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

465 comments for “Furnace Troubleshooting (the first step)

  1. 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

  2. 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!!!!

  3. 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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