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Old 01-09-2018, 11:54 PM   #1
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Heat pump/ AC

As a recent purchaser of a Thor Motorhome that came with 3 ac units on the top. When I try to get the heat pump side of the AC to come on it does not. This is our first MH and the dealer says bring it back which is nearly 200 miles away. Not the best option.. So given the knowledge here I will give it a shot. They are Dometic AC units with a dometic thermostat in the cabinet. When I put the number 1 unit into auto fan and heat pump mode nothing happens. The same with unit 2 and 3. If I put it into furnace mode the Hydronic heater in the bay comes on and heats yet nothing from the top units. If I change it in the fan mode to low med or hi the fan in that unit comes on but no heat from the top.
I contacted Thor with no help. I contacted Dometic and surprisingly again no help. Dometic did state a chip in the thermostat may be faulty and needs to be changed. Any ideas from the forum that might help me out here.. Oh yes, it was a used unit, but the AC units all blow cold air in the AC mode.
Suggestions??
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Old 01-10-2018, 12:54 AM   #2
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I learned a lot about my Tiffin M/H heat pump (Coleman Mach brand) this weekend.
One thing I did not know is if the room temp is more that 5* less than the thermostat's commanded temp, the heat pump portion of the A/C will be not allowed by the thermostat to engage, propane heater is called upon to eliminate the temp diferancal. Once temps are within the 5* split, then the heat pump is allowed to run. I do not know if yours operates the same way or not. But try running it on propane to warm the room then switch to "electric heat" (thats what my t-stat calls the HP).
Also I am sure that all 3 of your roof top units are heat pumps, I bet just your zone 1 is.

I learned about my system troubleshooting a loud banging noise my wife kept hearing coming from our heat pump. I pulled the cover off my unit to investigate and found that my squirrel cage blower was completely shattered (plastic impellers!).
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Old 01-10-2018, 01:35 AM   #3
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I have two units in my rig. One bedroom and one living but I believe my only heat option is furnace heat in the main living area. I couldn’t find an option for heat at all from the bedroom and only the heater vents in the floor blow heat. AC works great in both but heat seams to be propane only—no heat pump.


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Old 01-10-2018, 10:49 AM   #4
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Tanman,
Can you please list the make and model of your motor home, A/C units and thermostat? That will help with the troubleshooting. IIRC they typically only put the heat pump option on one or possibly two roof top units on a multi-unit system. Also what was the outside temps? The heat pumps only work down to about 40°, then they shut down.

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Old 01-10-2018, 03:13 PM   #5
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thanks for the response... I am still a little baffled.. the MH is a Thor Four winds Mandalay 45 foot diesel pusher. I did not take the upper covers off to see the model of the unit or check to see if they are all, in fact, a heat pump. Dometics told me they will not work below a temp of around 40 degrees but didn't really know what temp it was designed not to work. hell, that's when I need heat... It seems they don't put a defrost mode in the units to keep them from freezing up.. Go figure. Anyway, the main heat seems to come from the floor vents from a Hydronic heating system which seems to work ok but I am skeptical about how well it will work in freezing weather.
The Thermostat is a Dometic 5 button duo-therm model. Basically, 5 buttons on the right side with a backlit screen on the left. Mode, fan, up down and zone. pretty simple to figure out. One of the mode settings is heat pump (spelled out). When in the auto mode nothing happens. When I put the fan in the low med or high it comes on in the HP mode but no heat. hmmmm frustrating ... if I move it to furnace instead of HP, the floor furnace (hydronics) comes on with heat.
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Old 01-10-2018, 10:55 PM   #6
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Should be able to get the model numbers from the inside by pulling the grills. I do know that if you put the fan portion of the thermostat on anything but Auto it will run the fans on the upper units even if you have the furnace on. Never quite figured out why, unless it is to help move the air around. Until you check the model numbers you won't know what does or does not have a heat pump. As far as the 40° lower limit, that is true of most base model heat pumps even household ones, they have electric heat strips built in to "assist" when the temps get too cold. Until you get into the high pressure high efficiency units.

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Old 01-11-2018, 12:06 AM   #7
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If yours is set up like mine you should have two Dometic control panels. On mine the front one controls the front A/C unit & rear controls the other two. Like stated 40 is the lowest the heat pumps will work at.
I think you will find your Hydronic will work very well in cold temperatures.
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Old 01-11-2018, 03:28 PM   #8
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I wonder what fluid is used in the hydronic heater. Would it be water as in the old style water radiators or oil or glycol?

Hopefully water isn't used as the heat source, if it is the system will need to be drained prior to freezing temperature.

I would think the transfer medium needs to be identified before a problem arises.

Just something to think about...........
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Old 01-11-2018, 04:56 PM   #9
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If it's like the aqua-hot system in my Monaco it uses glycol. Mine is diesel fired to create the heat. It also uses the coach engine cooling system to provide heat.
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Old 01-11-2018, 06:35 PM   #10
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Quote:
Originally Posted by DesertRat View Post
If it's like the aqua-hot system in my Monaco it uses glycol. Mine is diesel fired to create the heat. It also uses the coach engine cooling system to provide heat.
Sounds like a cool idea all around.
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