memes

Heat pumps are not simple at all. They are extremely efficient but can’t produce a large temperature gradient so they need to run very long cycles (potentially remaining on 24 hours straight). Modern cold weather air source heat pumps also tend to have variable output (variable speed compressor, variable speed fan). This demands a more complicated thermostat that adjusts the heat pump up and down, possibly with PWM.

And then there’s the emergency/auxiliary heating from the furnace. The thermostat needs to have some intelligent logic to decide when the heating demand exceeds the capacity of the heat pump and call for the furnace.

If its a cheap crappy one, the compressor is on/off depending on temperature. Decent ones will have a VFD to manage the load of the compressor so it doesn't have to turn on/off all the time but just regulates the compressor load to match heating/cooling requirement. Both have their own controls, and you generally shouldn't mess with them.

They have their own microcontrollers usually to manage that stuff, including defrosting

I have a heat pump as well as a furnace (for auxiliary heating). The thermostat frustrates the hell out of me! For one thing it loses its date and time (yes it has a full calendar date and time as well as time zone) if there’s even a single second power outage. How hard would it have been to put a CR2032 battery and a diode in there just to run the clock when the power fails?

For another thing, the thermostat itself runs extremely hot. Just putting my hand on it, it feels super warm to the touch. The LCD touchscreen on the other hand has molasses-slow response time. It’s almost impossible to set the temperature on the first try without overshooting by 2 degrees.

Lastly, it is designed to be able to run both the heat pump and the furnace when heating load exceeds the capacity of the heat pump. The thermostat also has a sophisticated time of day temperature set point schedule system (with separate schedules for every day of the week). However, the damn thing does not correctly reconcile these two facts!

I have the system set for cooler temperatures at night and warmer temperatures in the day. When the morning arrives and the schedule hits the higher day time set point, the thermostat suddenly sees a multiple degree deficit vs the set point and then calls for emergency furnace heating because it thinks the heat pump is failing to meet heating demand!

This is so maddening and stupid! Why can’t I have the temperature set point just continuously and cyclically vary throughout the day and night like a sine wave? No, the dumb thing runs the heating and cooling schedule as a square wave and therefore runs the furnace every single morning in order to slam the temperature up by a few degrees to the day time set point instead of gradually ramping it up over several hours with the heat pump…