I can see that CoP chasing could become addictive haha
Ahhhh, I’m beginning to understand the room stat behaviour now…
It (or specifically, the programmer, but considering the room stat and programmer as a system) implements TPI - a crude way of reducing average flow temps for fossil fuel boilers and eliminating room temp hysteresis! The stat has a small (1.5 - 3.0K on this Honeywell kit) ‘proportional band’ where it will intentionally cycle the boiler on and off to get an average duty cycle sufficient to keep the room temp at the setpoint. So this on-off cycling three times an hour is intentional (although perhaps undesirable) behaviour.
The TPI system learns the response characteristics of the heating/building system, and adjusts the boiler duty cycle to keep to the fixed setpoint, where-as a ‘perfect’ controller would instead modulate the actual heat output to match. Of course boilers and heat pumps can’t modulate down all the way, and there are other real-world issues that get in the way…
Our Grant ASHP only accepts on/off control from the stat/programmer, and it doesn’t have a funky system like yours where we could adjust target flow temps on the fly (I’m not jealous, honest…) so I think we’re either stuck with TPI, a basic stat, or time control only.
However, I think if we get the weather compensation as close as possible to ‘perfect’, then the house will need ~100% of the heat pump output all the time. (I appreciate this will simply shift from thermostat cycling to the heatpump cycling its compressor, but as it drives its compressor with an inverter, this will be better - the inverter goes from 10Hz min to 70Hz max). Of course, in the real world, we’ll want to give ourselves some margin (lest we become unable to maintain room temps due to excessively low flow temps), but at least the TPI will ‘learn’ this and perhaps become more useful…
Here’s current behaviour: (I must admit, I adjusted the weather curve to lower temps already since yesterday)
Red is OAT, blue is CH demand on/off, yellow is heatpump power consumption. Overnight, while OAT was low, TPI was resulting in a duty cycle of ~25% (It’s currently set to 3 cycles per hour (20mins), and 5mins minimum run time - a minimum duty cycle of 25%, although I suspect any call for heat at all will result in that 5mins of run time. I set these settings, trying to reduce cycling… they were at defaults - 6 cycles per hour, 1min minimum on time).
As OAT increased, TPI duty cycle remains at ~25%, but the power consumption shows that the heatpump doesn’t run the compressor every time (this power consumption is as reported by the heatpump, so not necessarily reliable, and only to the nearest 100W, but it seems reasonably good. It shows 100W when the circulating pump is running, although I don’t imagine its actually consuming that much), indicating that the water temps aren’t cool enough to warrant the heat input. This is good - the heatpump is reducing its output based on increasing OAT, but not good enough - TPI is still only calling for its minimum heat. Reducing flow temps further will result in lower output, and at some point I’ll see the TPI increasing its duty cycle to compensate, meaning the heatpump will be closer matched to the building, and will see longer run times which should make it happier.
That all makes sense to me… can you see any glaring errors or misunderstandings?