@jelockwood The air-source heat pumps do modulate their output, but I don’t think they use OpenTherm. My installer fitted a very simple call-for-heat thermostat. It has some delays added so it doesn’t just flick on and off when it’s hovering around the right temperature.
However, I found (with my too-big heat pump) that it was coming on and off quite often because the heat pump couldn’t modulate low enough to provide just a little bit of heat. It’s not helped by my radiators which can release some heat but aren’t as good as underfloor heating. I suspect if you get a sensible-sized heat pump it would be OK. I see people like Trystan showing his modulating down and running constantly. I prefer to turn mine off, but that’s overkill.
The heat pump is trying to maintain a difference between the flow and the return. With an Ecodan I believe it stops itself when the return gets within 3 Celsius of the flow.
So I suspect you’ll find it doesn’t need OpenTherm because it’s modulating based on the flow temperature.
In my case I actually don’t even have any room stats. Each room has a local TRV and that shuts the radiator down when that room gets too warm. Eventually most of the radiators shut down and the heat pump decides it’s not worth carrying on and stops. My system turns the heat pump back on again when the temp in the pipes gets “low” according to a stupidly complex algorithm I wrote which takes account of wind-speed, solar gain, humidity, time of day, time of year and more.
For extra kicks, my system tinkers with the “target flow temp” to try and optimise how the heat pump works to improve the CoP. It’s still not as fine-grained control as ones like Trystan’s though.
You can see in this screenshot that my system is gently increasing the “Desired” space heating (the blue line) from 30 Celsius. This stops the heat pump trying really hard which is does if I start the cycle with the desired temperature that the high point of 38 Celsius. It’s getting me a few percent improvement on my CoP, but even if yours starts at 38 Celsius it’ll be able to modulate better than mine so it won’t race up to the higher temps.
