Atwood Furnace fan starts, won’t ignite- blows fuse.

[GaryS1953]

Just retired this week and leaving tomorrow for Qratzsite. Went to start the furnace today to warm the camper as we loaded it up. The fan came on as normal, but at about 20 seconds instead of hearing the whoosh of ignition I saw a spark at the wall thermostat and the fan shut off. There was an electrical smell, so I figured the thermostat was toast. I’ve replaced a couple over the last couple years, so I had a spare on hand and went ahead and installed it. This time when I turned it on the thermostat sparked again, but it also blew the 15 amp fuse on the 12 volt panel.

I know a furnace guy and he came out, said he didn’t know anythyabout 12 volt furnaces but he would take a look. After further testing he felt he had narrowed it to the gas valve, that turns on after the initial 20 second purge. My question to all of you experts would be could a faulty gas valve cause both the thermostat to toast and blow the 15 amp fuse? If not, what does this sound like?

I have a borrowed Mr Heater so will go ahead and leave tomorrow as planned, but hope to either fix it myself en route or get it fixed in Q once we arrive. We were planning on taking about a week to get there. Any thoughts on this would be greatly appreciated.

[GaryS1953]

dougrainer wrote:
You have replaced a couple of tstats in the past few years? THAT is NOT normal. Tstats rarely fail. So, your problem was there prior and now it is there for good. When the fan comes ON, The sail switch closes and sends power thru the Hi limit to the control board. Now, without the brand and model, we can only guess now. It is not uncommon for certain Atwoods the wire going to the hi limit switch touch the burner casing and melt and short. THAT would be my first thing to look and verify. Gas Valves shorting out? I have not seen such a thing in over 25 years and I have worked on thousands of furnaces in that time. Possible corrosion on the Control board. Most furnaces have a circuit breaker to prevent shorts from burning out tstats and such. Doug

Problem solved. I think you were right. Found a mobile tech guy in Louisville KY who worked us in between jobs, and found a yellow wire that chaffed, taped it up and reinstalled another extra thermostat I had laying around, all in about 15 minutes, at a VERY reasonable price. Both I and my furnace guy friend inMI had looked and never saw it. So happy to have a working furnace again.

If anyone needs him, the name of his company is B&S Mobile RV Service and Repair, in Louisville.

Thanks EVERYONE for the suggestions and help.

[ScottG]

Since it blows when it first comes on and before the flame can even try to ignite, I would suspect the motor is shot.
I helped a guy replace one like that last summer.

[dougrainer]

You have replaced a couple of tstats in the past few years? THAT is NOT normal. Tstats rarely fail. So, your problem was there prior and now it is there for good. When the fan comes ON, The sail switch closes and sends power thru the Hi limit to the control board. Now, without the brand and model, we can only guess now. It is not uncommon for certain Atwoods the wire going to the hi limit switch touch the burner casing and melt and short. THAT would be my first thing to look and verify. Gas Valves shorting out? I have not seen such a thing in over 25 years and I have worked on thousands of furnaces in that time. Possible corrosion on the Control board. Most furnaces have a circuit breaker to prevent shorts from burning out tstats and such. Doug

[BFL13]

GaryS1953 wrote:

BFL13 wrote:
Besides the 15 amp furnace fuse that blew, there are other "circuit breaker/fuses" in the furnace that might have blown this last time. Check for any blown fuse at the DSI board, or sail switch, but first find the short. Or if need to have fuse in to find it, have some spare fuses handy till you chase it down.

Ha ha! You’re a little ways away from me. I’ll probably have to wait till I get to Quartzsite to get this fixed. Can you or anyone else recommend someone somewhere there to take it to?

Not a clue, sorry. Do have a good trip, and no doubt there are people there who can help.

[GaryS1953]

BFL13 wrote:
Besides the 15 amp furnace fuse that blew, there are other "circuit breaker/fuses" in the furnace that might have blown this last time. Check for any blown fuse at the DSI board, or sail switch, but first find the short. Or if need to have fuse in to find it, have some spare fuses handy till you chase it down.

Ha ha! You’re a little ways away from me. I’ll probably have to wait till I get to Quartzsite to get this fixed. Can you or anyone else recommend someone somewhere there to take it to?

[BFL13]

Besides the 15 amp furnace fuse that blew, there are other "circuit breaker/fuses" in the furnace that might have blown this last time. Check for any blown fuse at the DSI board, or sail switch, but first find the short. Or if need to have fuse in to find it, have some spare fuses handy till you chase it down.

[Dutch_12078]

The gas valve won't open unless the fan sail switch is closed. Power to the valve comes when a time delay relay contact closes, so the timing indicates there's a short to ground somewhere between the relay and the gas valve.

[BFL13]

GaryS1953 wrote:
BFL13 - the furnace THOUGHT he had it narrowed down the gas valve, because the fuse only blew at the time the gas valve should opened the flame ignite. We never smelled gas and it didn’t ignite.

Got it, thanks.

Ok all I can think of is as above, the fan ran ok, sail switch closed and boom!

So check all those many wires on the blades to see if one has come off. I can't see where anything else can be shorted .

The big thing is in the first event, the 12v was ok until after the fan was running. So no short till then anyway. Next event is the sail switch closes, making more 12v positive wires after that come alive.

[GaryS1953]

BFL13 - the furnace THOUGHT he had it narrowed down the gas valve, because the fuse only blew at the time the gas valve should opened the flame ignite. We never smelled gas and it didn’t ignite.

[BFL13]

GaryS1953 wrote:

BFL13 wrote:
You have an 8531-111 or some such? Ok you obviously have a short in the 12v somewhere. It showed up after the motor ran the fan but then it happened. The sequence for ignition goes through a number of things so need to pin down where it stopped.

You said there was gas but no ignition, and then it shorted again? Please confirm you smelt gas. If you did that means the 12v got that far, but if you didn't really smell gas then the fault could be earlier in the sequence.

no, smelled electrical smell. Never smelled gas.

In the OP you said the furnace guy had the gas turn on. Please clarify. I understand the electric smell was the burnt out thermostat.

Anyway somewhere, one of the positive wires is touching RV frame ground which blew the fuse panel fuse. The furnace 12v is grounded to the RV frame. The thermostat just connects and disconnects a positive wire.

Bunch of wires on blade terminals to the various switches--maybe one came off a blade and is touching metal. The motor ran the fan so it must be after that when the sail switch closed and made the next or so positive wire live, and it is shorted. (My theory--not to be confused with actual knowledge 😞 )

[GaryS1953]

BFL13 wrote:
You have an 8531-111 or some such? Ok you obviously have a short in the 12v somewhere. It showed up after the motor ran the fan but then it happened. The sequence for ignition goes through a number of things so need to pin down where it stopped.

You said there was gas but no ignition, and then it shorted again? Please confirm you smelt gas. If you did that means the 12v got that far, but if you didn't really smell gas then the fault could be earlier in the sequence.

no, smelled electrical smell. Never smelled gas.

[GaryS1953]

midnightsadie wrote:
I,d take the southern rt warmer, hope you get it fixed.

yes, definitely taking the southern route, 65, then 40 to 30 to 20 to 10. Thanks!

[BFL13]

You have an 8531-111 or some such? Ok you obviously have a short in the 12v somewhere. It showed up after the motor ran the fan but then it happened. The sequence for ignition goes through a number of things so need to pin down where it stopped.

You said there was gas but no ignition, and then it shorted again? Please confirm you smelt gas. If you did that means the 12v got that far, but if you didn't really smell gas then the fault could be earlier in the sequence.

[midnightsadie]

I,d take the southern rt warmer, hope you get it fixed.