RVP AC/Heat pump in freezing temps

[sgkane]

We have the basement AC/Heat Pump and I’m currently have the Ecobee Si thermostat. I’ve had this setup for some time now without issue. This week we are in central Texas with a big cold from coming through with a high of 32 degrees. The problem is every time the thermostat calls for heat the heat pump will kick in and run for about 30 to 90 seconds then shut down. After several minutes later the propane furnace will kick in. Since it’s only about 32 degrees out all the heat pump does is blow cold air around the bus. I don’t remember this behaving like this before but then again, I’m not sure we’ve ever had to use it in freezing temps before.

My assumption on how all this is supposed to work is that the thermostat calls for heat from the primary source (in my case the heat pump). If its below 42 degrees the heat pump will say “can’t do” and sends power to the propane furnace. My guess is that the heat pump can’t tell how cold it is until after running for a short time, it fails and shuts down without turning on the propane furnace. After several minutes of not receiving heat, the thermostat calls for the secondary heat source.

If my assumptions are correct is there a way of adjusting the cold freeze thermistor or am I looking at replacing the whole circuit board?

 

[Laura & Charles]

What ours does, and the before this one is, when set for heat pumps (electric on thermostat), if the difference between the ambient room temperature and the setting on the thermostat is greater 5°F, it calls for gas heat to supplement. I’ve never noticed the heat pumps quitting. I also don’t think the heat pumps can make much, if any, heat when outside temperature is 42° or less.

 

[Mountain high]

We don't have a heat pump. From what I read they don't work in regions with subzero Fahrenheit temperatures. Currently using propane central air furnace, pellet stove, and electric space heaters.

 

[John Canfield]

If my assumptions are correct is there a way of adjusting the cold freeze thermistor or am I looking at replacing the whole circuit board?

No and no.

The basement heat pump as supplied does not have a coil defroster (unfortunately) so it will not blow hot air based on ambient humidity and temperature. If humidity is high, the coil might freeze in the low 40s. Low humidity will extend heat pump capability maybe in the low 30s.

The "freeze switch" is just a thermistor (actually two of them) which changes electrical resistance based on temperature. When the thermistor reaches a certain value it signals the original Coleman-Mach thermostat that "we have a problem " and heat might not be available so be ready to call for the propane furnace. The Coleman-Mach thermostat will eventually call for propane furnace after a period of time and trying the heat pump for three or so times.

Not sure if you were the one that replaced the Coleman-Mach thermostat for the ecobee SI or not. Here is my write-up of the ecobee conversion.

The ecobee SI is highly configurable so you can set it to call for emergency heat based on whatever you choose. Five degree spread is pretty much a standard to switch to secondary heat. Please review the ecobee SI manual - you should be able to tweak it to resolve your issue. Our Horizon has been sold or I would be glad to play with mine for experimentation.

Couple of other things if you are a bit of a hacker, you can add a simple dumb thermostat to directly call for propane heat which is what I did in our Horizon. You can call for propane heat by going deep into the ecobee menu - I think they give a warning that you can blow up the Universe by messing around in the installer menu.

 

[sgkane]

Thanks John. Yes I'm using the Ecobee and used your write up to do the install. I'm going to try the dumb thermostat option. I'm guessing I can wire it parallel with the Ecobee and just turn off the ecobee when we are in that situation?

 

[John Canfield]

Yes, just hook up the dumb thermostat to the gas furnace leads and set the ecobee to off.

 

[sgkane]

Thanks John. The Dumb Thermostat works great.

 

[John Canfield]

Very good, thanks for the follow-up!