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Old 05-03-2021, 06:56 PM   #1
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Heat Pump Wonít Start

I have three Coleman Mach 8, Model 47079A879, 15,000 BTU A/C Heat Pumps on my Tiffin Phaeton. Each has an RVComfort.hp thermostat. The front heat pump doesnít start but I can hear the thermostat click calling for heat. The furnace heat works fine and the A/C portion of the unit works fine so I know the compressor is working OK. Only found this out when we had a cold snap while in Red Bay.

I have had a hard time finding any documentation and am not sure where to start with diagnosis. Does the heat pump and A/C use a different start capacitor?
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Old 05-03-2021, 07:09 PM   #2
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I don't know about your Mach 8 but on my Duo Therm the heat pump is the A/C running ; with a reverse flow solenoid in the refrigerant system, so the evaporator is now the condenser .
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Old 05-03-2021, 07:29 PM   #3
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It should use the same start capacitor, all that changes is the reversing valve.

Are you saying the compressor never starts? In heat mode?

That would be odd at least with a home system the signal to run is the same itís just whether or not the reversing valve is engaged.
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Old 05-03-2021, 07:30 PM   #4
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Does the rear unit work? Also there may be a low temp cutoff that forces you to use the furnace if the outside air is to cold.
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Old 05-03-2021, 07:35 PM   #5
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Yes, rear works fine. I thought it worked when we bought the coach last June, but really wasnít familiar with the operation at that time. Middle works fine as well, only the front unit.
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Old 05-03-2021, 11:41 PM   #6
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Heat Pump Wonít Start

Quote:
Originally Posted by Biljol View Post
It should use the same start capacitor, all that changes is the reversing valve.

Are you saying the compressor never starts? In heat mode?

That would be odd at least with a home system the signal to run is the same itís just whether or not the reversing valve is engaged.


Correct, the compressor starts in cool but not in heat. Also, when past the temperature differential, it doesnít pass the duty to the gas heat like it should. Rather, it allows the temperature to continue to drop without starting the furnace.

Wondering if itís the RVComfort.hp unit itself.
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Old 05-04-2021, 05:51 AM   #7
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So the heat used to work on the front unit?

May be there a programming or dip switch to tell it itís heat pump but the fan comes on just no compressor thatís odd.

Are you sure the compressor is not turning on and itís just not the reversing valve that not engaged it could be putting out cool air.
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Old 05-04-2021, 05:56 AM   #8
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Here is a couple of searches I found in that t-stat


https://www.rvforum.net/threads/rv-c...furnace.37987/

https://www.rvforum.net/threads/prob...fort-zc.27950/
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Old 05-04-2021, 07:11 AM   #9
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Quote:
Originally Posted by Biljol View Post
So the heat used to work on the front unit?

May be there a programming or dip switch to tell it itís heat pump but the fan comes on just no compressor thatís odd.

Are you sure the compressor is not turning on and itís just not the reversing valve that not engaged it could be putting out cool air.


Nothing comes on, not even the fan. In cooling mode, the A/C works fine.
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Old 05-04-2021, 07:15 AM   #10
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Ok that makes more sense so I think one of 3 things.

A. The t-stat dose not think itís a heat pump.
B. The temp is too low outside to use a heat pump
C. The temp difference is to high to use the heat pump so it kicks the furnace.

3 items to start your troubleshooting.
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Old 05-04-2021, 07:28 AM   #11
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Try swapping your mid and front thermostats and see what happens. What you are describing sounds like either a faulty thermostat or a defective control board in the A/C unit.
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Old 05-05-2021, 07:22 AM   #12
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Heat Pump Wonít Start

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Originally Posted by Lt Dan View Post
Try swapping your mid and front thermostats and see what happens. What you are describing sounds like either a faulty thermostat or a defective control board in the A/C unit.


Tried swapping the front and middle thermostats, didnít work. Moved them back to their original positions and the middle heat pump fired right up. Front A/C fires right up, but no heat pump.

I donít see a control board per se, but there is a relay. Wondering if the relay has two poles, one for A/C and one for heat pump. It is also the low pressure switch. Itís part #25.

https://www.airxcel.com/DesktopModul...609c__1015.pdf
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