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Furnace Troubleshooting & Repairs

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Though most furnace problems should be handled by an HVAC professional, you can take care of a few tasks yourself.

Need furnace help NOW? Get a Local Heating Pro Fast!

Want to know how to repair forced-air furnace problems? If your heater doesn’t heat or works poorly, this furnace repair guide will help you troubleshoot and fix typical furnace problems.

Though forced-air furnaces are normally quite reliable, they can break down and, when they do, getting them fixed quickly is usually critical. With a little do-it-yourself experience and the proper guidance, you can troubleshoot and repair a variety of furnace problems yourself.

Once a year, vacuum out the area around the furnace’s blower. If possible, slide out the fan unit, clean each fan blade with a toothbrush, and then vacuum with a brush attachment on a vacuum cleaner. Look for oil ports on the motor, normally located near the motor shaft. If the motor has these, apply two to three drops of non-detergent motor oil into each port (you may have to remove a cover plate to do this). Most contemporary motors don’t require lubrication, but motors with oil ports should be lubricated once a year.

Gas Furnace Parts Diagram

The information in the following articles is for some of the most common types of repairs. If these look to be beyond your abilities, call a qualified HVAC repairperson.

If you suspect a furnace gas leak, deal with this immediately! If you smell natural gas in your home or near the furnace, do not light any matches or turn off or on any switches. If the gas odor is strong, immediately evacuate your house, leaving the door open. Turn off the gas supply valve, which should be located by your gas meter on the gas inlet pipe, by rotating the valve one quarter turn with an adjustable wrench so that the valve’s oblong stem is perpendicular to the inlet pipe. Then call your gas utility or the fire department from a remote location. Do not return to your home until you know it is safe.

No Pilot Light or Ignition

Older gas and combustion-fuel furnaces have pilot lights; some newer ones have electronic ignition. If your older furnace has a pilot light that won’t stay lit, the thermocouple may be loose or faulty, the pilot orifice may be clogged, or the pilot’s flame may be set too low. pilot light flame

You can clear a clogged orifice with a piece of thin wire. Be sure to turn off the gas to the furnace first. Also shut off the switch or circuit breaker that controls power to the furnace. Just poke the thin wire into the tiny orifice where the pilot flame normally burns to knock out any debris.

Some pilot lights have a flame adjustment screw. Refer to your owner’s manual, but normally this is simply a matter of turning the flame adjustment screw to achieve a full, steady 1 1/2-inch to 2-inch flame with no yellow in it.

HomeTips Pro Tip: By doing your own minor furnace repairs, you can save money and also get your home heated up much faster than if you had to schedule and wait for a repair person.

Your gas- or oil-burning furnace must receive fuel to work. With a gas-fired furnace, be sure the valve on the gas pipe is turned on (the lug or handle should be in line with the gas pipe). With an oil furnace, check the fuel supply.

On an electronic-ignition furnace, turn down the thermostat or turn the power switch off and then on again to reset the ignition control module. Listen for the sound of the spark or watch for the hot surface ignitor to glow (see your owner’s manual).

If your furnace has a pilot light, be sure the pilot is lit. Check your owner’s manual or the instructions posted inside the furnace cabinet for step-by-step lighting instructions. Usually this involves turning the gas valve to Off, waiting a couple of minutes, turning it to Pilot, pressing and holding it down while you light the flame, and waiting a minute or so, releasing it, and then turning it to On.

If the furnace won’t light or ignites but fails again, call a furnace repair technician.

Heating or Cycling Problems

If your furnace runs and provides some heat but not enough…

1Be sure nothing is blocking the flow of warm air.


2Be sure the thermostat is set properly. Try raising the set temperature 5 degrees and waiting a few minutes.

3Be sure the room heating registers are open.


4Check the filter. If it’s dirty, replace it.


5If these simple steps don’t work, have a furnace repair technician. check out your system; either the blower isn’t working properly or the system is out of balance.

Need help NOW? Get a Local Heating Pro Fast!

Call for free estimates from local pros now:
[telnumlink] 1-866-342-3263[/telnumlink]

If your furnace does not heat at all…

Most heating system failures are caused by thermostat malfunctions, a tripped circuit breaker or blown fuse, or—in the case of combustion furnaces—a pilot light that has gone out. If the heat doesn’t come on even when you adjust the thermostat above room temperature:

1Be sure the heater’s thermostat is set to Heat (if yours is a heating and cooling system).

If a circuit has tripped, reset it by turning it off and on.

2Be sure the furnace’s circuit breaker is on or that its fuse has not blown. Check both the main electrical panel and any secondary subpanels that supply power to the unit. If the circuit has blown or tripped, reset the circuit breaker by flipping it all the way off and then on again. Or replace the fuse. If the circuit blows again, there is probably a short in the electrical system providing power to the furnace. For this, you may need to call an electrical contractor.

3Be sure the furnace’s power switch is turned on; it is usually located next to or inside the furnace cabinet. If it isn’t on, turn it on, and wait for the furnace to engage.

4The motor may need to be reset because of an overload. Look for a Reset button near the blower motor’s housing and, if you find one, press it. If nothing happens, wait about 30 minutes for the motor to cool, and then try the Reset button again.

5Turn off the power to the furnace at the main electrical panel or subpanel. Look for a fuse in the power switch. If there is one, it may have blown. Replace the fuse (be sure to follow the instructions in your owner’s manual). If you don’t have an owner’s manual or are unclear about what it will take to do this, call a furnace repair technician. 6With a gas furnace, the pilot light may have gone out or the gas valve may be shut off. Check the furnace’s ignition.   7If the furnace still doesn’t work, be sure the thermostat isn’t faulty. See Thermostat Repairs.  

If your furnace cycles on and off too often…

The problem is likely to be with the thermostat—especially if you have a combustion furnace. For information on how to solve and repair this problem, see Thermostat Repairs.

If your thermostat has a small lever that moves along a calibrated scale that indicates “longer” (not the heat temperature lever), you can try adjusting this—the heat anticipator. Just set it one calibration mark closer to the “longer” setting if the furnace goes off and on too frequently or one mark away if the furnace allows room temperature to rise too high or drop too low before going on or off.

It may take several hours for the thermostat to stabilize at this setting, so wait a while and then adjust it again if necessary. If making these adjustments doesn’t solve the problem, consider replacing your thermostat.

When an electric-resistance furnace or heat pump turns off and on too frequently, the problem may be that the unit is overheating because of a clogged filter or a blower that is malfunctioning. First try cleaning or replacing the filter. If none of these measures do the trick, call a furnace repair technician.

Furnace Blower Problems

If your furnace’s motor runs but the blower doesn’t move air, the belt that connects the two probably has broken. Replacing it is an easy fix. First, turn off all power to the unit and turn off the gas at the gas valve that serves the furnace. Remove the door on the front of the furnace cabinet to give you access to the blower (it might be on a slide-out drawer.) Check the number stamped on the belt and get an exact replacement from a home center or heating supply outlet.

Replacing a blower pulley is an easy fix.

You can usually slip the belt on the motor’s (smaller) pulley first and then start it on the blower pulley. Rotate the blower pulley by hand, holding the belt in place but keeping your fingers from getting caught between the belt and the pulley. The belt should slip right into place. If it seems to be too tight or difficult to set in place, it may be necessary to adjust the motor mount to provide more slack. Then you can re-tighten the tension once the belt is in place.

Check the manufacturer’s specifications for proper tension—in most cases, the belt should deflect about an inch when you press down on it.

Finally, some fan motors and fans need oiling; some have sealed bearings. If recommended by your maintenance manual, oil the bearings according to the manufacturer’s directions.

A blower that runs continuously may be caused by two things—the thermostat mounted on the wall or the limit switch located on the furnace just below the plenum (the box that distributes heated air to all the ducts). The limit switch is designed to shut off the furnace if the air in the plenum gets too hot.

Check the thermostat to see if the Fan switch has been turned on. If it has, turn it to Off or to Auto. If it is set to Off or Auto, the furnace’s limit switch must be adjusted.

Call a furnace repair technician to adjust the limit switch, or follow the instructions in your owner’s manual to reset the pointers on the fan side of the limit control. The lower pointer should be set to about 90 degrees F., and the upper one should be at about 115 degrees F.

Noisy Furnace or Duct Work

Many heating ducts are metal, so they conduct noise quite readily from the air-handling unit to your rooms. To break the conduction of sound, you can have a heating contractor insert flexible insulation duct work between the furnace or air conditioner and the duct work runs.

If you hear a pinging or popping sound coming from the duct work, this may be caused by thermal expansion or by air blowing past a loose flap of metal. Track along the duct runs, listening for the sound. If you locate it, make a small dent in the sheet metal to provide a more rigid surface that’s less likely to move as it heats and cools.

If the furnace cabinet makes rattling noises when it runs, be sure the cover panels are screwed on tight. If they aren’t, tighten them.

Squealing sounds from a forced-air furnace generally occur when the belt that connects the motor to the fan slips.

If the blower is making a grinding noise, shut off the unit and call a furnace repair technician; the motor’s bearings are probably shot.

In most cases, the belt is improperly aligned or worn and needs replacement. Follow the instructions in your owner’s manual.

This job involves removing the access panel, loosening a couple of bolts that hold the blower motor at the proper tension, and realigning or adjusting the belt. In many cases, it’s a good idea to remove it and buy a replacement at a home center. When reinstalling the belt, be careful not to over-tighten it; this can wear out the motor bearings.

Oil Burner Problems

If the oil burner goes on and off too much, clean or replace the filter. If that doesn’t solve the problem, call in a furnace repair technician.

If your oil burner doesn’t work at all, check to see if it is receiving electrical power, if the thermostat is broken or turned off, or if a flame sensor in the burner or heat sensor in the stack has signaled the unit to shut off.

NEXT SEE: How to Buy a Furnace

Featured Resource: Get a Pre-Screened Local Heating Contractor

Call for free estimates from local pros now:
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About Don Vandervort
Author Image
Don Vandervort developed his expertise more than 30 years ago as Building Editor for both Sunset Books and Home Magazine. He has written more than 30 home improvement books and countless magazine articles. He appeared regularly on HGTV’s “The Fix,” and served as MSN’s home expert. Don founded HomeTips in 1996.

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  1. My open close flapper to my duct has fallen of and fell to the bottom and I think it is jammed cause no air flow is coming out my vents now for that duct lead. How do I go about getting thus out so I can replace the flapper? Any comments would help greatly. Thank you

  2. My forced air LPG furnace hums when it is not blowing heat. I’ve turned my thermostat down and even shut the thermostat off. Why does the unit continue to hum?

    • The control transformer is either loose from it’s mounting, or getting defective. This is a common issue I have ran into.

  3. I have a light switch in where my heater is. The switch is rattling and making noises. Can I just replace the switch or does an electrician have to?

  4. My lpg forced air furnace has a mini explosion on startup like there’s a buildup of gas at startup what would cause this?

    • Clean the ignitor pilot flame track/runner. This is a slit in the burner assembly that allows burning gas to run across and light all burners . That would be the first place I would look.

  5. Last week I smelled a burning plastic smell coming from the registers upstairs. I had a oil company come and do a tune up but couldn’t really find any thing wrong . last night and today when the burner kicks on room temp. air rushes out the vent with that same smell again, then the air and the smell stop? Any one have any ideas? I cant keep paying for a service every time it happens. They charge 150.00 a hour. Thank You in advance

    • It sounds like something is getting hot near the return-air register/duct, the furnace cabinet, or the ductwork that feeds the upstairs registers—I’m most suspicious of the latter if the smell isn’t being delivered equally through all house registers (be sure nothing has dropped into the register). I’m hoping it doesn’t have an “electrical” smell…this could indicate a more serious hazard. If this smell isn’t isolated to upstairs registers, the furnace may be overheating and shutting off. Please give me a little more info.

  6. I have a gas Lennox Conservalor III forced hot air furnace. Sometimes it
    shuts off and the temperature drops quickly. When I reset the Carrier
    thermostat installed by Con Ed, the fan will go on, but no heat. When I
    go down to the furnace I can hear/see the electric starter spark, but
    the furnace does not ignite. I then shut off the switch next to the
    furnace for a couple minutes and then switch it back on and the spark
    begins again and usually the furnace fire up, but sometimes it takes a
    couple of times.

    This problem occurs sporadically, but during this cool spell it has
    happened twice in two days. Obviously, if we ware on vacation then we
    would have suffered broken water pipes etc.

    One addition point is that the ‘clean air filter’ message is always present even though I change the filter.

    I did inform a company to come in to check it. They went through the
    same procedure and then just opened exhaust pipes and cleaned them out,
    but after awhile this problem reoccurred.

    Is there something between the spark and ignition of the gas that could be the problem?


    • Voltage issues in the ignition system may well be the problem. Fixing them will require a service pro—be sure to get someone who works with Lennox. Or the thermostat may have issues. Before you call a pro, check the wire connections for the switch on the side of the
      furnace. Wiggle the wires (don’t touch bare wires!) and make sure everything is secure.

    • Also clean or replace the flame sensor.

  7. I have a Day & Night electronic ignition gas furnace. A few days ago we noticed it continuously blowing cold air at the end of a heating cycle. I know the fan is supposed to blow a couple of minutes after the burner goes out but this kept going 10 or more minutes after no burner. In fact, I don’t know if it would ever shut itself off. When I checked the furnace I found a small flame (looks like it’s the small jet associated with the igniter) still on apparently keeping the blower going though the main burner was off. When the thermostat is moved all the way down this little flame shuts off and the normal start-up cycle can begin again when the thermostat is moved back to a normal position. Something is preventing this small jet from shutting off. I believe it usually does as soon as the main burner comes on. I expect this is some control module or gas valve issue?

    I’m giving it a rest (though this is a cold day around here) and I’m putting a new filter in. Besides that any suggestions?


  8. I have an odd heating issue. Lately, the outside temperature has been in
    the 10-20 F range. I set the thermostat at 68. Often I find the home
    temperature dipping below 68 but the heater does not kick in. If I
    manually decrease the thermostat “set temp” to below the present room
    temperature and raise it back up, then the heater kicks on again. if I
    turn off manually switch off the furnace and switch it back on, it works
    as well. I have changed the battery of the thermostat, and generally
    keep the filter and the surrounding area in the furnace closet clean.
    The Furnace is Rheem Quiet 80 and the thermostat is White Rodger. They
    are about 7 years old. Any thoughts on why this might be happening?

  9. We have an A frame air condition and the cold air will come on, but the heat want it just blows cold air what may be the problem the unit want come on out side

  10. My thermostat is shut to off and the heat still comes up into house and shoots to 75+ degrees. I have an oil furnace – any suggestions???

    • Kathy, this normally indicates a problem with the thermostat or thermostat wiring. Make sure the thermostat isn’t set to the Auto or Program mode with a separate setting from the Off switch.

  11. My Amana furnice will start to run fire the burners for about 3 seconds and then shut down. It will try again in about one minute with the same results and over and over again. I took the top panel off and looked inside the unit and the burners fire up but shut down. I looked at the thermostat and it is doing this when the set temerature is on the thermostat. I let cold air in the house until the temperature was under the set temperature and the furnace worked properly. Could this be the thermostat?

    • Same thing with mine too! Couldn’t figure out what’s wrong with it. It started doing this yesterday.

  12. Last resort before calling a repairman for a service call that I really can’t afford right now. I am a Mech Eng with an EE background so I’m somewhat knowledgeable about how to troubleshoot, I just don’t know much about furnaces other than what I have been able to learn on the web and youtube.

    I have a Bryant gas furnace, approximately 12 years old. The blower motor runs constantly but it does not fire. Here is what I have tried…

    Checked the thermostat setting first of course, switch set on heat and fan to auto and heat set higher than it is in the house.

    The filter is clean, put a new one in even though the one that was in there was only about 5 months old and not dirty. Have good airflow through the ductwork all over the house.

    Checked the house breaker (reset it off/on), then tried restarting furnace.

    Doing web searches on this problem usually leads to help that says one of the safety/limit switches are open. There are two limit switches on this furnace and I checked both of them, they have continuity across them so it isn’t that.

    I can smell gas when I turn the valve on to test the furnace. Not a lot of gas but definitely a slight wiff of it. I of course don’t leave the gas on very long to build up when testing but it was probably a full day before I realized the blower was running but not heating. In any case, since I can smell gas, I suspect it is not the gas valve.

    It has a spark igniter and I can’t hear it trying to fire but I don’t know if I should be able to. You can’t really see it where it is located without taking it loose from its bracket.

    The little inducer fan motor never starts when testing, only the blower motor runs.

    I don’t see a “flame sensor” rod anywhere, I suppose it could be part of the spark igniter assembly that I can’t really see. However, the flame sensor should really have nothing to with this particular problem from what I have read.

    I can only guess at this point but about the only thing left besides circuit board electronics is the pressure switch or the thermostat or the thermostat wiring.

    I think I will go to to lowes, buy a new thermostat, hook it up with quick, short jumper leads to rule out the thermostat or wiring. If that does not fix the problem, I can take it back. As a side note, I have continuity across the red and white wire at the thermostat base with the thermostat removed, that does not seem right.

    I don’t know how to test the pressure switch, there are three leads coming off of it and I don’t know if its low or high voltage or how exactly it is supposed to operate. I suspect I should get power across at least two of those leads when the blower is running.

    Any help would be appreciated!

    • Hey Ranger Ric, I was hoping an HVAC pro might weigh-in on this issue. By now, I’m sure you’ve got it sorted out. Did you learn what the problem was?

      • No, winter was over and I have not had a guy over to fix it yet. I’ll post if I remember after he fixes it… you might want to remind me in a few weeks.

  13. I had a Currier AC/Furnace (90%) and it was installed in 2008. My unit fan would start even though thermostat was off and/or removed. My nest thermostat wouldn’t get the power to the unit, so I called the HVAC that installed the unit and verified that it’s ‘Upper limit’ switch has gone bad. They diagnose the issue and installed new switch where they charged me close to $300. (What a rip off). Anyway, please check this first if you encounter the issue and you may be able to save some money.

    • Hiran, thanks for sharing this advice with the community. Is that Nest thermostat working well for you now?

  14. The temperature in our house often shoots up to 80 or more when it is set to 68. Noticed that hot air is blowing in.. switched off the furnace and ac for a while and waited an hour and then switched just the ac on and cold air started coming in. For almost a week, I switch the furnace ON just before showers and 2 hours later I switch it off or else hot air starts blowing in. What could be the problem? I cleaned and replaced the filter too

    • Sri, please see the info in the article above on adjusting the heat anticipator. Otherwise, try replacing your thermostat.

  15. My furnace just doesn’t start. I see a spark inside the HFR on a circuit board and the fan almost starts but then doesn’t. If I set the thermostat to fan always on the blower works.

  16. Furnace does not kick on until I soon the exhaust fan a few times, coincidence? Then it will run fine for a few cycles. Any suggestions

  17. I have a 25-year-0ld Lennox Aire-flo heater. The pilot light works but the heater does not kick on when it gets below the thermostat setting? Do you know what I need to fix? Would it automatically not work if the filters are too dirty? The blower works when set in the continuous mode.

    • This could be a dirty flame sensor. I just added a couple of videos to the article above to show how to clean this.

  18. I am repairing a 25 year old Day & Night HVAC system. I was able to determine the overtemp cutoff switch on the burner exhaust blower is defective. I can make the heater operate by temporarily bypassing this switch. I can buy a replacement but I have not been able to determine the correct temperature for it to open. I have been on several sites, including the D&N site; but cannot find the switch or information on it.

  19. My furnace turns off over and over. When I open the front plate and push in the button that indicates the door is shut it starts back up. I then close it and it works for a little bit and then goes off again. Do you know where I should start?

    • Sounds to me like a problem with the flame sensor. I’ll post some more info about this in the main article (above).

    • Try a shim on the blower door switch. Sounds to me like it is intermittent in operation. The flame sensor usually is either good or bad. but not always. Try cleaning the sensor with some fine sandpaper. But be careful as the ceramic ones are brittle.

  20. Last year, our blower motor belt was making a squealing noise, so I tightened it by moving the motor mount further away. Then, it started doing it again sometime after the start of heating season this Fall, and I tightened it again. I didn’t see any sign of the belt being worn, but perhaps I needed to study it more closely. We had an appointment to have our annual furnace maintenance coming up in November, so I wasn’t concerned. Then the belt actually came off, so I put it on, and moved the motor mount as far away as it could go. It never dawned on me that the belt might be failing, since it looked fine. Then it happened; the belt broke, around 8PM. What to do. I looked at the belt and saw the number 4L350, checked online, and saw that Home Depot, which would still be open, had them (except that in my haste, I didn’t notice that it said at the bottom “not available in stores”), for $6.45. Off I went to Home Depot. Fortunately, I thought to bring along the old belt, because I found a snow thrower auger belt, for about $20 which was a bit smaller than the old belt. But, since the old belt was actually too big now, this seemed like it was a good solution. It was a snug fit, and I had to move the motor mount to the closest position, but I was able to deflect it about 1/2″.
    Other than the fact that it cost $20 instead of $6.45, the snow thrower auger belt (OEM 754-0101A) seems to have been the perfect solution. Otherwise, we would have spent a rather chilly night and morning until I could get to a store that had the actual belt I needed (perhaps Sanels Automotive), which is nearby.

    • Hey Bruce, thanks for sharing this story. And kudos for some very smart problem solving. It always pays to take along the defective part when you go to buy a new one…and doing this really paid off for you!

  21. I have an 11 year old furnace. The pilot light is lit, it runs fine, the indicator lights are on showing the system is fine, the filter is clean, yet it isn’t blowing hot air. Just cool air. What is the issue? I can’t figure it out and it’s sitting at 57 degrees in my house right now…

  22. What is causing clicking noises from one or two of the zone boxes when my furnace kicks in?

  23. Don, thank you for your suggestion and link. So far I still have not been able to find the part I need there, or anywhere. I checked Carrier and other brands you listed but cannot come up with a part number. I have the part I removed and it was manufactured by Klixon many years ago. It does not have any numbers marked on it so I am still trying to get the P/N so I can order it.

  24. My furnace goes on, goes through the cycle, the air is blowing warm up through the vents, but the house is still cold. I don’t understand this since I spent 31K last year having insulation blown into the voids of my house and new siding put on with foam board underneath that. Old doors were replaced and all voids closed and insulated. The furnace was serviced last month for the beginning of the winter season and it checked out okay. I am also diligent about changing the filter, making sure to mark it on the calendar and on the filter itself. Any idea why this is happening? I thought maybe I am just getting old and feel the cold more easily but my brother stopped by the other night and he noticed it felt cold in here too. The thermostat shows the house at the temperature it was set at so I know it’s not that either. I am at a loss why it feels colder in here now than before I had the house insulated. This is beyond strange. If anyone has a clue about why this is or what it could be I’d greatly appreciate it hearing it. Thanks.

    • Sounds like the problem could be drafts caused by the movement of air through the house. Is your house 2 stories? If so, the warm air downstairs could be rising up, causing a draft. Also, if you have a fireplace, make sure the chimney flue is closed when not using it. Try closing doors to rooms—as a test—to see if this helps.

  25. Hi all, I have a carrier 58PAV model furnace. I have replaced the controller pcb, gas valve, flame sensor w/o any luck. I am getting the 24 volt AC to the gas valve for a second, then it goes to o volt. My inducer motor is working, inducer sensor has 24 volts on both sides. My igniter glows really good. It goes thru the cycle a few times but my gas never comes on. Filter is new, unless there is another one somewhere else. Any help would be greatly appreciated.

    • I hope an HVAC pro responds to this…

    • Check the gas valve for a small switch that will not allow the flame to light the burners. A flame sensor doesn’t come into play unless the gas switch is on. I have found this to be the case many times when the home owner accidently turns this small switch off or forgets to turn it on after replacement.

      • Pat, thanks very much for helping out with our visitors’ questions—it’s really great having an A/C pro weighing-in! You’re a valued part of this community!

  26. Hello. I have an electric furnace, in a mobile home. Have tried repeatedly to get someone to come fix it, seems as though my job not big enough for them. For awhile now, the blower kicks on but the heat doesn’t. I have always been able to turn it off and on and eventually it would work. Now it won’t. One repairman said he believed it was the sequencer. Can’t afford to buy part if it won’t fix it. Repairmen never show up and need heat for my son and myself. Any ideas?

    • Make sure the furnace has power—not just for the blower, but for the heating elements (check the fuse panel or circuit breaker). Look for a reset button on the furnace (you may need to look up the manufacturer’s directions online). Next, consider the thermostat—you may need to adjust it or get a new one. It could be a defective heating element—you’ll need a pro for that.

  27. I have electric furnace/ac ac works fine. Heat was working fine one during night but this morning wont kick in

    • See my answer to Kristi, below.

  28. Our 28 year old Bryant plus 90’s air induction motor broke and now the blower seems to not work as well. Is there a way to test the blower? If they’re both broken, I’ll just replace the furnace. Thanks in advance!

    • I’m not sure if I’m understanding the question. It’s easy to tell if a blower is running (you can hear it) and blowing air—just put your hand in front of a register and feel for blowing air.

  29. Its taking few cycles for heater to blow hot air. Sometime I have to reset/restart thermostats few times for fan to blow hot air. Any suggestions?

  30. I’m having issues with my gas furnace and created a video showing the issue. I believe it’s a bad thermocouple, but there may be other issues as well. https://youtu.be/Q8ancKFOna0

    • A bad thermocouple would shut down the pilot. I would guess that your heat chamber is overheating from a lack of cooling air. Check you filters and possibly your AC coil getting plugged.

  31. I have a Janitrol/goodman HVAC Model: GMP075-3.

    When it’s cold the furnace runs a while, then shuts down for a minute or so, then starts up again. It goes through this cycle most of the morning while the inside temperature finally reaches the set point on the thermostat. I was thinking flame rollout or pressure switch, but I happened to be looking at my thermostat when it shut down and the thermostat lost power. It went dark and a few seconds later the furnace shut down. After about 30-50 seconds the power came on to the thermostat and the furnace started up again.

    Could my problem be as simple as replacing the 24V transformer, or is there something else I should be looking at?


    • Check the blower door switch. Sounds like it intermittent. This will shut down the control power to the thermostat as well as the whole furnace.

      • Thanks for the reply. Turned out that I have two air return paths, and one of them was blocked. The heated air was 190° and the primary safety was opening up. Moved the furniture from the second return (and changed the filter) and the hot air from the registers is now in the 95-115° range. Problem solved.

  32. In 1978 I purchased a townhouse. The HVAC is a Day & Night Company (Division of Carrier Corporation) Model Number 24/8 RUP 601B with Serial Number DEENG 18476. It is an
    “all in one unit” (A/C, heating, and water heater). The water heater failed long ago. A separate water heater is now being used.

    I am still using the A/C, heating portion of the unit.

    The clip on left side holding the air filter to the unit broke off. I would like to obtain both a
    replacement clip and a replacement air filter. Day & Night’s website directs me to a local dealer. Local dealer will not sell me parts. Can anyone offer any leads to a source for parts?

  33. Hello. I have a gas furnace that blows hot air well enough. The problem is it blows how air through one of these “holes”, for lack of a better term. One of these holes is connected to the A/C condensate pipe through which condensed water drains away, and the other isn’t connected to anything. It’s through this second “hole” that lots of hot air blows out, making the area around the furnace toasty warm while the upstairs is considerably less warm. I’m used to the basement (where the furnace is) being the coolest part of the house. Now it’s the warmest. What could be the problem? I’ve checked the filters and they’re ok. Thank you.

    • Wow, I’m having a hard time picturing this. If 6 inches or so, it sounds like a duct was removed. If this is the case, the hole needs to be covered with a piece of sheet metal and duct tape. I hesitate to recommend this, however, because I don’t know what the hole is.

    • Was a humidifier removed at one time? This would be my guess. Get some sheet metal and some screws and plug the hole.

  34. Our heater appears to start normally. The little blower motor starts to increase pressure then the burner will lite. It seems like the main blower starts too early and the flame goes out. Or maybe sensor does not see the flame and closes gas valve. Fan comes on to vent any gas, since it thinks flame did not lite. Flame sensor dirty possibly?

    • Sounds like a dirty flame sensor, as discussed above under “If your furnace cycles on and off too often…”

  35. Older gas central furnace is making a double click noise every 5 seconds. Seems the system is still working. Any ideas?

  36. I have a concord furnace sometimes it will start right up other times it goes to start up pilot goes on flames start then it goes out & tries to refire after a little wait it starts up again the furnace is about four years old

    • See the video above under “If your furnace keeps shutting off after about 5 minutes…”

    • clean or replace the flame sensor.

    • Another flame sensor possibility to me. These sensors are a more common problems than in the past due to no pilot flames as in the past.

  37. I have a 3 year old furnace that has been working randomly for a couple months now. Some days I wake up and my house is warm and others not so much. I have it set on heat/auto and even if I increase the temperature several degrees it will not kick on as it should. Sometimes it will, but will take 15 or so minutes and sometimes it won’t kick on at all. Also, if I move the switch from auto to fan it will kick on but blow cool air out of the registers. I had a professional look at it and he said it was a loose wire and fixed it. Apparently not though because it’s still doing it. Any suggestions?

    • Did you pay the professional? If so, I’d call him to come back and get it right. You might need a new thermostat—which is relatively cheap to buy. More likely, however, is a problem with the flame sensor or pilot. When you switch to “fan,” blowing unheated air is what it’s supposed to be doing.

  38. Before You Call The Repairman – Check These First. It sounds unbelievable, but 20-30% of calls to the repairman are due to very simple and overlooked things that you should check first before calling anyone. Before you start any major work on a heating or cooling system, take these simple preliminary steps first:
    1. Is your filter dirty?
    2. press the reset button
    3. Is your door cover on correctly?

  39. My furnace has a buzzing noise that appears to be coming from the circuit board and the board flags a code 12 (igniter has open). The vent blower turns on momentarily, then shuts off and only the blower remains on (but no flame or heat) with off an on buzzing from the board. It is a York Affinity 9.v 2 stage high efficiency furnace. Should I follow the code and replace the igniter or is this a faulty board misleading me?

  40. I turned on my furnace today and it turns on, but, it doesn’t seem to ignite or turn the blower on. It is an electric ignition, the gas is on and no smell of gas and I’ve hit the reset button and changed the filter. Does it take a while to heat up the filament and if so, how long does it take, on average?

  41. I have a Robertshaw electronic ignition and installed a nest thermostat in June. Heating and cooling worked after installation. Used AC all summer but now the igniter comes on and sparks consistently whether Thermostat is calling for Cooling, Heating or in Off position. Not sure where to start troubleshooting.

  42. I just turned on my oil fired forced air furnace for the first time this season and I get a strong smell of oil coming out the floor registers? what would cause this?

  43. My 1997 Goodman heater was working fine, but now it will not ignite. I’ve cleaned the “firing pin” (my layman term) with sand paper and used a straw to dust off the sensor, vacuumed as much debris as I could get to within the “flame zone”.

    The repairman said I needed a pressure switch….(according to the blinking error lights, he was right), but that didn’t fix the problem–though he didn’t put it on, I did, because I couldn’t figure out why I paid for parts and labor and definitely had a repairman under my feet for two days, but no part attached for some reason (note: the lights quit blinking). “Snaked” the gunk out of my PVC outlet, poured in some bleach to clean out the rest. I’ve “reset” the system more times than I can count and still no luck. Any thoughts?

  44. I have a gas furnace. When i set the thermostat to a certain temperature, the furnace will kick on and air will blow through the vents. After a little while the furnace will stop blowing air but the motor keeps running. I assume the motor is still running because I can hear a humming noise coming from the furnace. The air will stop blowing before the furnace reaches the set temperature. If I lower the temperature below what the actual temperature is, the humming will stop. Then I can increase the temperature a couple of degrees and the furnace will kick on again and blow air. However, in a little while the furnace will stop blowing and the motor will continue to hum until I lower the temperature on the thermostat. Sometimes after repeating this cycle a few times the furnace will work correctly but that doesn’t last. In a few hours the air will stop blowing before it reaches the set temperature. What could be causing this issue?

  45. my thermostat goes up to 80 degrees on its own after i set it at 70

  46. Have a heater that wasn’t blowing warm air or heating up house. Repair man said he replaced the fuse but needs a new fan and safety switch. 600$. But the heater now works normal since he replaced the fuse. Should I cancel or continue further with the suggested repair? He said the fan is to be running at 1.8 amps and runs between 1.6 and 1.8 so why would that need replacing along w safety switch if the heater is working fine now? Thank you.

  47. Big thanks for your tips and guidance! I believe you have covered almost all the issues related to a furnace. I will try to repair it myself next time. Keep up the good work!

  48. Hi, we just installed a new thermostat, (digital). We had an old Honeywell round one. We don’t have cooling system, only oil heat. When we disassembledo old thermostat, there were only 2 wires.
    Black and white. So we connected blackck to black, white to white. Therm seems to work. But, room temp is 80 and thermo set at 68. Heat is coming up. We changes thermo because of this, but it’s doing same thing. What else can be causing this? Help, please!!


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