Originally Posted by Pahaska
In my 28, I find that the heat pump can keep the trailer very comfortable down to the high 30s. With the Dometic Comfort Control, there is an automatic switchover to furnace at about 36 or 37 degrees. There is a sensor on the outside unit that detects when the switchover should occur. Here is central Texas, I almost never use the furnace.
The heat pump also goes into periodic defrost cycles at colder temperatures. You will hear the inside fan stops, but the compressor keeps humming for a couple of minutes, then the inside fan starts again.
I would agree with Pahuska that the heat pump provides adequate heat down to the upper 30's, but I do find the furnace to be quieter. One side benefit to the automatic switchover to furnace when the outside temp drops into the 30's is that the furnace provides some heat to the underfloor area where the HP is producing warm air only from the top down.
The 1st night we used the HP, I woke up in the middle of the night thinking something was wrong. I could hear the compressor humming along but the blower wasn't running. By the time I was fully awake, I realized it was in defrost mode. During defrost, the unit runs as if it were in cooling mode, but without the blower circulating cold air. This allows the outside unit to heat up melting any accumulated ice up there. Also, if you are used to the A/C condensation draining out below the body, in heat mode everything is backwards, so the melt from the defrost cycle runs off the roof along the drip rails.
You may also get some strange looks from your neighboring campers as they wonder why you're running the otherwise conventional looking rooftop A/C when its 40 outside. :-)