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.

So! How does the heat pump map these three discrete levels to continuously variable motor speeds? Does it use a bang-bang controller which jumps between, say, 30% and 100% intensity on calls for low and high heat, respectively? Or does it perform some sort of temporal smoothing, or try to guess the desire set point based on recently observed behavior?

How the heat pump interprets 24V signals is often hinted at in the heat pump’s manual. Lennox’s manuals, for instance, describe a sort of induced hysteresis mechanism where the heat pump ramps up gradually over time, rather than jumping to maximum. However, Durastar omits this information from their manuals. My HVAC contractor was also confused about this. After weeks of frustration, I tried to reach out to the manufacturer directly, and remembered that heat pump manufacturers are like paranoid wizards who refuse to disclose information about their products to everyday people. Only licensed HVAC professionals can speak to them. I wasted so, so much time on this, and have two secrets to share.

First: “licensed HVAC contractor” is not a real requirement. Many states have no licensing program, so you are just as licensed as anyone else in, say, rural Indiana. The trick that folks in construction use is to simply lie and tell them you’re an HVAC installer. As a midwesterner I do not like this, but it is apparently the only way to get things done. Durastar’s contractor support number is 877-616-2885.

Second: I talked to an actual Durastar engineer who immediately understood the question and why it was important. He explained that they use the thermistor on the air handler’s inlet as a proxy for indoor temperature, and learn the set point by tracking the 24V thermostat’s calls for heating over time. As long as the thermostat maintains a stable set point, the heat pump can run at a nice intermediate rate, trying to keep the indoor temperature close to—but not reaching—the inferred set point. That way the thermostat never stops calling for stage 1 heating/cooling, and the heat pump avoids short-cycling.

Finally, if the industry could please get its act together and make a standard protocol for communicating thermostats, we could all be free of this nonsense. I believe in you.