Dometic controller: HP won't come on; furnace does

[MikeGriffin]

I have a 15DS4369. We had left all three zones off last night and this morning it was 50 degrees inside the coach.

I went to the Dometic controller and turned Zone 1 and Zone 2 to "HP", which should turn on the roof Heat Pump units. They didn't come on. Instead, and this happened right away, the Zone 1 furnace kicked on. (Zone 3 remained off).

According to the thermostats, Z1 and Z2 were both 17 degrees colder than the HP setting. Since the HP's weren't turning on, I turned on the burner to help the furnace a little bit. A half hour later, the furnace is still running and neither HP has come on.

When I look at the controller, Zone 1's mode is alternating between HP and FURNACE. You only see this when the backlighting is off. When you touch a key and the backlighting turns on, you only see HP.

I checked power and it looks okay. We're on 50A service. There are no error codes.

Update: In the back of my mind, I remembered that there was something about the HP's not kicking if it was colder than XX degrees. As I was looking around the Internet, I saw that this is true, and that for many coaches the magic number is 55 degrees. As I was reading, the furnace went off and the HP's came on.

Anyone know the lowest temperature at which the HP's will come on in our units?

Thanks!

[Mr_D]

Heat pumps become inefficient at 40° or so. Then the system automatically switches to the furnace. Also, if the thermostat is set for more than about 5° higher than the actual interior temp the furnace will come on to bring the temp up quicker.