Furnace worked yesterday ... today no fan (hence no heat)

[Eagle & Bear]

We are full timing in our 1976 29 foot Int. Ambassador and now traveling through Wyoming. Although it was in very good condition when we bought it, I spent a lot of time restoring updating & upgrading everything.

I originally pulled the furnace (NT 32 FCA) out from under the sink (kitchen stove also has to be removed) and had it bench tested and adjusted last summer. It had little to no rust and may not have been used more than a few times in the past 30 years.

In November I fired it up. It worked but it produced a lot of soot. Pulled it out again in March so the service dpt. could adjust the fuel/air mix. I also had them replace the "board", sail switch and gas valve so that I would not have to pull it out again. I reinstalled it. It has worked great up until now.

Got up this morning (6/9) and turned the thermostat on. The only sound was the thermostat switch clicking closed. All wire connections are good. No debris around the intake slot or board. Cook stove burner lights. I don't want to pull the furnace again. Any ideas?

[Fyrzowt]

You probably checked it, but - circuit breaker?

[azflycaster]

On my 75 TW the wires for the thermostat are in a closet in the bathroom. I have had one of the connectors (push together) become loose and nothing would happen.

[Eagle & Bear]

Thank you for the quick response. Breakers and fuses are all OK.

[startrekker2001]

I have a 78 31' Excella. The furnace was replaced by the PO as a result of a recall. I found that the lighter would not light when ... wait for it ... the bathroom lights were on. It seems that the lights which are screw-in light bulbs like 120v only 12v, draw a lot of power and apparently the lighter for the furnace is very sensitive.

I will hear the blower come on, and about 60 seconds later the lighter will click. The burner will light only if those lights are off.

Vic

[azflycaster]

You should have 12 volts DC on one of the wires to the thermostat, both wires if the contacts are closed.

[Eagle & Bear]

Thanks Vic. They did install a new igniter. It appears that competing drawdown from other components will not allow the igniter to create an arc. Weird problem but at least I did not have to pull the furnace!!

[startrekker2001]

Glad I could help.

[HowieE]

Does the purge fan come on when you close the thermostat? If yes it should run for about 15 seconds and then the ignition should come on and the burner fire. If you do not have a fan I would look for an open electrical problem leading to the furnace.

If the fan come on and runs for ever without the burner firing. You may have air in the gas line. I have this problem often when I change bottles. What I do is first turn a stove burner on to try and move gas forward. If that fails I crack open the flare nut at the furnace till I smell gas and them try again.

Now someone is going to chime in here and say opening gas inside the trailer is dangerous, illegal, and fattening. If you agree with them don't do it. If you want the furnace to work try it. You will allow no more gas into the trailer than what is delivered before you light a burner on the stove and you have lived through that.

If you were able to get it to fire by turning off some lights I would test the battery voltage and the voltage at the furnace with the lights on and off. If there is a voltage drop of more than a volt I would check and clean the battery terminals. Lights should not effect it if all the connection are good.

[HiHoAgRV]

One of the units that I had simply refused to fire up if there was less than 12.2 volts at the heater. The fan would spin up, the igniter would click, then nada, fan forever but no heat. Usually I had a full 13+ volts at the battery so don't bother checking there...check at the heater.
I ran a 10 gauge wire from the breaker panel to the heater to replace the 14 gauge factory wire and also removed 1 of the 3 intermediate connections. HEAT!
I'm not familar with the different model numbers, but my '77 has 3 wires, power, ground and TURN ON THE HEAT (it comes for the thermostat)

[HowieE]

Originally Posted by HiHoAgRV

One of the units that I had simply refused to fire up if there was less than 12.2 volts at the heater. The fan would spin up, the igniter would click, then nada, fan forever but no heat. Usually I had a full 13+ volts at the battery so don't bother checking there...check at the heater.

Now you got me thinking. If the furnace is that sensitive to a voltage drop I would look at the voltage at the furnace, with what ever lights you had on, before and during the purge cycle. The fan will draw a bit of power and may drop the voltage at the heater enough to cause this problem.

I have had several occasions where my refrigerator would not fire on gas while on the road. Even though there was no other load on the batteries at that time. If this happens again I will test the voltage at the refig. before I replace th costly igniter.

[Eagle & Bear]

Howie,

Thank you for the help. I had a similar problem with the fridge. At some time in the past the igniter was removed. Since I had no idea how to get replacement parts, I drilled a 3/4 in hole in the burner housing at a level that allowed me to see and light the burner manually. I made a small "swing" cover to cover the hole. It works great!