda like it wants to turn on then shuts off. This is continuous. During this past summer I just turned off the furnace. Help.

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      sicxto cruz 12 months ago

      question;

      will there be anything wrong with the furnace if while everything supposed to be off that part of the house seems too hot ? or would have been a plumbing issue?

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      Franc75 17 months ago

      Hello, My Hot surface ignitor works intermittently while my thermostat is on the cold setting and the room temperature is higher than the set temperature. The Induction fan kicks on and then turns off, it does this a few times in a row and then stops for a while then it does it again. 2 AC guys have come check it and they replace the main control panel, and install a new thermostat and still doing it. I will appreciate your input on this since I'm at lost with this unit.

    • Cre8tor profile image
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      Dan Robbins 2 years ago from Ohio

      You need a technician. No offense but booms on gas ignition with smoke alarms going off...you need someone to look at this ASAP.

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      raymond 2 years ago

      My rheem furnace goes boom at ignition and the only stays lit for 45 seconds....then cuts off the fire alarm comes on also

    • Cre8tor profile image
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      Dan Robbins 2 years ago from Ohio

      Scott - I'm sorry for such a delay but it sounds like there is an issue at your control board. I've never seen an issue like yours but it seems the board is allowing voltage to go direct to the inducer without command so that's why my initial reaction is this. Thank you for reading.

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      Scott 2 years ago

      The reason I noticed anything was wrong with my furnace was when I turned the system thermostat off, the inducer motor stayed on. My furnace light was flashing that the roll out switch was open. I replaced both roll out switches with new ones, but it is still flashing this message and having the problem that the inducer motor won't turn off even when the system is off. I really don't know what to try next. Any ideas?

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      Dan Robbins 2 years ago from Ohio

      ADam Geary - Electrical testing would have to be run to see exactly what is causing the issue. I see no reason it should be if the furnace isn't asking it to.

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      ADam Geary 2 years ago

      why does my electric furnace heating element cycle on and off after furnace heats and shuts off

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      TeddydaBear 3 years ago

      Thanks anyway for responding.

    • Cre8tor profile image
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      Dan Robbins 3 years ago from Ohio

      @TeddydaBear - I read it...just wanted to make sure. At any rate, I'm not real comfortable suggesting you add anything to the tank or alter the regulator. What you have isn't a real cut and dry situation and as much as I'd like to provide you an answer, I'm inclined to say this may be something you want to at least call someone out and let them offer advice after checking some things first hand. It may cost you a trip charge but then you sound like you can learn from them and take it from there. Might cost you a few extra bucks but you'd still save and be confident nothing you do will cause further problems. Sorry I haven't been of more help on this one. Thanks for reading and giving it a shot. Let me know what you find if possible.

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      TeddydaBear 3 years ago

      While I appreciate your comments and help.

      I don't feel as though you read my text thoroughly. I mentioned that the unit has been in place and running fine for about 13 years.

      It is only after replacing the 500 gallon tank

      with a 1000 gallon tank and new regulators

      that this problem came up.

      I visually can tell by opening the valve and trap

      (Pipe plug) at the end of the gas line (just

      before the line enters the furnace) that the

      flow of gas is 1/2 of what it was when the

      system was new. This has nothing to do with

      the furnace. It has to do with either water in

      the LP itself....or a malfunctioning regulator.

      The furnace is starved for fuel.

      It cannot achieve operating temperature if

      it is only getting 1/2 the propane necessary

      to operate.

      I just wanted to know if there is a way to add

      more ethanol to the tank as an extra drying

      agent, or which regulator is causing the

      restricted air flow (e. g. malfunctioning or

      freezing up).

      Of course it was properly set up for LP and

      adjusted properly. I had an HVAC guy come

      in and check my work as well as it needed to

      pass all state and local codes.

    • Cre8tor profile image
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      Dan Robbins 3 years ago from Ohio

      @TeddydaBear - Being online I can only do/say so much. They stopped making these the same year you installed yours and first I'll ask, when near the unit, do you smell say a rotten egg like smell? Perhaps the exchanger is bad and it needs replaced. If clogged, particularly the secondary, the unit will act like this. I just replaced one doing very much the same thing and there's no quick fix.

      If this has always been intermittently a problem...you did convert the unit to LP when installed right? A gas burning furnace is set up for NG usually unless specified out of the box and would need an LP kit and adjustments to the gas valve to use LP.

      You should have a code light on the circuit board that can narrow this down. Read the code like morse code and there is a chart on the unit to help with potential problems. We would have more to go on then. I'll be watching here today so if I can assist with the code interpretation I will do my best.

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      TeddydaBear 3 years ago

      I need help accurately diagnosing what is

      wrong with my furnace!

      I have a carrier 9200 weather maker on LPG.

      During mild periods of weather (around 30 or

      above) the furnace cycles normally.

      Usually, when colder....it shuts off and goes in

      lockdown/out mode.

      I went from a 500 gallon tank to a 1000 gallon

      tank and new regulators.

      I installed the furnace, ductwork, tank, lines and

      programmable thermostat myself, 13 years ago.

      So I don't think it is workmanship.

      I suspect two things. A malfunctioning or

      freezing regulator, or water in the propane.

      One thing I checked is visually inspected the

      force the propane has while exiting the trap

      below the shutoff valve to the furnace.

      It is approximately 1/2 the force I remember

      when the system was new. This leads me to

      think the regulator either on the tank or second

      one on the house is restricting the flow to the

      furnace.

      If left to run the normal program from the

      thermostat.....it will come on and run for a short

      amount of time, but flame out.....and lock out

      after a few minutes.

      But if I override the program and put temperature

      on "hold" and set it very high.....say 85 degrees?

      The furnace will run. But will never achieve the

      high temperature set. It will run constantly,

      all day long and only achieve maybe 72 degrees.

      This could be due to inefficient burning of LPG,

      starving for fuel and unable to achieve the temp

      called for by the thermostat.

      Question is........because I now have a 1000

      gallon tank, I do not use all the propane every

      season........Does the drying agent (ethanol)

      become ineffective over time? Thus allowing

      moisture to accumulate in the tank? Or is it

      simply a malfunctioning regulator? And which

      one is malfunctioning? The one on the tank?

      Or the second regulator on the house?

      Can any "extra" preventative be added to the

      tank to keep it dryer?

      Help! It is really too cold to experiment now.

      But I am burning off my propane too fast as the

      furnace is running all the time now to keep the

      house warm.

      I am also unemployed.....so big expenses are

      too be avoided.

      Help!

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      pattyejones 4 years ago

      Hey Cre8tor, thanks for the reply.

      After reading your post I went to the basement and checked to see if the fan was getting any juice by disconnecting the plug and using my tester - nada. When I reconnected the connector to the inducer motor it fired right up. I watched it go through all the steps you outlined.

      So I turned the power off and traced the wires from the inducer motor back to each connector and cleaned them with a little sandpaper and it's working now. This weekend I plan on going back down there and cleaning up the rest of them. The basement I have is more like a pit in the ground and pretty damp and dusty.

      This unit was here when I moved in 16 years ago, I'm hoping to keep it going a little while longer. Now, armed with a little more insight, thanks to your post, that might just happen.

      Cheers!

    • Cre8tor profile image
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      Dan Robbins 4 years ago from Ohio

      Well, I like that your thinking in order of process but if the inducer isn't working, the furnace wouldn't/shouldn't fire at all. Without knowing what it's doing it's hard for me to guide you beyond that however I feel quite confident in saying the furnace won't run if the inducer isn't getting power and will shut down if there is a problem there during a cycle.

      Thanks so much for reading and commenting.

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      Patty Jones 4 years ago

      Thanks for posting this. It's a tremendous resource!

      I have been having intermittent issues with my furnace for a few years now. I poke around inside and it starts working for a while, then stops. I have cleaned the flame sensor, checked the ignitor, they seem good. But it seems, after reading your post, that my inducer motor and fan may be the real problem, or maybe it's not getting power all the time.

      Since that is the first thing that's supposed to happen and it doesn't, that gives me a real place to start. I bypassed the thermostat but it didn't help so tells me the thermostat isn't the problem.

      The fan seems free, not frozen, so maybe it's not getting power? We'll see.

      Thanks again for taking the time to put this together!

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      Marcy Goodfleisch 4 years ago from Planet Earth

      Can I just order an entire book of your hubs? You are like an encyclopedia of terrific knowledge for maintaining our homes! Thank you for yet another helpful article - I know my furnaces are getting old, and I honestly don't know what to do about them! Or I didn't before reading this. Voted up and up.

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