Furnace only works when resetting fuse

[DazedNConfused]

Good evening....

On our family camping trip this past weekend, it was cool enough to use the heat. I could turn the heat on, set the thermostat, and the blower would spin up followed by nice toasty warm air. However, when the proper temperature was reached and the system turned off, it would almost never come back on again. I had to pull the 12v furnace fuse, reinsert, and it would instantly fire up again. Maybe once or twice it cycled on its own, but without fail, each night we'd wake up cold, I'd crawl out of bed, pull out and reseat the fuse, furnace would kick on, and I'd crawl back in bed again. Ths would repeat throughout the weekend. Any ideas?

Thanks,
Rob

[tpi]

One thing you may want to try is remove thermostat and cycle the furnace by touching the wires together. Simulate the action of the thermostat, run it till interior is warm enough. The break the connection for a bit then try again and see if the furnace restarts.

I had exactly the symptoms you describe turned out to be a defective mechanical thermostat.

I do say the limit switch lockout is a good possibility. Checking for restriction good idea.

DazedNConfused wrote:
Good evening....

On our family camping trip this past weekend, it was cool enough to use the heat. I could turn the heat on, set the thermostat, and the blower would spin up followed by nice toasty warm air. However, when the proper temperature was reached and the system turned off, it would almost never come back on again. I had to pull the 12v furnace fuse, reinsert, and it would instantly fire up again. Maybe once or twice it cycled on its own, but without fail, each night we'd wake up cold, I'd crawl out of bed, pull out and reseat the fuse, furnace would kick on, and I'd crawl back in bed again. Ths would repeat throughout the weekend. Any ideas?

Thanks,
Rob

[DazedNConfused]

Golden_HVAC wrote:
My guess is that you might find it is reaching over 150F air output, and locking out. Did you have a chance to check the temp of the air yet?

Higher than normal air temps can be caused by a bocked vent, or near the furnace if the hot air is leaking into the cabinet that the furnace is located in, then the air returning to the furnace might be say 95F on average rather than a normal 60 - 70F. I had to seal one of my supply air ducts that was leaking hot air near the furnace, and also installed some reflex insulation around the hot air chamber, to prevent so much heat from gettting back into the return air and fan intake.

Cooler air into the furnace supply air fan means cooler output air, and thus will run longer without going into lockout. Also if the furnace has only 3 ducts, say 4" ductwork each, that might be barely enough air flow, 4 or 5 ducts might flow a lot more air, and the output air will be in the normal 120 - 140F range, the thing will stop locking out.

Good luck,

Fred.

I'm fairly certain you're on to something here. I don't have a quick-read thermometer so was forced to use my grill meat thermometer....so it took a while for the temps to settle....but here is what I recorded this morning. Everything is degrees F

Inside RV temp was 66 degrees before turning the heat on. I turned on the heat and then let it run for a while...then recorded these temps:

Living room vent (which is about 16 inches from the return) = 178
Master BR vent which is about 13 feet from return = 109
Bunkhouse vent which is 14' from return = 118
Return temp = 108

Air flow at the vent closest to the blower is very strong -- as I would expect -- with less flow at other vents, but still seems like adequate air flow everywhere. However, I've never owned an RV with ducted heat so I really don't have a reference. But I can't see how it was a good idea to put a vent a mere 16" from the return. I would think the return is pulling in some of that air coming right out of the vent and is the reason for the high return temps and causing the overheat/lockout to occur?

[Golden_HVAC]

My guess is that you might find it is reaching over 150F air output, and locking out. Did you have a chance to check the temp of the air yet?

Higher than normal air temps can be caused by a bocked vent, or near the furnace if the hot air is leaking into the cabinet that the furnace is located in, then the air returning to the furnace might be say 95F on average rather than a normal 60 - 70F. I had to seal one of my supply air ducts that was leaking hot air near the furnace, and also installed some reflex insulation around the hot air chamber, to prevent so much heat from gettting back into the return air and fan intake.

Cooler air into the furnace supply air fan means cooler output air, and thus will run longer without going into lockout. Also if the furnace has only 3 ducts, say 4" ductwork each, that might be barely enough air flow, 4 or 5 ducts might flow a lot more air, and the output air will be in the normal 120 - 140F range, the thing will stop locking out.

Good luck,

Fred.

[Old-Biscuit]

Here is a generic rv furnace guide....operation & troubleshooting
LINK

Typically 'one cycle' operation is due to inadequate combustion air in/exhaust out......overheats and high limit switch opens
Even a new trailer furnace can get blocked due to mud daubers/spiders etc.
Remove outside furnace cover/panel and check combustion air in/exhaust out for blockage/debris

[DazedNConfused]

Came across this interesting article that talks about the operation of the furnace, its various safety features, etc. Thought I'd share: clicky

[DazedNConfused]

I didn't get the chance to take the temperature at the vent this morning...but will do so tonight. I do know that there is more air flow at the register closest to the blower motor -- which sounds like it's under my fridge and about the center of the camper -- and not as strong at the front and rear -- which I would expect. I don't know how to jump the thermostat....but can follow directions if there is an example of how this would be done.

Thanks for any further information. I'll be hitting the manual and will do some initial troubleshooting before handing off to the dealer -- who is a 3-hour round trip. It's a new camper so it has plenty of warranty left.

[epusher]

Probably not moving enough air (heat) and tripping the overheat lockout. You may have a duct plugged or crimped.

[path1]

If it was mine, with my set up. (some of the newer stuff has too many gadgets to do a simple jump test to check continunity) I would jump the t-stat with a wire when it should come back on. If it does come back on when running a jumper wire. I would then trouble shoot t-state for broken or chaffed wire or replace the t-stat. That's my a short guess. My first step would be to get out manual and check the trouble shooting check list. Maybe under burner fails to re-ignight.

[Golden_HVAC]

Sounds like it is going into some sort of lock out, and turing off the power is the way to re-set it.

You can check the output air temperature, and say if it is reaching over 150F, let us know how hot it is getting, that might be a clue to why it is locking out.

Also if the propane tank is low on propane, or the regulator is freezing up, it might cause a lack of gas pressure for a little while, then back to normal later, causing the flame to go out while the burner is on, and lack of flame can cause it to go into lockout. Yet I have almost ruled this out while typing it, because it locked out so many times, yet you still had propane to run the furnace hours after that first and second time. ALso the refrigerator was not effected, so the propane system is probably working normally (unless the Refrigerator check light came on as well).

Fred.