Posted by: @technogeek

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My current heating setup is a Samsung 12Kw High Temperature Quiet heat pump fitted in parallel with a Grant Votex Blue flame oil boiler, all controlled using a Honeywell Resideo TR6_HW smart thermostat.

As the HP was fitted in June of this year I have been going through the process of fine tuning the weather compensation control, now that the outside environment is getting to proper winter temperatures and through testing, finding out the capabilities of my old 1970's / 80's rads, as these were an unknown in the initial calculations.

Following the HeatGeeks procedure and using little Govee thermometers to verify the temperatures in various parts of the house, I was confident that the system was now functioning optimally, house reaching 21C, Good Lady happy until... I reintroduced the smart thermostat into the control loop... Good Lady no longer as happy due to feeling colder than before.

Hmmm I thought, so I placed one of my Govee thermometers right next to the thermostat to monitor what was going on and have now learnt something new!
Smart thermostats, from what I know, have algorithms in their programming to avoid temperature overshoot which is a waste of energy when these devices are being used with fossil fuel systems. As fossil fuel systems push a large amount of heat energy into the building during each cycle, the thermostat compensates for this (in my case) by shortening the cycle time the closer the room temperature gets to the target temperature.
So the result is, when the room temperature reaches approximately 1C to 1.5C within range of the target temperature, the thermostat changes its behaviour by reducing the cycle time.

Now this has the effect on my oil boiler of slowing the heating rate to 0.5C / hour for the final 3 hours of heating, meaning the room finally gets to 21C within 30 minutes of going to bed, so I have now adjusted the start time to enable most of the evening to be 21C.
However as we know HP's are not so great at producing large quantities of heat quickly per cycle and need to run for longer times per cycle to achieve the required flow temperatures. This was not so in my case, the thermostat throttles back the HP to the point the flow temperatures only get within 4 to 5 degrees of the required temperature on each cycle, resulting in less heat to the room and a cold Wife.

Two options were open to me at this point, increase the weather compensation curve to hopefully cater for this but completely ruin the COP or make the thermostat think its colder than it is. Opting for option 2 I adjusted the temperature offset to -1C to see how that helped which I am pleased to say, as a temporary work around, has the desired effect. The thermostat keeps the longer cycle times until the room gets to within 0.5C before slowing the rate meaning the final stage now only takes 2 hours not the 6 hours I was experiencing with the HP.

This leaves me with the question, are current smart thermostats on the market actually suitable for HP's? Is it time to seriously consider a control system such as the Homely?

Or you could just use the controller that was designed to control the heat pump. Simples. 😎 

@derek-m I wish it was! 🤣 Sadly due to the HP and controller being located within the outside boiler room, running the data cable for the built in thermostat to the lounge was not an option plus the Resideo is still needed when I use the oil boiler 🙁

@Technogeek  I've recently been attempting to characterise my gas-fired central heating and radiators prior to a possible transition to an ASHP and stumbled across the same problem with my 'smart' thermostat. I had turned down the boiler output - which has control algorithms of its own - removed the overnight temperature setback and found we were variously throwing on extra clothes or ripping them off once the outside temperature started going low.  The situation varied from room to room, probably not helped by unbalanced radiators and the odd 30 year old TRV deciding to stick again. I suppose it may help convince my good lady that we really do need to replace what we have 😉 .

@derek-m Admittedly, we have only had the Homely smart controller for a few weeks but it seems to have settled down now and is keeping the house at the temperatures we require at different times - without any problems; the radiators seem to be warm most of the time now and our consumption now when the temp. has dropped to 1 - 3 degrees most days is lower than when we were at 8 - 9 degrees and using the Heatmiser Neo on/off stat. I had spent some time carefully tweeking the LWT and WC settings on our Daikin EDL08 hp to get the best out of the system - but Homely has improved on this considerably! Well worth considering I think. Regards, Toodles.