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Furnace Troubleshooting (the first step)

I recently had a furnace come in- complaint was that nothing happened- no fan, no heat.. nothing.

Whenever I have a furnace problem, the first step I take is to go to the thermostat. This one had a simple heat only, mechanical thermostat. I pull the cover off, switch my multimeter leads to read amperage and hook up the leads across the thermostat terminals.

On this furnace, I immediately read a 1 – 1.4 amp draw, which dropped quickly to around .2 amps. This told me the time delay fan relay in the furnace was good (on most furnaces built in the past 20 years, this will be a valid test- the exceptions will be new models with “fan control” circuit boards, or models where the board has been retrofitted with a Dinosaur brand fan control board).

After about 40 seconds, the blower came on, and the amperage rose a bit to around .5 amps. After another 45 seconds, the amperage rose again to about 1 amp.

What these readings told me was 1- the relay was good and the furnace had power (the initial high amperage reading was the heating element in the relay heating up), the second amperage jump (after the blower came up to speed) told me that the limit switch and sail switch were both OK, and power was getting through to the circuit board, the last amperage jump told me that the gas valve was getting power, and the circuit board was opening it. (A side point is that the final amperage reading- 1 amp- is the setting that the “anticipator” should be set at on mechanical thermostats).

For that job, I was lucky- the only problem was a bad connection in the thermostat- I repaired that connection and the furnace worked fine, but I hope you can use this method of troubleshooting to diagnose furnace problems (if you have a combination heat and AC thermostat, the same method can be used, but you have to find and break the correct thermostat wire to the furnace, which is usually easiest to find right at the furnace, and will nearly always be one of the two blue wires).

–Chris

CC BY-SA 4.0 Furnace Troubleshooting (the first step) by Chris Bryant is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

505 comments for “Furnace Troubleshooting (the first step)

  1. Carlton Sheley
    May 26, 2016 at 5:36 pm

    On my Suburban 35sf furnace, the fan starts immediately & the temp doesn’t get above pee warm. Also, the furnace turns on/off as the thermostat calls for it. Is there a thermocouple that’s stuck closed telling the fan to come on because the furnace has reached operating temperature when it really hasn’t?

  2. William W Lorimor II
    May 10, 2016 at 2:29 pm

    That burner looks different from the one I have . I can find pictures on the Internet but I can’t seem to post it her. I would have to describe it as looking like a tongue .I know that it’s strange but that’s what it looks like to me.do you still think that it could be plugged as well.
    Thanks for your help

  3. William W Lorimor II
    April 29, 2016 at 5:24 pm

    I have a 1986 shasta trailer with a hydro flame furnace that I works but it doesn’t seemed to get as hot as it should . I have gone through it looking for a cause but don’t seem to find anything wrong. All other propane appliances work great.any suggestions?

    • April 29, 2016 at 6:40 pm

      Hi William,
      A lot depends on the model, but an 86 could be the DC8200 series, which were great, reliable models. The one maintenance item on them is the burner gets rusted. If it is this model, you should be able to see the flame through an observation hole- the flame should be blue, with a fairly “hard” flame- not lazy and yellow.
      These burners can be cleaned- the “waffle” plates can be removed and wire brushed. It is a bit of a pain to put it back together. but not impossible.

      Of course, a thorough cleaning is probably not a bad idea- a partially clogged orifice` and a heat chamber coated with soot will cut down heat measuably, and a 30 year old furnace should be inspected for rust or cracks in the heat chamber.

      Hydroflame burner

      — Chris

  4. April 27, 2016 at 12:47 pm

    I have a 2010 Keystone Retreat 39FDEN, problem just started when I turn heater on in trailer the blower comes right on and runs for about 30 to 40 seconds and then unit shuts down, I do know that it is not igniting, just want to know could it be the pc board or what else do I check.

    • April 28, 2016 at 7:50 pm

      Hi Frank,
      Because it is coming on and shutting off so fast, my bet would be the sail switch or limit switch are bad. The sequence of events is the thermostat calls for heat- the board starts the blower, the sail switch closes the circuit which includes the limit switch, a ~30 second purge cycle (just blower), a try for ignition- if it doesn’t ignite, it goes through another purge cycle and tries another couple of times.
      Because it is not running more than 30-40 seconds, the board is not seeing power through the sail and limit switches, so it shuts down. Depending on the furnace make and model, it might be an easy fix, or it might involve pulling the furnace.

      — Chris

  5. Chris Johnson
    February 23, 2016 at 3:23 pm

    My thermostat calls for heat the burner does not kick on. Then after the fan delay expires (120 seconds) the fan runs and blows cold air. This happens only intermittently and when I turn the thermostat up or down and hear the click that say “call for heat” it will then start the burner. It is a brand new Thermopride oil lowboy. The service tech came out and replaced the fan and limit control just because he really didn’t know what the problem was. It went almost a week in which it was OK as it warmed up and didn’t kick on as much. Once it got cold again and was running frequently the problem started happening again. The thermostate has been replaced and that didn’t work. Thanks in advance for any help you can offer.

  6. Marti Keating
    February 6, 2016 at 4:21 am

    Thanks Chris. That lady on the smart phone found you for me. It was the limit switch on a suburban furnace in an ice castle. Fishing on the ice in South Dakota in total fog at 10 o’clock at night. made a good weekend better. Thank you. My wife thanks you even more.

  7. Tom
    February 2, 2016 at 12:41 am

    I have a Suburban P-40 furnace, it will turn on and heatup but then shut down, anywhere from after a couple minutes to 10 minutes. The furnace is mounted in an enclosed area in a park model trailer. The furnace will try to re-ignite but goes into shut down after a few tries. The blower does not stop and after the unit cools some I can turn the thermostat down all the way then right back up and the process will repeat.
    If the furnace is removed from the cabinet and run on an open bench it will never shut down on it’s own. The module board and hi-temp switch have been changed but the problem continues. I see no obstruction in the air flow, it even happens with the front cover off. The local service shop has given up and I am getting there too.
    The problem started late in the season last year and has never gone away not even to work one time correctly.

    • February 2, 2016 at 2:10 pm

      Hi Tom,

      That’s a tough one- does the whole unit shut down- blower and all, or does the blower keep running? Troubleshooting would be easier if you had room to install the Dinosaur Extend-a-life kit, which would let you test power to the board with the unit installed. If everything does shut down, I might suspect the transformer over heating and shutting off.

      — Chris

      • Tom
        February 2, 2016 at 10:19 pm

        Thanks for your reply Chris.
        The blower keeps running and only stops after I turn the thermostat down and leave it down.
        I can hear the gas valve energizing and see the electrode arcing numerous times before lockout, the burner will not re-ignite until I reset the board by turning down the thermostat and then back up, which in turn resets the board. At this point the furnace will ignite again for a few minutes, then the process starts over again. I have ordered a new oriface and sail switch out of desperation, the oriface threads did seem stripped.Now with the furnace out, I have also cleaned the electrode, flame sensor and ground probe, but will wait until the parts arrive to see if any of this makes a difference.
        I am a retired machine tool and industrial maintenance electrician so I understand this circuitry, it is actually very simple compared to machines I have worked on over the years.That is why this frustrates me.
        I like your suggestion about the control transformer and when I reinstall the furnace, if I still have a problem, I will monitor the 24 volts. I can hear the gas valve energize and see the electrode arc. I wonder about that one, unless the voltage drops just enough to not fully open the gas valve. At one point, I disconnected the gas supply line from the large home tank and attached a gas supply line from a 20 pound tank. It still shut down, with doing this, I think I ruled out the regulator. Never have a problem with the pilot lights or burners of the gas stove.

  8. rolls
    January 19, 2016 at 12:04 am

    I am changing my thermostat from a manual to digital I am having torble with the wires I can send you a pic of both I was smart enough to take a picture of the old one I need a clue thanks rolls

  9. mike
    January 18, 2016 at 12:58 am

    I have a brand new nt30. worked great when the installer put it in. Now it wont turn on. I played with the circuit breakers. When I turned off the breaker that runs the inverter, the furnace turned on. It comes on with straight battery power but will not turn on when plugged in and direct power?

    • January 26, 2016 at 1:57 pm

      That is an odd one- I would look closely at the power at the furnace- measure it with a digital meter on both DC and AC scales.

  10. January 9, 2016 at 8:55 pm

    Hi, we are in a 1992 skyline Layton with a Honeywell furnace, it was working fine this am then tank got low and we switched to the full tank, temp here is very cold and snowing, everything else is working on propane , fridge, and cook stove with good flame, but heater will not come on at all even tho we have checked all the wires and shut off the heater and turned it back on, we have 2 ele. heaters running now but are on a 20 amp circuit and it still won’t keep it warm in here, so we have looked at all the connections for the furnace and like I say it was fine and working well this am till we switched the tank over, we have been using the same tanks for the past 2 months and keep them full, what do you think is wrong??? We are cold and not liking this old rv for sure but it has more room than our camper and seems to be more of a pain to be in this time of the year, any suggestions would be appreciated..thanks

    • January 9, 2016 at 11:27 pm

      Hi Folks,

      I’m betting you have a Honeywell thermostat, not furnace, but if nothing comes on, look at the 12 volt fuses, and there may be a 12 volt circuit breaker on the furnace itself.

      — Chris

  11. ben
    January 7, 2016 at 7:54 pm

    Hello,

    I have a Suburban p-40 heater (about 15 years old).

    The unit was not igniting on a regular basis. I changed the following:
    1. Thermostat
    2. Control Board
    3. High Limit Switch

    Now the unit lights and heats the room, then the fan continues to run forever until the thermostat is turned down to the off setting.

    Any ideas what might cause this? (Defective sail switch or fan relay timer?)

    Thanks! ben

    • January 8, 2016 at 12:54 pm

      Hi Ben,
      The only 2 things that could cause that are the time delay fan relay and the thermostat. Even though the thermostat is new, I would double check it, simply because turning it off shuts the blower. If not for that, I would say the relay.
      Try testing for AC voltage on both of the (according to my diagram) white thermostat wires at the furnace after the burner shuts off but while the blower is still running. I suspect you might be getting some voltage there- just enough to keep the relay from shutting off.

      — Chris

  12. Robert
    December 30, 2015 at 6:15 pm

    Do you have any information on troubleshooting a Hydro Flame Model #FA-7825DS? And know where I can get replacement parts. Mine will not ignite at all. Thanks for your help and any information that you may provide.

    • December 31, 2015 at 1:24 pm

      Hi Robert,
      I have the service manual for that model at http://bryantrv.com/docs2/docs/hydroflameold2.pdf– it’s a pretty straight forward design. A couple of common problems, the burner will get rusted (it has some waffle type plates that are plain steel), it doesn’t like to run without the outside vent assembly (gets too much air).
      Parts are still available at the RV Dealership website, among other places.

      — Chris

  13. Patty
    December 28, 2015 at 9:15 pm

    My 1991 nomad,the pilot is on but when i trun on heater is just blow’s cold air,has been working fine?

    • December 29, 2015 at 1:26 pm

      A make and model of the furnace itself would help. On a pilot type furnace, the sail switch and limit switch would both be suspect if you get no full flame.

      — Chris

  14. Joe Riniker
    December 15, 2015 at 6:19 pm

    I have a 2 year old Suburban furnace model NT-30 SP that will not come on at all. Not even a sound of anything trying to work. I can get it to work when I connect the 2 wires at the thermostat, so I changed that with no luck. There are no blockages of any kind, and it is obviously getting power. I had the heat only thermostat that came with the new furnace, and replaced it with a similar one from Honeywell that still did not work. This is the first I have tried to use it since last winter, so it has been sitting awhile. Not that it should make a difference, but this is installed in an 1985 Airstream Excella trailer.

    • December 16, 2015 at 12:41 pm

      Hi Joe,
      It has to be a thermostat problem- if the furnace runs fine when the wires are connected together- which is what the thermostat does. I would measure for continuity across the thermostat – you should have continuity when calling for heat. Make sure the anticipator is set at the lowest level and that the shutoff switch is turned on.

      — Chris

  15. Scott
    December 7, 2015 at 2:57 pm

    Hi Chris
    Just a weird issue I have
    Sf35q works awesome but maybe a couple times a day it will not start again after running or will do a 30 sec aproxx time sometimes more or less but the blower blows and then it shuts down also locking out the thermostat. … worse the colder it gets outside
    It only has a bottom duct vent …. which found out I had to repair the ducting so all good I thought … nope still does it…. like its over heating… and limiting out
    I have ordered more side ducts to run…. another limit switch….. trying to figure out how I can safely get some cold outside air to mix with inside air to maybe cool down the heat exchanger a little more so it can get the trailer up to heat…. it’s almost a flip of the coin when it’s going to lock out …. mostly I wake up to chilly Temps. …. we have 2 300 lbs bottles hooked up
    24 and 7 baby

  16. Mary
    December 1, 2015 at 5:38 pm

    We had same problem. The gas regulator on propane tanks and hoses was bad. The rv tech put a test or on cook stove and it was on 2 pressure should be on 10 or so. The cook stove worked and I wondered why my oven was not baking right. It was not getting enough gas either.

    • December 2, 2015 at 12:49 pm

      That’s a good tip- a quick test would be to light a stove top burner and turn on the furnace. If the flame dies out- or even gets smaller- when the furnace is trying to ignite, the regulator is a good place to start.
      The only caveat is that the stove is not a great place to test a regulator- it’s OK for a quick test, but stoves have a built in regulator set at a 10″ water column, and the main regulator needs to be set at 11″, so you really cannot adjust a regulator from the stove.

      — Chris

  17. Kerri
    November 25, 2015 at 2:56 am

    I have a Suburban SF-20F furnace. The blower runs but the ignitor does not come on. You can hear it click like it is trying to light but it never does. How do we check this?

    • November 27, 2015 at 1:53 pm

      You cannot usually hear the ignitor itself on that model, but you can hear the gas valve make a clunk. The SF series really need to be removed for most problems and to access the burner. The main problem with that model that can be easily fixed is a bad or oxidized limit switch. There is an inside removeable cover tha has the limit switch right behind it- often simply unplugging and re-plugging it will fix a “no fire” issue. Unfortunately, if yours is trying to ignite, that probably is not the problem.

  18. Don
    April 6, 2015 at 9:54 pm

    I have a Suburban SF20 rv furnace.. I checked the power and have power to the limit switch.. The fan will not run and the igniter won’t light.. Can you give me some ideas on what the problem might be??.. Everything worked fine in September.. I parked the rv and now the furnace won’t do anything.. I’ve taken the furnace out of the rv.. Do I have to have propane going to the fan to test it??..

    • April 7, 2015 at 12:10 pm

      It all depends on whether you have a fan control board, or the fan is controlled by a separate relay- I’m figuring you have a fan control model. For this model, the thermostat closes, feeding 12 volts to the thermostat input on the board, *and* to the sail and limit switches. The board knows the sail switch should be open, because the fan isn’t running. If the sail switch is closed, and the board is getting power through it and the limit, it will not start the fan, as it knows there is a fault. If the fan does start, if the limit switch is open, it will shut down as well.
      It doesn’t have to have LP to do a basic test, but without it, it should start and try to light 3 times- after the third unsuccessful attempt it will shut down on lockout.

      Hope this makes sense!

      — Chris

      • Ralph Wozniak
        October 19, 2015 at 8:25 pm

        Have same problem with both front and rear furnaces, Tiffin said it is a control board in roof top coleman mod 8535, Cannot find anybody to verify or get a part number. Tiffin said they don’t carry part and have no Part number. When I connect the blue wires at furnace the roof top and furnace both start with thermostat in off position. Puzzled and frustrated, Help, Tiffin said board controls both front and rear furnaces. Ser number of air unit is 010409287.

        • October 22, 2015 at 1:55 pm

          Hi Ralph,
          What thermostat do you have? I assume it is a single thermostat with zones. There are 2 zone control thermostats that I can find- one at this link to a pdf, and the older model at this pdf file, but everything coming on at once suggests that power is being supplied to the wrong wire- if the thermostat is getting its power from the furnace, try switching the blue wires around- one wire from the furnace is hot, the other cold.

  19. brian scarbrough
    January 19, 2015 at 12:00 am

    i have a 2004 Dutchmen 31bh i had 2 wires on my power plug to arc and had to replace my end. Ever since this my heater blower turns on and will not turn off even when i turn thermostat to off. It is blowing cold air the igniter isnt working. Only way i have found to turn it off is pull the 15 amp fuse that works heater and refrigerator. any help is appreciated!!!!

  20. Anonymous
    December 23, 2014 at 3:57 pm

    I still have a situation no-one seems to understand. if the thermostat sits down with the screw tightened down on it furnace will not ignite. if I raise it up with a washer under it , it works. ????

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