Durastar Heat Pump Hysteresis
In which I discover that lying to HVAC manufacturers is an important life skill, and share a closely guarded secret: Durastar heat pumps like the DRADH24F2A / DRA1H24S2A with the DR24VINT2 24-volt control interface will infer the set point based on a 24-volt thermostat’s discrete heating and cooling calls, smoothing out the motor speed.
Modern heat pumps often use continuously variable inverters, so their compressors and fans can run at a broad variety of speeds. To support this feature, they usually ship with a “communicating thermostat” which speaks some kind of proprietary wire protocol. This protocol lets the thermostat tell the heat pump detailed information about the temperature and humidity indoors, and together they figure out a nice, constant speed to run the heat pump at. This is important because cycling a heat pump between “off” and “high speed” is noisy, inefficient, and wears it out faster.
Unfortunately, the manufacturer’s communicating thermostats are often Bad, Actually.™ They might be well-known lemons, or they don’t talk to Home Assistant. You might want to use a third-party thermostat like an Ecobee or a Honeywell. The problem is that there is no standard for communicating thermostats. Instead, general-purpose thermostats have just a few binary 24V wires. They can ask for three levels (off, low, and high) of heat pump cooling, of heating, and of auxiliary heat. There’s no way to ask for 53% or 71% heat.
Ecobee Settings for Heat Pumps with Resistive Aux Heat
I’m in the process of replacing a old radiator system with a centrally-ducted, air-source heat pump system with electric resistive backup heat. I’ve found that the default ecobee algorithm seems to behave surprisingly poorly for this system, and wanted to write up some of the settings that I’ve found yield better behavior.
A disclaimer. I’m not an HVAC professional. I have two decades in software operations, a background in physics, and far too much experience inferring system dynamics from timeseries graphs. This advice may void your warranty, burn your house down, etc.; everything you do is at your own risk.