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Cuda67bnl · 12-23-2022, 02:14 PM

I used our coach last weekend for a hawking trip. Came home and winterized it Sunday afternoon. Wife just went out to get something out of the basement. Came back inside and told me there was a fan running above the bay door. So I went out, and the furnace blower was running.

I had left the thermostat on, but it was set to the heat pumps(65*), not the furnace. Propane is off. Why/how was the furnace blower running if propane was off and the thermostat was set to the roof units? Batteries were pretty dead, so no idea how long it’s been running. I’m letting the generator run for awhile to get the batteries charged back up.

Is the stat wired in such a way that the furnace automatically lights if you lose AC power? If so, I didn’t think the blower would continue to run if there was no propane getting to the furnace?

shootist · 12-23-2022, 02:40 PM

Board is sending it a signal to stay on, the temp sensor maybe failed and kept it on , can't be too many causes. It has to stay on a minute or two after the furnace shuts off to cool the exchanger down quickly.

Old-Biscuit · 12-23-2022, 02:41 PM

heat pumps can only 'heat' down to 40*F or so....if temp varies by 5*F (?) the furnace will be turned on
No propane....fan starts/purges/3X attempts to light off
BUT furnace controls should have gone into Lockout and turned fan off after failure to establish flame

Mike8253 · 12-23-2022, 02:45 PM

When asking for heat from your heat pump, if you set the requested temp on your thermostat to 5 or more degrees above what your thermostat is currently sensing in your house, the furnace will automatically come on to assist until you get within that delta. If the propane is turned off, just the blower will run until it gives up for lack of combustion.

Cuda67bnl · 12-23-2022, 05:44 PM

I’ve turned my heat pumps on to 70* when it’s been high 40s to low 50s in the coach. My furnace has never came on while the heat pumps are on. Only time my furnace has ever ran, is when I’ve had zone 1 on the thermostat set to furnace. I guess my lesson in this, is to make sure I turn the thermostat off next time, lol.

Old Scout · 12-23-2022, 06:04 PM

Jimmy, I tend to agree with what you are saying. The Dometic heat pumps on my 2003 didn't have the 5-degree delta feature. I never turned off the propane tank but depending on a low enough ambient temperature, I could see a situation where the Dometic HP could lock out and auto switch to the furnace. In this case, if the propane was shut off, I would think the fan would continue to run. I am not aware of a fan shut-off feature on furnace if burner failed to light.

Old-Biscuit · 12-23-2022, 06:10 PM

3 try ignition board
After three (3) Trial-For-Ignition attempts with no ignition of the main burner, the module board will de-energize the gas valve immediately and
blower will run for 3 minutes and then shutdown in lockout mode.

TDR---Time Delay Relay
. After three (3) Trial-For-Ignition attempts with no ignition of the main burner, the module board will de-energize the gas valve immediately and blower motor will continue to operate

OP........sounds like you have a Furnace that uses the TDR vs Fan Control Circuit Board

Cuda67bnl · 12-23-2022, 06:26 PM

Quote:

Originally Posted by Old Scout

but depending on a low enough ambient temperature, I could see a situation where the Dometic HP could lock out and auto switch to the furnace.

Even with no AC power to the heat pump? I guess since the thermostat and furnace is reading/running off of 12v that the lack of AC makes no difference?

Sideshow · 12-26-2022, 11:06 AM

Jimmy,
I had a similar issue. Our ALPINE is relatively new to us and learning quickly thanks to the great people on this forum. We dry camped in the desert and used the furnace a bit.
A couple times No mode was selected on the thermostat and the furnace came on. Never figured out why but like you made me scratch my head. I double checked the thermostat setting to make sure no mode was selected on zone 1 or 2 and still came on.
I ended up turning the thermostat off with the switch under the thermostat and never came on again.
Cheers
Bob

Cuda67bnl · 12-26-2022, 06:58 PM

I experimented a bit on Saturday. It was in the teens outside, and 20* in the coach. I turned the propane on, and set the thermostat to heat pumps. The furnace came on….. I started the generator so the heat pumps would have ac power, but they wouldn’t even try to come on. I turned genny off, hooked up shore power, set thermostat to furnace, and let it run for a couple hours. When I went back out, it was 40* in the coach. This time when I set the thermostat to heat pumps, it lit up as heat pump/auxiliary heat, and the furnace started again, but no heat pumps. Stopped genny, put thermostat back on furnace, and let it run for awhile again. Came back out and it was 50something* inside. Started genny, set thermostat to heat pumps, and they fired right up, and didn’t show auxiliary heat. What I learned, is the heat pumps won’t even try to turn on when it’s that cold outside. The thermostat reverts to furnace when you don’t have shore power to the heat pumps. The thermostat reverts to furnace when it’s too cold, with shore/genny power. And if you turn the thermostat off, you won’t run your batteries dead, lol.