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Warm air heating furnace troubleshooting:
How to diagnose & fix hot air furnace heating system problems: hot air or warm air heating system troubleshooting, inspection, diagnosis, and repair.
Diagnosis and correction of NO HEAT or NOT ENOUGH HEAT conditions for warm air or hot air furnace and air duct heating systems. We explain how to recognize and fix common heating system operating or safety defects and how to save money on home heating costs.
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If your furnace isn't working, what can you fix yourself and what should you check before you call a heating service company? Carson Dunlop Associates' sketch hows the main layout of a basic warm air heating system.
[Click to enlarge any image]
If you don't know whether your heat is provided by a furnace (hot air) or boiler (hot water), or whether your fuel is oil, gas, or electric,
and whether your heat is hot water, steam, or warm air,
see HEATING SYSTEM TYPES.
If your heating system uses water or hydronic or hot water or radiator or baseboard hot water
heating systems, instead of the article found here you should
see HEATING LOSS DIAGNOSIS - BOILERS.
Note: some hybrid or mixed heating systems combine both hot water and hot air to heat a building, such as
water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger
(that looks like a car radiator) - described
at HEATING SYSTEM TYPES.
If you have power and fuel to the heater, next check these controls
Then the furnace draft fan began to pulse several times upon starting. After making sure the control module showed a steady green, I asked a friend and he suggested that I wiggle all the spade connectors to be sure there is good contact between the copper and aluminum. It seems that there only needs to be a little corrosion to cause problems. That seems to have done the trick. . - M.F. 12/24/2014
Some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).
For a detailed guide to inspecting, diagnosing, maintaining & repairing mixed, hybrid, water to air heating systems
see FAN CONVECTOR HEATERS - HYDRONIC COILS
These buildings will have both a hot air furnace and a completely separate hot water heating boiler installed. In this case these are completely separate heating systems and usually each serves different building areas.
Continue reading at FAN LIMIT SWITCH - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AIR HANDLER / BLOWER UNITS - home
Or see FURNACE CLEARANCE DISTANCES if you are installing a new gas furnace
Or see FURNACE CONTROLS & SWITCHES
Or see FURNACE DIAGNOSTIC CHECKS, GAS for a gas furnace homeowner checklist
Or see HEAT WON'T TURN ON
Or see THERMOSTATS, HEATING / COOLING - home
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Furnace fan powers up, ignitor coil heats up, but burners don't ignite. I have four (4) limit switches on the furnace. If I flick the switch on each one to reset them and restart the furnace, it works fine again for about half a day. However, when the furnace attempts to power up for a second cycle, the same thing happens again (burner don't ignite) unless I reset the switches again. Any help? (This is getting old and I'm not looking forward to the trip to the basement to reset the limit switches every 12 hours.)
Thanks - Jerry 1/31/12
Check the control wiring connections first; As you are describing a gas furnace, it sounds as if the gas valve is not opening.
have a lennox elite series 29, blower comes on but heater doesnt and i have continuity on all four heating elements. any idea whats wrong - Mark 10.28/12
I HAVE A LUXAIRE TWO VENT PIPE 90%afue downflow/horizontal gas furnace HI EFFICIENCY HEATER.The intake sucked in a leaf or leaves during the hurricane into the burner box, or they leaves are either stuck in the intake pipe or somehow is blocking the proper flow of vent air thru the system.My diagnosis view port is showing a 3 flash led pattern.The secondarary vent blower continues to run but will not allow system to start.Is their anyway to clear the debris and restart the system - Tom 10/29/12
For safety you need service by a trained technician.
Hello my friend (a newly graduated hvac technician) and I have been trying to help a local woman with heating troubles. She has a 'Kerr' Oil Fired Warm Air Furnace. We have tried on several occasions to help by troubleshooting in order to make it/ keep it functional. It seems to work for a heat cycle or two and even though it starts, each time the system has needed to be reset. The things that have been done so far include cad cell tested and cleaned, air flow adjusted, tested/ adjusted limit settings and changed over to new thermostat. Can you direct us as to what could be the source of the problem? We really can not afford to hire a technician...and would sincerely appreciate any assistance you can provide. - M.D. 12/28/2013
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. You will find additional depth and detail in articles at our website. That said I offer these comments: Hi Mary, I'd like to help diagnose the Kerr furnace you discuss, but with so little information I'm not confident about what specific details I can recommend by email. I'll make a few suggestions, then you may be able to tell me more and I may be able to comment further.
While there is nothing harmful in the steps you've taken so far, they sound a bit random to me. I'm doubtful, for example, that changing a thermostat would have much to do with a furnace going off on reset - except for a very odd case of a furnace that kept switching the call for heat on and off very rapidly.
And your cleaning the cad cell is a fine idea, but doesn't consider that if the cad sell was dirty then the flame is probably running smoky and there is an adjustment or fuel or nozzle problem, nor does it consider the possibility of a bad cad cell that needs replacement.
When a heating system keeps going off on reset usually there is a problem with fuel or combustion: dirty oil filter, defective fuel unit, air in oil lines, cracked or improperly set electrodes etc. You can take a look at two diagnostic pathways at InspectApedia, one for furnaces and the second focused on oil burners. See
where we include diagnostic flowcharts and very thorough debugging steps.
You should also have at hand the installation and set-up instructions for the Kerr oil fired furnace you mention - though you didn't give me a model and serial.
Kerr oil furnaces are produced by Granby Industries, of which Kerr Energy Systems produces both forced warm air heating furnaces and heating boilers. Contact information: http://www.granbyindustries.com - for furnaces is Granby Furances Inc., 12118 Hwy 209, PO Box 637 Parrsboro NS B0M 1S0 Phone: 902-254-2543 Fax: 902-254-3863 gfisales@GranbyIndustries.com
I would contact the company with the furnace model and serial number to obtain installation instructions if you don't already have them. There are several Kerr furnace models: Gemini, Eclipse, Compact, Paradigm (High efficiency), Summit, Scotty, & Scotsman - so you can see that you'll need your model and serial number to obtain the right installation and maintenance instructions.
But most often a problem with an oil fired furnace that keeps going off on reset will be traced to a fuel, electrode, cleaning, or adjustment problem that is more generic to oil burners rather than to the specific furnace model.
(Feb 17, 2014) sandy said:
I have a Chrysler air temp gas heater that won't heat the house has a new thermostat but need to move it way up and then there is buzzing and never enough heat. please help it is 15 degress here
Sandy, what is buzzing? If it is a control on the heater the control may replacement; if it is the low voltage thermostat that's buzzing that may need to be replaced; Have you called for service?
(May 21, 2014) Marty said:
I recently finished a basement. The customer is complaining that they are not getting any warm air in the basement. We confirmed that air flow to the basement was low and complete a camera assessment of the trunk lines that were installed. They are free of obstruction and there are no misaligned trunks. Outside of normal degree variation of below grade space, do you have any ideas on root cause? Would you agree that the issue is most likely in the furnace and/or blower versus the trunks we installed? What solutions might you suggest? x
Let's look at things that would obstruct air flow. Don't forget to include
- dirty air filter
- dirt, even a little dirt, on those blades of the squirrell cage fan
- inadequate return air
- defective fan, blower drive belt, blower assembly, voltage
(Jan 7, 2015) jimmy said:
I installed a limit switch with the bypass tab on.now nothing works.what would be the most
Likely thing that I burned up .I replaced the switch and still nothing.
Sorry to read that aggravation Jimmy. Check fuses at the furnace and also check to see if the blower motor has a reset button tripped.
(Jan 23, 2015) Tom said:
Hello, I recently installed a new T-stat for a customer who wanted to control their T-Stat with their cell phone. They have hot water radiant heat along with forced air that works in conjunction with the boiler. They have several zones that are just two wire stats. They had one upstairs that controlled the ac unit and the furnace. They had your normal R,W,Y,G wires all hooked up to the appropriate terminals. They had an additional wire that I hooked up for the common so the T-Stat could be powered up. Everything seemed fine when I was there but I received a call from the customer stating that their heat hadn't shut off and it was 94 degrees when they got home...yikes.
Not sure what happened here so any thoughts would help. I had taken apart one of their two wire stats so maybe that thing wasn't working right...although it was just an old mercury stat so not sure what can go wrong with that. When I hooked up the common I followed what I believed to be the common side of the transformer and followed the wire to a group of wires nutted together. It powered up the stat so I thought it was correct. My thoughts are its maybe the old stat downstairs, as that is a heat only. I have to find out what stat is keeping it on first. Not too familiar with these systems working with a furnace so I'm not sure what I did but maybe it will be apparent once I look at it. Thanks
Just last weekend in Chicago we helped a brother-in-law install another Nest thermostat. Nest has a website and cellphone app that gives nice remote control of the thermostat as well as providing data on house temperature when you're away.
We lucked-out: the thermostat wires (there were four, two cooling, two heating) plugged right into where we thought they should by following the color codes.
In your case I'm not sure what that additional wire was; some thermostat models require an additional power or voltage wire to operate or power the thermostat. You might check that with your DMM or VOM. But it does sound as if something is calling for heat.
Don't forget to check for shorted or crossed thermostat wires (or an un-wanted jumper) anywhere in the circuit. That can leave you permanently calling for heat.
More details on thermostat wiring are in our article series on that topic - see
12 Feb 2015 Hoodoo said:
I have a Trane model bld080e936b1 gas furnace that is not igniting. The thermostat seems fine and if I jumper the R and W wires together I get nothing. It has a standing pilot light that stays lit just fine. The thermocoupler seems to be ok. When I tested the R to G with a multimeter I get 27 volts. Same with R to Y but R to W gives only 2.4 volts. I checked the R and C connections on the transformer and I have 27 there too. Any thoughts?
marc santora said:
my garage heater. it was clicking on but not firing up.i made sure i bled system,i replaced tip and adjusted the electrode,replaced transformer, cad eye too.i got it fierd up and ran it for few hrs. then turned off power because it was 30* out and thermostat only went down to 50*. next day went to start up and nothing. reset button won`t reset when pushed in. i checked rest button on motor and wasn`t popped. i replaced relay switch brand new. and it wont even kick engine over. i have electric which i test witha testergoing to every thin including thermostst . scratching my head and cold can u help me
If wiring is correct and nothing is inadvertently grounded I usually suspect a bad igniter or a bad igniter control module.
(Oct 8, 2015) Lance Cundiff said:
I have a 1987 Goodman furnace with no AC change thermostat and now the furnace won't do nothing but the limit switch will turn on blower on manual but will not light when thermostat is on Auto pilot light is lit very confused
(Oct 17, 2015) Jon said:
Do I have forced draft on my mobile home unit it is a heat pump and why do I need it
John if your mobile home heat pump is working then there is an indoor air handler that is blowing cool or warm air into your home. You would not need forced draft unless your backup heat uses a gas or oil fired heater that operates in that manner.
(Dec 28, 2015) Jasongutman said:
I have a gas furnace the blower is stuck to manual and i cant get the burners to fire. I installed a new thermostat but cant figure out how to make the burners fire but the pilot is lit
Some thermostats include a FAN -ON switch that will keep the fan on. Others don't. Check to see if you have that switch.
But to me it sounds as if you have a wiring error; Search InspectApedia.com for FAN LIMIT SWITCH to read about how to turn off the manual -on switch on your blower; also check the thermostat settings; that's where I'd start.
(Feb 19, 2016) jose said:
I replaced nozzle, oil filter, air filter, and cad cell. my forced air furnace runs for 3 to 4 hours and wont start again only cold air comes out. I reset it and it starts again no problem until hours later the same thing. could it be my thermostat?
I don't think so. A bad thermostat only calls for heat ir doesn't. That's not going to explain a heater tripping the rest button.
(Feb 19, 2016) jose said:
Thanks, could it be the relay even though it runs for hours
If the furnace is going off on rest its safety device is telling you that the oil burner is not producing a proper flame - an ignition problem. That can show up under varying conditions, can be intermittent, and basically merits a visit from a trained hvac tech. You wouldn't normally need to replace a cad cell though on occasion they do go bad. Check the new cell for evidence of soot on its face or for a loose connection
Watch what happens when the burner tries to start. No flame? Should go off on reset
(Dec 14, 2016) Anonymous said:
I have no flame
Check fuel supply, then ignition - or primary control or its wiring.
(Dec 25, 2016) Arch Stanton said:
Thanks for the info,the problem was corroded electrical connections.
(Dec 16, 2016) deanna said:
my heater is blowing cold air
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