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Old 01-21-2022, 04:14 PM   #1
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Heat Pump Operation and Furnace Fan

I noticed something about my heat pump and I wonder if it is normal. The first thing I know is normal, if we get up in the night when the inside temperature is quite a bit colder than the thermostat setting the gas furnace runs at the same time as the heat pump till things warm up, then the heat pump takes over, this is what I would expect.
Now if we set the thermostat before going to bed and the temperature drops below the set point during the night, the heat pump will start but the gas furnace will not, to be expected, however the blower fan for the gas furnace will run. In fact the blower fan for the gas furnace always runs when the heat pump is running whether the burner is lit or not, and is stays running for a short time after the heat pump cycles off just like it would if the burner was lit, but it's not, the fan is just recirculating room air.
Is this normal? Is it a safety feature as I'm sure the blower fan also is connected to the combustion exhaust fan, does the thermostat automatically start the fan in case the furnace is called upon at any point during heat pump operation?
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Old 01-21-2022, 05:50 PM   #2
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If your RV has the RV Comfort thermostat, when set to electric heat it will signal the furnace to provide heat IF the current temp is more than 4 degrees difference from the set temp.

If you leave it on, once the temp reaches the set point the heat pump will come on and off and you'll never get far enough below the set point for the gas furnace to run. It will go between 1 degree less and 1 degree more than the set point. UNLESS it's cold enough (below about 40) that the heat pump can't put out enough heat to keep the temp at the set point and eventually the difference will hit 5 degrees and the gas furnace will come on.

The furnace fan is not manually operable, it comes on immediately when the furnace is signalled to provide heat and will run for at least a few minutes before turning off even if the thermostat stops signalling the gas furnace to provide heat. This is to make sure that that the furnace blower always runs longer than the gas burner is on and cools down the furnace to prevent damage from overheating.
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Old 01-21-2022, 06:03 PM   #3
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Originally Posted by powercat_ras View Post
If your RV has the RV Comfort thermostat, when set to electric heat it will signal the furnace to provide heat IF the current temp is more than 4 degrees difference from the set temp.

If you leave it on, once the temp reaches the set point the heat pump will come on and off and you'll never get far enough below the set point for the gas furnace to run. It will go between 1 degree less and 1 degree more than the set point. UNLESS it's cold enough (below about 40) that the heat pump can't put out enough heat to keep the temp at the set point and eventually the difference will hit 5 degrees and the gas furnace will come on.

The furnace fan is not manually operable, it comes on immediately when the furnace is signalled to provide heat and will run for at least a few minutes before turning off even if the thermostat stops signalling the gas furnace to provide heat. This is to make sure that that the furnace blower always runs longer than the gas burner is on and cools down the furnace to prevent damage from overheating.
Thanks but as I said in my post I understand all that, what I don't understand and the reason for my post is why does the furnace fan always run when the heat pump runs, even when the furnace is not being called upon for heat, and is that normal?
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Old 01-22-2022, 05:51 AM   #4
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> why does the furnace fan always run when the heat pump runs, even when the furnace is not being called upon for heat, and is that normal?

No, it's not normal. The furnace only has 12 volt power and on/off from the thermostat input connections. So, unless modified by someone, if the furnace fan is running then the furnace is going to be trying to put out heat. There is no input connection on the furnace to ask it to run fan only without heating.

So, I don't know why.
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Old 01-22-2022, 08:07 AM   #5
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To get the best answer on this, I would think it may be different on different models and brands, so a deep dive into the brand and model of the specific furnace would be the way to go.
I find most will have online troubleshooting guides and those are a really accurate way to get the correct answer.
I would guess (without ever testing it!) that it is working the way supposed to but that is often where we need to check what we read. Way too may folks making desiions on guesses and getting in real trouble because they believed what they read on the internet!

Neither Randy, nor I are trying to mislead you but a check on your specific Rv may get better answers. You may have the one year they tried doing it different?
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Old 01-22-2022, 09:10 AM   #6
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Originally Posted by Morich View Post
To get the best answer on this, I would think it may be different on different models and brands, so a deep dive into the brand and model of the specific furnace would be the way to go.
I find most will have online troubleshooting guides and those are a really accurate way to get the correct answer.
I would guess (without ever testing it!) that it is working the way supposed to but that is often where we need to check what we read. ?
Thanks Richard,

The furnace is a Suburban model SF-20, the thermostat is a Coleman and the AC/heat pump is a series 49000 made by Airxcel which I believe is a Coleman Mach as that is referred to throughout the operator's manual. I have studied the manuals and nothing shows that the furnace blower should operate unless heat is called for. When it has done this there was no problems with the furnace or propane supply and lowering the setpoint would cause the burner to light. The fan was always set to auto. It is unlikely anyone has messed with it, certainly not the folks that owned it prior who were not DIY types. Perhaps it is the way the thermostat was originally wired. I'm going to do more checking on this in a day or two to see just what it's doing.
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Old 01-22-2022, 06:14 PM   #7
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A review to se that I'm getting the rightideas?
You have a pretty standard RV funrace sending heat to outlets near the floor but with rooftop AC also having the heating option which sends electric heat through ceiling vents.

Not currently able to check that as I get error messages when checking, but believe that is correct.
When running on heat from the ceiling heat pump, the furnace with the floor vents is blowing, even though not lighting.

I agree that this does not sound normal but in looking at where the signal for the furnace blowing to come on would seem to be at the point where the ceiling wiring and furnace wiring are both near each other and that seems to be only at the thermostat.

I might suggest taking the cover off the thermostat and looking closely at the connections and wires to see if there might be some sort of cross connection like a wire tail sticking out too far or maybe the wires mashed together as the cover is put on or as the thermostat is placed on the wall?

That would seem to be the cheaper out but it may also be a defective thermostat but I like to look at the chep options first.

Ever optomisic but often disappointed!
So the
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Old 01-25-2022, 11:32 AM   #8
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update

Well I tried to replicate the problem and I couldn't, everything works as intended. It is possible that I didn't realize the gas heat was working and only checked after the burner shut off since the blower stays on for quite a while after the burner shuts off. It was a somewhat cold morning (36F) when I first noticed it and I suspect the heat pump was not keeping up and the furnace kicked on without me realizing it, or maybe for some reason the furnace was not lighting or staying lit. Trying to replicate it at home the ambient was close to 60F so heat pump didn't need help until I raised the setpoint more which started the furnace which worked as the chart describes.
Thanks for all the help and sorry to have cried wolf so quick, I should have done more investigating first.
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Old 01-25-2022, 01:32 PM   #9
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Your explanation in your update post is the most likely scenario. We had the same system in our Bounder motorhome. The T-stat was an RV Comfort (Coleman) with a Coleman Mach Heat Pump. At times the furnace fan would run, even though the temperature was not below @40 degrees F requiring the furnace to supplement the Heat Pump. The issue was not in the T-stat but the control board in the Coleman Heat Pump AC. Replacing the board solved the problem. You'll need the model # from the Heat Pump for the proper board.

As mentioned in another post the furnace fan will run before and after the furnace lights. The furnace is normally equipped in what is known as a 3 try board. The fan will come on, the furnace tries to ignite, if no ignition the fan will shut off, then go thru the same procedure twice more before shutting down.
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Old 01-25-2022, 01:46 PM   #10
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That's good news that it seems to be okay for now----I think!
Always good to get a firm answer but then in this deal we get boards in three different items and those boards all have to work together.

I've never looked real close at what, where, and how the boards decide who is going to do the heating! Likely not something we could ever figure without doing some board changing and hoping for the right one before we go broke!
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Old 01-25-2022, 02:04 PM   #11
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I had some inside knowledge and help with mine. I worked for Camping World as both a service writer and in charge of the small parts room. This was before CW became part of Freedom Roads dealerships and was ruined by Marcus Lemonis. Got to work closely with some very good RV techs and part suppliers, so I learned quite a lot from them.
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Old 01-25-2022, 08:31 PM   #12
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Well, I learned something tonight!
I got home today from a trip to Orange Beach where the temperature dropped below 30 a couple of nights. I ran my heat pump until the temperature got down to 42 or so then manually switch to gas for the night. Nice to know that the heat pump will automatically switch over to gas heat without me manually changing it. But I did learn that running a small electrical heater on low at night was all we needed. We like to sleep “cool” and set the thermostat to 65 overnight. With the little electrical heater, the gas furnace never came on.
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Old 01-26-2022, 12:55 PM   #13
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Well, I learned something tonight!
I got home today from a trip to Orange Beach where the temperature dropped below 30 a couple of nights. I ran my heat pump until the temperature got down to 42 or so then manually switch to gas for the night. Nice to know that the heat pump will automatically switch over to gas heat without me manually changing it. But I did learn that running a small electrical heater on low at night was all we needed. We like to sleep “cool” and set the thermostat to 65 overnight. With the little electrical heater, the gas furnace never came on.
We have found the small electric heater is often enough to get us by at night but found the temperatureswings with the builtin thermostatwere more than we liked as it got too cold before kicking on an then might gettoo hot.

Our solutionwas to pick up one of the non-programable digital thermostat used for house furnaces and retrofit it to the heater.
Many may point out that the normal house thermostat works on 24V, so be awre and get the one which uses and internal battery for the unit operation, not 24 volt type! Honeywell makes a good pretty cheap one if buying new.

The big advantage is that one can set the thermostat where you are and the heater in the other end of the RV as well as the much smaller swing of digital.
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Old 01-26-2022, 04:50 PM   #14
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Originally Posted by bigb View Post
I noticed something about my heat pump and I wonder if it is normal. The first thing I know is normal, if we get up in the night when the inside temperature is quite a bit colder than the thermostat setting the gas furnace runs at the same time as the heat pump till things warm up, then the heat pump takes over, this is what I would expect.
Now if we set the thermostat before going to bed and the temperature drops below the set point during the night, the heat pump will start but the gas furnace will not, to be expected, however the blower fan for the gas furnace will run. In fact the blower fan for the gas furnace always runs when the heat pump is running whether the burner is lit or not, and is stays running for a short time after the heat pump cycles off just like it would if the burner was lit, but it's not, the fan is just recirculating room air.
Is this normal? Is it a safety feature as I'm sure the blower fan also is connected to the combustion exhaust fan, does the thermostat automatically start the fan in case the furnace is called upon at any point during heat pump operation?
After reading several responses I didn't see where your question was answered. Forgive me if it has been.
I see you have the call chart posted. That thing is a bit tough to follow. Here is what I have learned.
The gas heater fan only runs if it has a call for heat. Once it fires it must go through a purging cycle to cool down and clear all the gas out of the burn chamber.
If you set your thermostat a 3*F or less above the room temperature reading on the thermostat only the heat pump will operate. If you set the temperature 4* or more degrees over the room temperature the heat pump and gas furnace will be called into action.
Heat pumps work down to about 39* or a bit less if your lucky. After that you have to go to gas.

I hope this clears things up a bit for you. I had the same questions when I started RVing in '05. Took me a while to understand the gremlins.
Happy trails.
PS: If your LP tank has fuel and you just can't seem to get your furnace to put out heat when it is below 30* you have butane in your tank. How do I know? I'm in AL. The nights have been cold and I kept "running" out of propane. The tanks didn't feel empty though. Went to a different Ace hardware. He weighed the tanks and confirmed they had fuel. He topped them off at $6 a gallon! I'm running on an extend-a-stay. Now I have a cube heater next to the tank to keep it 'boiling' when the temps drop. Propane will give off vapors down to -40* or so.
The challenges of RVing make it all that much more fun. You never have time to get board.
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Old 01-26-2022, 04:50 PM   #15
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Big B,


I'd call Winnebago and ask a tech there. They've always given me good info.
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Old 01-26-2022, 05:00 PM   #16
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PS: If your LP tank has fuel and you just can't seem to get your furnace to put out heat when it is below 30* you have butane in your tank. How do I know? I'm in AL. The nights have been cold and I kept "running" out of propane. The tanks didn't feel empty though. Went to a different Ace hardware. He weighed the tanks and confirmed they had fuel. He topped them off at $6 a gallon! I'm running on an extend-a-stay. Now I have a cube heater next to the tank to keep it 'boiling' when the temps drop. Propane will give off vapors down to -40* or so.
The challenges of RVing make it all that much more fun. You never have time to get board.
Thanks Rick for the info. By the way I learned a trick from my brother in law how to make the propane flow when cold, put an electric blanket around the tank. He was an HVAC contractor in Jackson Hole and once when it got really cold the propane fueled customers started calling in to report no heat, his guys bought up all the electric blankets in town and got things flowing again.
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Old 01-26-2022, 05:04 PM   #17
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Big B,


I'd call Winnebago and ask a tech there. They've always given me good info.
I will if any more issues but I think everything is working as it should for now.
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Old 01-26-2022, 07:55 PM   #18
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Thanks Rick for the info. By the way I learned a trick from my brother in law how to make the propane flow when cold, put an electric blanket around the tank. He was an HVAC contractor in Jackson Hole and once when it got really cold the propane fueled customers started calling in to report no heat, his guys bought up all the electric blankets in town and got things flowing again.
That's a great idea, Brian. But I don't have one. There's another problem for those with MSW inverters. The standard stick & brick electric blanket won't work. The dirty sine wave power burns up the control. There is a RV electric blanket that will work. My cube heater, plugged into the engine heater outlet, will have to do. Not much snow here and temps only go to about 22*for the low. It's strange how I can't recall this problem with propane as a kid growing up in northern NH. -20* was not unusual, but I know it got colder at times.
One other thing about your heat pump. If left to fend for its self, if the room drops 5* or more because the heat pump can't produce heat, the furnace will take over. All of this is in your users manual. Pretty nifty setup.
Blessings,
Rick
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Old 01-27-2022, 07:01 AM   #19
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Beware of wich LP is in your tank: if your last fill was done in a southern state, it is butane instead of propane. Butane stops evaporating at 32F while propane stops evaporating at -10F. In most camping conditions it won't make a diference to the user who lives in southern Texas or Florida.

So why is butane replacing propane in southern parts of USA?
Because butane uses a cheaper process to extract and is easier to liquify, therefore should cost less to the end user!
But it will cause your furnace to fail attemps to light up in cold weather.
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Old 01-28-2022, 07:59 AM   #20
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Beware of wich LP is in your tank: if your last fill was done in a southern state, it is butane instead of propane. Butane stops evaporating at 32F while propane stops evaporating at -10F. In most camping conditions it won't make a diference to the user who lives in southern Texas or Florida.

So why is butane replacing propane in southern parts of USA?
Because butane uses a cheaper process to extract and is easier to liquify, therefore should cost less to the end user!
But it will cause your furnace to fail attemps to light up in cold weather.
You're correct except for propane. -40*F is the point it stops producing vapor. One other aspect to consider is the amount of fuel in the tank. The less the tank has the harder it is to get usable vapor at low temperatures respectively. So, if you have the misfortune of getting butane unknowingly, as is my case, and your 40# tank is down to about 3 gal, the temperature drops to 33*F, the amount of vapor available to your systems with butane is not sufficient to sustain continued burning. Hence the need to provide heat to the tank. With this blizzard coming in, you folks in FL and areas in the south under the gun need to get heat to your tanks while you can just in case you don't have propane on board. A incandescent bulb drop light and a tarp is a will help greatly.

Be safe. Stay warm.
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