Troubleshooting Common Furnace Problems With an HVAC Expert
Figuring Out Furnace Problems
Before you call out the pros, why not take a shot at fixing that furnace yourself?
I have been in the HVAC industry for over 22 years. There are a lot of things that can go wrong with a furnace, but many issues are easy for anyone to figure out and fix. Even if you need a part, don't be discouraged—many of the most commonly replaced parts are easy to install and can usually be bought from a local supplier. Your odds for getting the heat back on sooner rather than later are actually not too bad. Below, you'll find...
- A furnace troubleshooting checklist.
- Tips on common furnace repairs.
- A video guide to help you fix it yourself.
Furnace Troubleshooting Checklist
- Check the thermostat settings.
- Check for power via the breaker, switch, indicator light, and fuse.
- Check that the inline gas valve is open.
- Check filter.
- Check ignitor for ignition.
- Check flame sensor for ignition confirmation.
- Check for flue/chimney blockage: pressure switch.
- Know when to call a service technician.
Below, I explain each of these processes in detail. We start with the simplest problems and then test for more difficult issues. It's important to be methodical and systematic in your diagnosis.
#1: Check the Thermostat
I know it may sound silly, but before we get carried away, let's check the thermostat for a couple of things. You might be surprised how much money per year is spent having guys like me come and turn up peoples' thermostats or replace their batteries.
Thermostat Problem Checklist
Check...
| It Might...
|
|---|---|
for a jumbled digital display
| need replacing (we can test this to be sure)
|
for a flashing battery indicator
| need new 'AA' batteries
|
temperature setting
| not be turned up high enough
|
function selection
| not be turned to "heat" (switch off and on again to be sure)
|
inside for loose or touching wires
| just need tightened or tidied up
|
Mercury-Controlled Thermostats Gotta Go
If you have an old thermostat that senses the temperature with a mercury bulb, get it replaced as soon as possible. It's not nearly as accurate as newer digital models and since it contains mercury, it's not safe.
How to Test a Thermostat
Testing your thermostat is really easy. All you need is a small or medium sized screwdriver and a short (6") piece of 18 gauge wire (what we'll call a "jumper wire"). If you go to the hardware store, ask for "18/2."
Simply go to the furnace, remove the doors, and follow these steps.
- Locate the circuit board where the thermostat wires connect to the furnace.
- Disconnect the wires connected to the "R" and "W" terminals. (The wires should be red and white, but aren't always.) Push them safely to the side.
- Connect your jumper wire between the "R" and "W" terminals.
- Re-secure the door and see if the furnace works.
Did it work?
NO: Then your thermostat isn't the problem.
YES: Then it's likely your thermostat is bad and needs replacing. It could be the wires connecting the thermostat to the furnace, but if they aren't loose, brittle, or broken it's unlikely.
NOTE: Do not leave the jumper wire on the furnace to run it. It will make the furnace run continually without any temperature control which can be extremely dangerous and cause other problems. Only use the jumper wire to see if the thermostat is faulty.
ALSO: These instructions of for testing a regular 24v (low voltage) circuit only. In other words, don't use these instructions if you're troubleshooting a line voltage circuit. (120v or higher)
#2: Check for Power to the Furnace
If you hear the furnace trying to light (ignite) or hear the blower running but aren't getting any heat, then you can skip this step, because you have power. If not, then check the following...
- Did a breaker trip?
- Is the service switch at the furnace on? (Most units have this mounted to the furnace or ceiling just above.)
- Is the little red light on the furnace's circuit board on? (Most modern furnaces have an indicator light to tell you it's getting power, and if you're lucky, tell you what might be wrong.)
Got power?
YES: Okay, then let's move on.
NO: Then you may need an electrician or HVAC professional. Before you call, though, let's check the fuse on the circuit board.
Did the Furnace Blow a Fuse?
Most modern furnaces have 3 amp fuses on the circuit board (although it might be located elsewhere on the boards of certain furnaces). It looks just like the standard size fuse you find in your car and should be easy to find.
If you have an indicator light and it's not on or flashing, this may be your problem and is easy to determine and fix.
Simply pull the fuse off the board and look to see if the link inside is burned and/or broken. If so, you should be able to pick one up at the local hardware or automotive store. DO NOT try to substitute the fuse with one of another amp rating.
#3: Make Sure the Gas Is On
I've been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents a lot of grief (and about $150).
Did You Check the Filter? (I Had to Ask.)
Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?
#4: Check the Flashing Red Light
If there's a little indicator light on your furnace's circuit board (usually visible through a tiny window on the furnace door) and it's flashing, it's trying to help.
This light blinks a sort of Morse code, a number that corresponds to a chart located somewhere on the furnace. It's usually on the doors that you remove from the front. For example, a "dot...dot...pause...dot...dot...pause" would be the number 2, which the chart may say indicates something like "ignition failure" or "pressure switch open."
Understanding the red blinking light on your furnace will help you navigate through the following possible issues that have a possibly simple solution.
#5: Check for Blockages
Furnace Pressure Switch Problems
Sometimes, the little red light indicates an open pressure switch, which often means a flue/chimney blockage. It's also possible the pressure switch is bad, but before we decide that, let's see if the chimney is blocked. Birds and other small animals have been known to take up residence in flues when it's particularly cold. Check for this (and/or icing) by...
- removing the chimney pipe from the top of the furnace to see if there's anything in there. If you have a high efficiency furnace, this may be difficult, but animals are less likely for you. Also check outside to make sure the ends of the pipes are clear and not iced over.
- making sure the little fan hooked to the flue (the inducer fan) is working. If not, you'll probably need a service tech.
DO NOT get up on the roof to check the chimney unless you are completely confident and comfortable in doing so. It's not worth the risk, since usually the problem isn't up there. On occasion, birds will build a nest in the chimney, but if you haven't had problems so far this winter, it's not likely your problem. This is something that becomes apparent early in the heating season.
#6: Check Furnace Ignition
This is easy:
- Watch and listen to your furnace as it begins to cycle. Just before you hear the gas valve click open, you should see the hot surface ignitor begin to glow or hear the click, click, click of your spark ignitor. (You might even see this spark.) If you don't see or hear these things, the furnace will stop the cycle.
- If your ignitor isn't working, the gas valve won't stay open until you replace the ignitor because the flame sensor is telling the valve that the gas isn't being lit.
NOTE: The igniter part can be tricky because it could be the pressure switch that won't let it light so you may have to run some checks on that switch to be sure which is causing the issue.
- If the igniter glows or sparks then make sure the gas is on. I have been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents grief and about $150, so be sure the gas is turned on as well.
- If the ignitor is working but the gas valve didn't open, I am inclined to have you call a heating and cooling technician to work with a gas valve. This is not a simple repair and can cause dangerous issues if not done properly.
- If you have a pilot light, look to see that its flame is lit. If not, this is likely your problem. Try to relight it just like you would a light an old water heater pilot. If it won't light, then you probably need a new thermocouple. These are cheap and can be bought at most hardware stores. (Measure how long of one you need before you go.)
A Furnace Flame That Keeps Going Out
If your furnace lights the burners and then shuts off just a few seconds later, this is almost surely a fault of the flame sensor. This is one of the most common furnace problems homeowners face.
Cleaning and/or replacing a flame sensor is one of the most common and simple heating and air conditioning repairs one can make. Without a doubt, you should give this a try on your own before calling an HVAC technician.
#7: Know When to Call an HVAC Professional
Trying to fix a furnace yourself is great, but know when you need an HVAC professional.
Experimenting can be dangerous and cause more problems than you already have.
Beyond the items we've discussed, there are many other things that could be wrong with your furnace. I have identified the simple fixes above, and if you still haven't diagnosed the problem, messing around any more could compound both the problem and the expense. Although you may not want to, you should consider calling a professional HVAC technician.
Please understand that any time you work with a furnace, there is a certain amount of danger that requires your attention and care. Never perform work on a furnace if you're not sure of what you're getting into and that you've taken all the safety precautions that you can with gas, electric, and sharp edges.
I hope we were able to solve your problem but if not, at least you will have a much better understanding of your furnace which will help you with your upcoming service call. You'll be a very informed customer when they come knocking.
~ We're all in this together ~
Questions & Answers
© 2014 Dan Robbins
Comments
- Author
E Thomas - It very well could be. When there are funky readings and weird activities that just don't seem to tie together for logical conclusion, the board is a pretty regular suspect.
I have an issue with m HVAC unit where the blower runs non-stop, but the AC (or the furnace) wouldn't turn on. I have made sure the thermostat is fine. On the control board, there is no voltage between R and C. The fault LED is flashing an error - Open RO Switch. I have confirmed that that the RO switch circuit is closed. Could this be a bad board?
- Author
I really can't say exactly what is happening but if it started with the new exchanger they should be fixing or at least figuring out what the issue is for you. I know propane at the wrong pressures can produce soot...perhaps it was wrong before he came but a bad exchanger had covered that up somehow? Do you know for sure that the unit was converted to LP? Units almost always come set up for gas and then need an LP conversion kit to make it work with LP which means changing out the orifice and valve spring as well as balancing the gas pressure to whatever the unit requires. If these things aren't done it will have your unit all goofed up.
Hi, last February I had a Heat exchanger replaced in my Miller Propane Furnace. The Furnace starts ok to where ever the thermostate is set for, The problem i am having, is the furnace is turning off and on about every minute about 10 times before staying off. The roof vent is completely covered in black soot and it is all over the roof, None of this occured before the tech changed the heat exchanger. What could be the problem?
- Author
Jerry - I cannot say for you to do that. I do not have enough info to recommend you do anything with those wires other than what they're intended for. If the problem is new then the wires are not likely the issue. Kill the breaker to make the unit stop.
is there any way i can unhook them to wires and run them some where else to see if the unit will shut down or not with just them two wires?the unit only has heat hook to it.
- Author
Jerry - I would imagine the green is hooked to the white terminal then those 2 wires would give you heat but they may have used a device call "add a wire" and then it may be all they needed to do the a/c when that was installed as opposed to pulling new wire. That is my guess.
all the wires on the honeywell digial thermostate are attached didnt hook them up myself but where it comes into the wall to th pigtail plug wiring harness there is only two wires hooked up a red and i believe a green one and all others where cut short there
- Author
Jerry - I don't believe the stat is faulty since there would be other features that would shut down the unit if it overheated and if it won't shut off then it sounds like something is mis-wired perhaps.
have an intertherm electric furnace hooked the power up to it set the thermastate and it was all ready running waiting for it to turn off at th temp i set it but it never shut off and the blow was still running on hight and i could see the coils still glowing red tryed everything to shut it down till i pulls the disconnect over the cartages fuses and it shut down could it be the digital thermastate being faulty?
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Mrs. Klann - While I can't say what's causing this I can say that the unit should be shutting off then because if the flames run without the blower it will overheat and shutdown. If that isn't happening then I would quit running it because it could become a safety hazard. You'll need to call a technician however to find out what's the cause of this. Maybe a bad limit switch or circuit board but again, I can't say for sure based on this little bit of information and not testing the circuit.
My furance flames come on-but blower wont run-unless I keep turning it on by thermostat switch??
- Author
Jesse - The answer to this all depends on your mechanical abilities. For someone who is familiar with taking things apart and putting them back together I'd say yes, it's a pretty easy task. If you're not, then it could be more hassle and lead to more cost than it's worth. If you do, don't forget to buy a new capacitor with your new motor. Best of luck!
I think I have a blower that is starting to fail. Last year I could hear it making a squeaking sound but would always come on. Now it won't come on and the red light is blinking.
Is replacing a blower easy or should I get an HVAC tech?
- Author
Shawn - it could be so many things. Try and catch the code it blinks if you have that LED on your unit next time it breaks down. That would help narrow it down. If not, he really should call a tech for you.
- Author
Bill - I'm not sure what would cause both the fuse to blow and then when that's replaced still not run. Being brand new, I would certainly call the installer and ask them to come figure out and fix the issue.
I have a airtemp gas furnace brand new it ran all night on and off only ran 5 min then went off got up this morning nothing found a blown fuse on the board replaced fuse now ignitor glows but will not light
I have an intertherm furnace, the thermostat died, changed the batteries and it works but the heater won't come on, the diagnostic light shows no problem, I turned off the switch and restarted it and nothing
- Author
Joe, Did you resolve the issue at the thermostat? Just turning it off and on may not have solved the problem. Otherwise, without more information or personally running tests, I'd have a hard time offering any sound advice.
I have a Lennix G27M 4–100a-2 and is turning on every 10 minutes and working for approximately eight minutes and shutting down, this is after finding out that the thermostat was not working properly and I turned off the thermostat and turned it back on, what's going on please help and thank you
- Author
If the igniter is coming on but not lighting then the gas valve isn't opening. That could be because the pressure switch isn't being satisfied or it is failing. If the igniter itself isn't working then you'd have to replace it. I've written other articles that may help you narrow down your problem and perhaps make the repair.
Hello, my furnace was coming on and cycling about 5or 10 sec. later. I cleaned the flame sensor and now nothing. the ignitor switch now coming on slower than usual and now the flames aren't coming on. it just shuts back down,
- Author
Perhaps a burnt out heating element? Definitely an issue though since if there's an odor like an electrical burn then something is shorting out and I'd have a tech come sooner than later.
We have a Coleman electric furnace and it clicks like it’s turning on but it’s not actually kicking on to blow air...and it smells like something is burning when it’s trying to click on
- Author
It's possible and if so then make sure you get stainless steel burners since you have LP...they'll last longer.
I have a 14 year old Rheem outdoor combination propane furnace/ electric air. In the last six weeks or so, on 3 or 4 occasions when I got up in the morning my house felt cold. When I looked at the temperature on the thermostat it read 61 when it was set at 68. After I turned the heat off and on, it went back to working fine until the next incident. The repairman says it needs a new burner. This does not make sense to me. What do you think?
- Author
You should have a chart on your furnace that answers that for you. I do not have these memorized. Every brand and unit has it's own coding.
- Author
If the heat strips are working too hard then the breaker trips. For some reason the amperage is spiking and therefore it's working to hard, there's a short, or there's bad voltage. You need to know what is overloading the circuit. Probably a bad element or going bad. Too much resistance and/or heat due to bad amperage causing breaker to trip.
My electric furnace blows hot air for about an hour and then the furnace breaker trips and it starts to blow cold air. The house breaker is fine. It's the breaker on the furnace that keeps tripping. What could it be?
I have an amana furnace and its flashing a red light 6 times
SO SO Helpful!! THANK YOU
Hello, my furnace is showing a 4 red light error indicating the high limit switch has opened. When the tech arrived he saw the error and tested the switch. According to his test the switch was closed (though the 4 red light error continued to show). He examined the rest of the unit and found that the capacitor was starting to fail (he replaced it). Within a half hour of his departure the 4 red light error appeared again. He believes that it is the circuit board that is the issue. Unfortunately for me he had a few other jobs to deal with before returning to me. He told me to reset the furnace (turn off and on) and remove the access panel to further reduce the heat build up. Since doing this the furnace has run fine.
My question is, does this appear to be a circuit board issue? The furnace has been professionally cleaned very recently (last week) and the filters are brand new. Any insight would be greatly appreciated.
- Author
Cj - I'm sorry but there just isn't enough here for me to offer any real sound advice.
- Author
TD - I really can't say from here. You'd have to run tests to see if that's the cause.
My nest was not getting enough power so I used extra wire to hook common to thermostat. Then a couple days later the nest says no power from R. The furnace board has green light. I tried removing the common wire I had connected before and still no power from R. So I'm not if what I had done before caused this new problem. Anyway I took the extra wire I had used for common and connected that to R on the board and R on thermostat. Now furnace works however fan will not turn off and I have to keep recharging my nest with USB every day. Is there something wrong with the board or do you think it's the thermostat?
Furnace stopped working in the middle of the night, turned it off, turned it back on, nothing. Went into the garage and unplugged it, then plugged it back in while it was on and it began blowing air. Flames never came on, but the furnace continues to blow air. I removed the cover for the circuit board and checked the red light on it after turning the furnace on, the light stayed a constant red. I conducted the jumper wire test which resulted in the thermostat not being the problem. I am stumped, because I replaced almost every part on the furnace within the past 5 year's.
- Author
Jose - There is not enough information here for me to offer advice. I've written quite a few articles that you could read and see if you can narrow down the possibilities a bit.
- Author
Shirley - I can't offer any advice based on this. I would really need to physically inspect the unit for a stuck open damper, back draft, or something of that nature.
- Author
Ian - Check your filter and if the a/c coil is clogged you could be trapping too much heat in the plenum. Or the limit switch itself could be bad. These are where I would start.
I have a rudd force air furnace heater..thats warm up a little then wont start back again unless i switch button to off to heat again..cant find where filter is located
I have a free standing gas fireplace .Will start is and it may work real good for a couple days .Then all of a sudden I look and blue flames are dancing all around in a swirl .I get scared and turn it off ,
My Lennox SLP98 modulating furnace has a red blinking decimal light when on idle mode but it gives an E250 error code( Limit switch circuit open - check why limit is tripping, overfired, low air flow) when it fires up. How do I fix this fault?
Thanks
- Author
van - Well the ignitor is good but beyond that I cannot offer any advice based on this information. Sorry.
my older carrier gas furnace ignitor glow but the won't turn on
Thank you so much for your help. We will contact landlord again and send letter addressing also.. Merry Christmas
- Author
Pamelarenae - I wish I could offer more help and given what you've told me I would say there is definitely an issue and the landlord MUST have someone come back if it's "black as coal" because it's not normal or healthy if it's spewing it all over the home. There would definitely be an issue that needs addressed under those circumstances and by law, the landlord must address the issue.
Its natural gas and he does not believe duct work has been done. He said " The unusual amount of dust that I was getting was actually not dust, it was soot that the furnace has been spewing all over the house" ..the filter is black has coal. Landlord will not send her contrator back out, plus contractor only speaks russian n sasly my friend does not know russian to ask questions. Landlord said in spring she will have ducts cleaned.. it is 2 story townhome, furnace is in basement not in same room with washer dryer if that helps.
Thanks for answering question above.. im waiting to gear back from friend. I know he has natural gas and has commented on how dirty everything gets in his home very fast. Ill write again after he responds to my text. My friend moved in about time furnace was put in so im not sure he will know alot about past duct work. Thank you again.
- Author
Elisabeth - I'm not sure what "steady on" means for your particular furnace. There should be a code chart that says if that means "normal operation" or a particular failure. Is it lighting? If so, then I'm not sure how you'd have cold air. I know that sometimes air can perhaps feel colder than it actually is but if there is a flame in the unit, I assure you it's not as cold as you might think unless for some reason your a/c and heat are coming on together. (Don't laugh...I've seen it before.) Sorry I can't be of much more help based on the information you've provided. There are just too many possibilities to narrow it down based on this. Thank you for reading.
My furnace pilot light is on, red light and yellow light are steady on but I'm get nothing but cold air. I've checked the filter and everything else I can think of. My formate is a Nordyne model # M1MB 070A BW. PLEASE HELP
- Author
Dave - I answered the other question but here there are a lot of moving parts and I'm not sure what you're dealing with. If the fire is on, the air cannot be cold. If the furnace is failing, then the blower will come on as a default and then yes, you would have cold air but after 3 times, it should go into lock out. After all you've done, I would recommend having a tech look into this since it will likely require some tests that I cannot run from here. I'm sorry.
- Author
Ryu8 - Glad to hear!
Problem fixed. Thank you so much.
I have HVAC furnace model number KG6RC 080C-12B. I was experiencing problems where the inducer motor would kick on but after 1-3 minutes turn off. The LED on the board had two flashes which indicated "inducer motor on with pressure switch open." After hours of troubleshooting which included removing tubes from pressure switch and making sure obstructing. Checking for obstruction in exhaust pipe and running shop vac through each end, cleaning out condense tubes I still had two flashes. I finally removed tube from pressure switch to combustion box and ran heat cycle and it worked.however during that cycle warm air would only blow for 15-30 seconds and then blow just cool air. Once I reconnected the tube it would go back to the two flashes. I noticed some grime and residue below the inducer motor so I removed it and noticed the plastic gasket was cracked and brittle. I removed the gasket it fell apart once I touched it, cleaned up the area and reseated with high temp silicone sealent . I no longer get the two flashes and the furnace runs through it's normal cycle BUT I still have a problem where the furnace is only pushing out cool air. The flames do not stay on long at all. I did remove the flame sensor and cleaned it but I fear I may have cleaned it too much by way of over use of sand paper.
I replaced the flame sensor and the limit switch and still having same issue.
- Author
Ray, Sounds likely the pressure switch is either bad or doing it's job because the chimney is blocked or the fan is running too slow to satisfy the pressure switch. Pressure switch is where I would start my assessment.
- Author
Vick, It sounds to me like if this problem occurred at the same time as your repairs, you've gotten something in the wiring incorrect. I would advise you revisit this to be sure. Is it possible you have the transformer wired to the board in the wrong spot thus applying constant power to the circuit?
furn inducer comes on then indoor fan comes on and blinks 5
I installed a new gas valve and transformer. I have no thermostat control. Furnace running constantly
- Author
Colleen - If that is the case you definitely have an airflow issue but what that issue is, I can't be certain from here. It sounds as if there is a lack of return air but what's tripping as a result and causing the unit not to light...??? I suppose if it was REALLY sucking hard for air it could be pulling the gas away from the burner bar, downward, and to the point where it's not getting to the igniter but I would wonder why now and not before. Have you done any work to the system lately?
- Author
Ryu - If it clicks and nothing else happens, it may be the inducer motor. That is the first thing to come on and if there is a problem there, it won't do anything else.
Furnace was blowing cold air, had a tech. come out, he checked everything and all was ok, but if the door was off the furnace would light, put door back on and it would not ignite. Tech thought it may be an air flow issue or door sensor fault, but really couldn't figure it out???
Thank you for your time Dan, you are so nice to try to help everyone saving money here. When I turn it on. The control board red light is on and have one clicking sound on the board after 1sec. The light is still solid on, but nothing going to work. Is it the board problem or some cable problem ? Looking forward for your reply. ^_^
- Author
Rachel - I am really not sure what the cause is here. The heater definitely shouldn't run without the fan on for 10 minutes though. That I'm sure of. I'd recommend calling a tech and if possible, I'd be curious what they find.
- Author
Steve - It would be hard for me to say "definitely" but it does sound logical to change the Honeywell module and is probably much easier for you to purchase at a reasonable cost and even if that's wrong and you end up buying a new control board and doing that, you'd still probably come out ahead financially if doing yourself.
- Author
Ryu - Did the light go back out on the board? It almost sounds like it's shorting out but I can say for sure. Electrical testing would need done to narrow down the possibilities. Trying to see where power is lost or getting too much/little voltage. Sorry. I know that doesn't help much.
Thanks for the advice Dan. I tried to check the voltage from the board to the ignitor. My furnace has a Honeywell S8600 ignition control module. I checked the voltage from mv/pv to pv and did not get 24v. After I light the pilot with a lighter and the burners come on, I checked mv/pv to mv and did not get 24v either. Should I be replacing the control module or do u think I need to replace the board instead? If I’m not getting 24v coming into the ignition module, in that case would you say it’s definitely just the board that needs replaced?
My central gas heater will not blow air during the night. It works just fine during the day. It's 40 outside and my house is freezing. The thermostat says it's 66 inside but it feels colder. I turn the desired temp up to 80 and still nothing blows out of the vents unless it's daytime. I have changed the air filter, reset the schedule in the thermostat, checked the breaker box. I cannot figure this out. I hear the heater kick on when I turn it on, but nothing blows out and 10 minutes later the heater kicks off on its own.
I checked all the step is fine. I replaced my old dead control board(no light) with a new one. When i turn it on. There are only one clicking sound. Then nothing happen. What is that problem ?
- Author
Steve - Since you can light it with a lighter, the pressure switch doesn't seem to be the issue or it would not let the gas valve open. You may not be getting the proper voltage from the board to the spark igniter thus it's not lighting the gas on it's own. This would trip the flame sensor and cause the blower to come on as a safety default. This would typically happen a few times then cause the unit to go into lock out for a set amount of time before it tries again unless you reset the breaker. So, it might be the board. I would check the voltage from the board to the igniter to see.
Dan,
I have an icp furnace with an electronic pilot/ignitor assembly. The furnace comes on when call for heat, the inducer comes on but the pilot won’t light by itself to turn on the burners. If I use a lighter it will light and the furnace will run for a few minutes and the burners will shut off. Then the furnace will run continually without lighting the burners. I replaced the pilot/ignitor assembly and that still did not fix the problem. Do you think this might be a pressure switch problem? What do you think I should be checking or testing. Thanks.
- Author
First, that's the ignitor that's glowing not the flame sensor...that's on the other side of the burner assembly. Second, it will blink this way perhaps until it goes into lockout and then it will blink a lockout code. I can't say for sure but it sounds like the pressure switch. Maybe not the part itself but that whatever is going on is preventing it from getting the pressure it needs to see for the valve to open. Could be a blocked chimney, maybe the inducer motor, perhaps the part is bad...there are a few possibilities and this is just what I'd be checking if I were there.
The system is a gas unit and it won't light. The flame sensor glows but the flames never appear and then it shuts down and starts over. The led lights and fault code list says that after 4 tries it will lockout and it never did a lockout. The led light kept saying operations normal. What could this be.
- Author
Glenn - It sounds very similar to a flame sensor issue. There is another article here I've written detailing how to check it.
My furnace does the following: gas burner ignite for about 15 seconds, then they turn off and the fan begins. Fan runs for about 4 min. I get a short burst of warm air through the vents for about 30 seconds then it cools (room temp). It repeats the process every 5 min. Any ideas on what’s happening?
- Author
Rebecca - I'm not sure what the noise difference is in the gas but typically if it lights and then shuts down immediately it's the flame sensor which I've written another article on. Otherwise, there are a lot of things that can cause that and would need to know exactly at what point in the cycle the unit is shutting down to offer any thoughts.
Hi Dan, thanks for having such a great blog available for people like me who don't know what's going on! I commented earlier but didn't get a response, and was just wondering if you had any thoughts?
Basically, I have an HE furnace that has problems sometimes igniting the burners (at least, that's what the blinking red light code is telling me). I've had this problem for a while now; the furnace tries three times to light the burners, and if it can't, it then just blows cold air for an hour, after which it tries again. I've been listening to it for months and I can now tell if the furnace is going to fail by how loud the gas sound is....if it's soft, it won't work, and if it's louder it'll work. I really haven't been able to predict any kind of conditions that will cause it; I'd say 80% of the time the furnace works fine, but I can't figure out what's causing that other 20%. Any thoughts?
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Gene - The leaks in windows and doors is no fault of the furnace and if they are worse than last year then it would make sense that the home isn't heating well. When is the last time you've had a clean and check?
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Rich - Hard to say from here but being that new your parts are under warranty so I'd let a tech handle it. Also, the blinking light should give you an idea of what's happening.
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Abdullah - I'd check to make sure the orifice the gas is coming out of is clear of obstruction that may cause low gas output....fine for a lighter but not for igniter. Then if still persists, it may very well be the igniter is going bad or not getting the right voltage so it's not getting the gas lit.
My gas valves are working because when I manually light the furnace when the gas is released, the valves are fully lit and do not turn off. Not turning off also indicates it’s not the flame sensor otherwise if even after manually lit, it would shut off. The problem I’m having is the igniter IS TURNING orange but furnace is not catching the flame and after a short period of time, the gas stops being sent and igniter turns orange again and tries to fire up and it just doesn’t want to catch. Maybe the igniter is faulty? Even though it is turning orange, only the bottom portion is turning orange and not the whole thing from top to bottom?
It's a LP furnace and it is sucking the cold air from the doors and windows through the cracks and open area's, an the furnace feels like the heater ain't working right as I got it on 80 and the heat coming out of the floor vents is warm not hot like it was last year at this time of the year
my furnace won't go over 64 & only runs for a short period of time. Its a new furnace 1 year ago. Is there a fan limit switch or something I can ck or maybe a thermostat issue?
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It shouldn't. No. For what it's worth, the green wire is to control the fan. You could perhaps have a short somewhere or a place where that wire is making connection with a live wire. Likely red. It could be in the wall. Sometimes these wires can be brittle in the wall and pulling on them to replace the stat can cause the connection to short. These are guesses but are what I would be checking.
We have two zones. Upstairs/downstairs. Exactly the same equipment. I used the working system to double check the wiring. All is good. If the 'signal' from the old thermostat had the fan on could it be stuck in that mode?
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I think you're correct that the fuse would've blown if you crossed them up. That said, I'm not sure why the blower continues to run unless there is a crossed wire somewhere in there.
Last night I attempted to replace my thermostat with an Ecobee 3 smart thermostat. I thought I turned off the power but it turns out I did not turn off the furnace. I am sure I touched the bare wires at some point when disconnecting and reconnecting the thermostats. I called Ecobee technical service and they helped me troubleshoot to find out that the thermostat was not getting the power it requires. After looking on the Internet I think I most likely blew the fuse on the control board because now the thermostat gets no power. I haven't checked yet. The thing is the furnace blower is running constantly on the slow setting. Would the thermostat not getting power (24 volts) becuase of a burned out fuse also allow the fan to run constantly?
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sb cole - Perhaps you don't have power to the unit. The fuse is blown. The transformer perhaps or the board is bad. There are a lot of possibilities that would require troubleshooting the electrical circuit to find out where the fault is.
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Jen - I can't say for sure but it sounds as if the board may be the problem. Electrical troubleshooting could answer that.
red light does not come on, what does that mean
Thanks Dan. I searched and did indeed find a loose wire in the wall behind the thermostat. Tightened it up but still have the same problem. A little more troubleshooting and I found when I checked voltage at the furnace, running 28 between R & C. When the induction motor starts, it remains unchanged and furnace ignites properly. As soon as the blower motor starts, voltage drops to 0 and flames go out. About 12 seconds later voltage climbs back to 28 and it all starts over again. No other loose wires found.
Thanks! The code says it was the pressure switch, but we replaced that part and we cleaned the flame sensor. I am thinking maybe it is a faulty ignitor.
I had an initial error code that indicated the flame sensor. The furnace would sequence to the flame ignition and the after a few seconds go out. It would cycle through this 3 times and then time out. I replaced the sensor bar and now have a new code indicating sticking gas valve. I had the gas valve replaced. And still have the same code.
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Jerry - It's possible the fan is running slow and thus not creating the vacuum the pressure switch wants to see so you could be right. The other possibility is you have a birds nest in the chimney...it happens a lot over the summer and no one knows until it's cold out. Otherwise, it sounds like you've been checking the items I would have too so I'd be looking at the same things and if none of that, perhaps an issue in the control board if you're sure about the other things.
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Kenny - If you've removed the wires and it still runs with heat and not just the blower, then I'd be leaning towards a short in the board. The blower will run if the unit is hot and you stop the stat until it cools down then the fan would shut off.
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Jacque - I don't think the transformer would do that since it's in a different part of the circuit and would have just started because you have a new stat. Sounds like a loose connection maybe that the blower is shaking the wire and if even for a second, it would cause the unit to recycle. That's just a guess but would be the first thing I would check.
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James - This would definitely require someone on site with know how. Sorry I can't help.
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Jenna - I and they likely can't be sure if the problem doesn't occur when they are there but if you have the LED troubleshooting light, it should tell you approximately where in the cycle the problem occurred. Might be flame sensor or pressure switch related but those are just guesses that could stem from and underlying issue. Try to check the light code before resetting next time.
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Aletta - Not sure how that would be possible unless you have very poor windows, insulation, roof, etc.... These items would let your heat escape and perhaps give the impression that the furnace is the issue when it's not.
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John - You may have to hold that button for up to a minute or so before letting go to make sure the thermocouple is heated. If that doesn't work perhaps you have weak gas pressure or pilot flame which you would need a tech to measure the gas and make those adjustments.
I installed a new smart thermostat. The furnace starts up normally. But as soon as the main blower starts, the thermostat loses power and reboots. After couple of minutes the furnace shuts off and restarts the lighting process, again causing the thermostat to reboot when the main blower starts. Wondering if a weak transformer is the problem where it has enough juice to run the thermostat or the blower, but not both.
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