Originally Posted by RogerR
Heat pumps in general will not work below 32 and poorly below 40 degrees. House style ones have auto shutdowns somewhere in that region. Your furnace should have picked up the slack but if you ran out of LP or you had it manually shut down then that could be your cause of a chilly night. I use a small electric heater backstopped with my LP furnace set a few degrees colder. When the electric heater can't keep up the furnace will kick it.
The reason that an RV
heat pump shuts down when the outdoor temperature gets near 32F (0C) is because they lack a defrost cycle that their residential and commercial counterparts have.
When a heat pump is in heating mode, the heat extracted from the air being pulled through the outside coils causes a frost buildup on the coils. Eventually, the frost builds up to the point where restricts, or even blocks, the airflow through the outside coils. In mild weather (above freezing) the frost melts naturally between heating cycles. Residential and commercial heat pumps invoke a defrost cycle periodically to melt the frost that's accumulated on the coils. Here's what happens'
- The fan on the indoor unit stays on to circulate air while defrosting.
- The fan on the outdoor unit and compressor shutdown.
- The reversing valve changes the direction of the refrigerant so the heat pump is now in cool (air conditioning) mode.
- The compressor starts, but the fan in the outdoor unit remains off while the while the warm refrigerant heats the outside coils to melt the frost buildup.
- The emergency/auxiliary resistance heat turns on to temper the cold air passing through the coils on the indoor unit.
- When the heat pump senses the coils on the outside unit have completed defrosting, the compressor shutsdown, the reversing valve changes the direction of the refrigerant so the heat pump is now in heat mode.
- The compressor and fan in the outside unit restarts, the emergency/auxiliary resistance heat turns off and normal operation resumes.
So, you've read this and now you're thinking, "Why didn't they just make the RV heat pumps with a defrost cycle?". Here's why.
You have only 30 or 50 amps of service coming into RV. I''m sure everyone has at least once tripped a breaker while the A/C, electric HWH, refrigerator, coffee maker were all running and then someone plugs in a hair dryer, heats something in the microwave or uses the toaster... Where's the electricity for the auxiliary heat when that RV heat pump does its defrost cycle?
It's probably a rarity to trip a breaker at home with these same things running because most homes have at least 100 amps service minimum. My home has 200 amps.
A few more things about heat pumps. The lower the outside temperature drops, the less heat a heat pump produces. Thermal heat pumps get around this issue because the temperature in the ground isn't affected by changes in outside air temperatures.
Residential and commercial heat pumps run and provide heat when the temperatures are below freezing. The temperature outside here is only 5F (-15C), yet my heat pump's thermostat says it's 70F (21C), the set point temperature, inside. Once the temperature outside goes below 15F (-9C), my heat pump runs continuously. I'm sure it's not making enough heat so the auxiliary electric resistance heaters are cycling on periodically to make up for the deficit. I've used heat pumps in both the homes I've owned over the last 45 years so I'm quite experienced.