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38979Re: [classicrv] Duo-Therm furnace issues again!!!!

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  • Todd Barnard
    Feb 4, 2014
      Hi Rob, good to hear from you again, and you're on the right track.

      If the pilot stays lit, no matter how long the motor runs, and you never see any sign of the main burner coming on, the we are definitely looking for a handful of thins to check.

      As Warren mentioned, its always good to check your gas pressure, but most pilot types will at least attempt to light the main burner when the fan comes on, even if your pressure was terribly low.  Electronic ignition types are much fussier here.

      There are 2 safety switches that can go bad or stick, and not let the valve open, the valve could be getting power and just not opening or the valve could be opening, but an obstruction in the burner or main burner orifice could be not allowing enough gas to come through.

      Make sure you you have a solid 12vdc going to the furnace. If the fan is not turning at 75% of top speed, the sail switch will not be tripped and not send power thru the circuit.

      I'm sure you are used to thinking of  the Sequence of Operations in your mechanical work, so let me give you a run down on what happens with your furnace.

      Closing the thermostat contacts sends 12vdc to the fan relay allowing it to start the motor.

      You appear to be good to this point, again barring a voltage problem. T-stat, relay and the motor are not that fussy about voltage so far as just starting.

      Power is sent to the hot side of the sail switch, and once the fan reaches the potential I mentioned before, the sail switch is tripped. (normally open, now closed) .

      The job of the sail switch is to ensure that the motor is turning fast enough to take heat away from the chamber and to ensure the proper air to gas mixture, as the fan also feeds the combustion air to the burner. I sure you remember a fan on each end of that motor.

      Once the sail is tripped, it sends power to the limit switch (Normally closed). Its job is to shut power to the valve off if the chamber gets too hot. If the switch is functioning, then it sends power on to the valve.

      Depending on the model, you might be able to check the leads coming to the valve for power(12vdc) after the fan is running. If you have juice there, then the switch are doing their job.  If you have no power here, and cant access either switch from the front, then you should pull the furnace and hook it up to a good 12vdc source on the bench.

      There will likely be 2 blue wires on the harness, or a blue that gets jumped to another wire on the harness. Connecting them is the same as closing the thermostat.  Your fan should start and you can begin to trace the circuit as I described above.

      BTW, some models send power to the Limit before the Sail...it makes no difference.

      Let us know the model when you have it and I'll try to find a schematic.

      Good luck!

      On Mon, Feb 3, 2014 at 11:05 PM, <sirrobyn0@...> wrote:

      Well, last February I had some issues with the fan on my Duo-Therm furnace and a very nice man here on this board named Todd was kind enough to coach me though removing the furnace and replacing the fan motor.  It has been working well up until last weekend.  I went to start the furnace like I always do.  Got the pilot flame lit up, hit the switch on the wall thermostat and the furnace fan kick on shortly there after just like normal.  But heres the thing, a few minutes later I realized that nothing but cold air was coming out of the furnace.  I took popped the front vent cover off the furnace and the pilot flame was still going but the main burner was not.  I know the main burner won't kick on if there is a low voltage situation at the furnace so I checked to make sure I had good power at the furnace while I did.  I pulled the furnace out and checked all the electrical connections which were all fine.  I know basically all I see on the unit is the gas control knob and the sail switch, I have no idea if either of these could be at fault.  All other propane operated stuff seems to be operating normally so I think we are good there, although I don't know how to test the  regulator, or if that could even be causing this issue. 

      I'm open to any and all suggestion.  This is a 1977 Duo-therm Furnace a factory installation in a class C motorhome.


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