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Old 10-19-2022, 06:42 AM   #1
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Weird Heating Issue

'07 Essex. We pulled in last night ahead of the cold front. The front two (zones 1 & 2) heat pumps ran fine all night and kept us warm. Early this morning that all changed...
Now all zones (1, 2 & 4) will start the heat pump but shut them down again after a few minutes. When that happens, I notice that the furnace heat register fans start up as if I have set that zone to Furnace. Outside temperature was below 35 degrees when this was happening. The ePlex panel still shows the zones set to Heat Pump.

We're warm due to the Hydro Hot furnace system working OK, but what would cause the heat pump units to shut off? Is it an ePlex 'feature' or is something not right? I know heat pumps loose efficiency in cold temps, but it seems to me that they should keep running if the control unit tells them to.

I will wait for warmer temps this afternoon and try the heat pumps again, but this is weird....
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Old 10-19-2022, 06:52 AM   #2
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Heat pumps will not work below ~40F. It sounds like your coach is trying to use the Oasis furnace and shutting down the heat pumps due to the outside temperature.
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Old 10-19-2022, 07:52 AM   #3
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On our Aqua Hot, our furnace fans will sometimes engage when heat pumps are first turned on. They shut off after a minute. As explained to me, it is to offset the start up of the heat pumps when they are near their low temp efficiency. In other words, its too cold (low 40s) to use the heat pumps.
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Old 10-19-2022, 08:06 AM   #4
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It sounds like one or all of your heat pumps try to heart with the heat pump. But after some time they have no success raising the temp and they go to backup heat. Most likely what they are designed to do.

Or, The second option if the ducts are connected between the two dissimular heating systems. it could be they are going through a defrost cycle.
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Old 10-19-2022, 08:51 AM   #5
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Most RV heat pumps are set so that if the heat pump is running and the room temperature drops 3 to 5 degF below the thermostat set point, it will shut down the compressor and bring on the furnace heat. It will repeat this cycle 3 times and then lock out the heat pump and run on the furnace. At this point you will need to shut off the units at the thermostat and it will reset.

You need to read the thermostat and A/C-heat pump manual for the sequence of operation. An RV heat pump will not function well below 40 degF. The outdoor coil will ice over and stop transferring heat.

Many of the units will have a temperature sensor or pressure sensor on the outdoor coil to prevent the coil from operating below about 20 degF coil or refrigerant temperature. The split on the coil is about 20 degF so that will mean the outside air has to be 40 degF or higher.

By definition, air conditioning, refrigeration or comfort cooling is defined as the removal of heat from one area and rejecting to another area where it is not objectionable. In A/C mode, the heat is removed from the cooled space and rejected outside via the condenser. In heat pump mode, the heat is removed from the outside air and rejected to the inside of the RV to warm it. As the air outside gets colder there is less heat for the heat pump to use and it will start to freeze the coil which will further reduce the heat transfer.

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Old 10-19-2022, 03:29 PM   #6
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Many thanks for the replies. Came back home today with warmer temps and the heat pumps work just fine.
Learned something new, so today was a good day
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Old 10-19-2022, 05:17 PM   #7
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If you have Dometic AC units on the roof, the furnace is controlled by the primary Dometic control board. The Dometic heat pump will operate until air temperature over the coil drops to around 32*F (which will always be lower than the actual ambient temp), at which point the control board will lock out heat pump operation and transfer to furnace operation to reach set point. This is a fully automatic process and you cannot override it.



It is exceedingly hard on the Dometic scroll compressors to operate them in temperatures under 39*F. The rate at which the coils begin to freeze up and the frequency of defrost cycles take a toll on the compressor and reversing valve. You hear that sloshing noise in the roof? That's your compressor trudging through liquid freon as the exterior coil freezes over and the reversing valve operates. You are saving yourself no money nor electricity running a Dometic heat pump below around 42*F, the efficiency isn't there any more. In fact, you're contributing to their early demise as the bearings in the scroll compressor are eventually exposed to a mixture of liquid freon and lubricant- something not intended. Dometics also don't have emergency heat or coil strips to enhance their low temp performance like many home heat pumps.
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