I had very helpful advice from several forum members back in January, however still experience heat pump short cycling, the background is as follows.

1. 2024 new build house in central Scotland. 260 sqm floorspace, well insulated and triple glazed with 36 sqm of south facing glass, so lots of solar gain to contend with. Heat loss of 8.6kW at -4 degrees from the SAP design calculations, which I now believe may be overstated by 1.5 to 2 kW after I adjust for the very large ventilation losses that were assumed.
2. Grant Aerona 17R32 heat pump (nominal 17kW, but I believe a bit lower in practice), Grant LLH, UFH to ground floor with seven zones, 4 rads. to first floor, electric UFH to bathrooms. The heating system was installed by my builder who subcontracted the heat pump installation, connection and commissioning to a local Grant registered engineer. In my opinion the commissioning was poor and left setup as for a conventional boiler. (45 deg. C flow temp to rads, 30 deg. C to UFH, whole system on/off three times per day)
3. Our first experience of cold weather operation was in December 2024 when high electricity consumption started my self-education journey into heat pump setup and optimisation. I found that the return to the HP had no flow regulator fitted and with advice from the forum, decided this needed correcting as a key first step to improving control of the system. This was also confirmed by Grant technical support.
4. The Grant installation engineer said the flow regulator "would not make any difference as they all cycle on and off anyway” and after waiting three months and him still not doing the work my builder used an alternative (Daikin) engineer to fit it in May 2025. By then the weather was not ideal for testing, but with the flow regulated to around 18 L/min the HP modulated down to 600W at a fixed flow temp. of 31 degrees and ran for a two-hour test without cycling at OAT 10 degrees. After several similar results I was pleased and waited for winter to start setting the weather compensation.

In spite of this good result, by this time I had learnt that my HP is probably significantly oversized, and that the design of my heating system is not really to best practice. But without significant changes I will probably need to accept some compromises in its operation.

My (compromise) thinking and initial target is to run at 29-33 deg. C flow temp. controlled using w/comp. for the radiators as that seems to be sufficient to maintain first floor temperature, and blended down to say 27 deg. C for the UFH. If possible, with room stats set at high level, just to protect against overheating when we have winter solar gain.

With consistently low temperatures in the last week, I have run more tests with the following observations. (I disabled W/Comp to avoid any variable flow temp. targets during the tests).

1. At OAT of 6 deg. C and above the HP modulates down to 600W at my 31 deg. C flow setpoint and will run continuously without cycling, (as in May 2025 tests) and even with only half of the zones calling for heat.
2. At below 5 deg. C and all zones calling for heat the HP short cycles, best cases after 12-15 minutes, worst cases after only 6-7 minutes. I have the same result at 0 deg. OAT
3. On one test at 13:00 hrs, 4 deg. OAT in the shade, the HP in sunshine with its thermometer reading 7 deg. OAT it modulated down to 600W and ran without cycling.

I am surprised (and confused!) by these results as they appear to go against what I have understood about short cycling in warmer v colder temperatures.

I understand that at lower OAT a HP will consume more energy to achieve a given flow temperature, and that I am probably trying to run the heat pump at the bottom end of its operating window. But if my HP will modulate down to 600W at 10 deg. OAT, why will it not modulate below 1500W at 5 deg. C or lower? It continues to run at 1500-1800W, may finally modulate to 1200W for around 30 seconds but the flow temp. rises 2-3 degrees above the set point within a few minutes and it then stops.

It’s as though the control system is saying “it’s really cold now, so work extra hard and never drop below 1500W”. I can see no parameters in the Chofu manual that would cause this behaviour.

My thoughts are to take this low OAT short cycling issue to Grant technical support, but I would value some guidance from the forum before starting that discussion.

My questions are:

1. Please could anyone explain what may be causing short cycling when OAT drops and if there might be a solution?
2. Please could anyone with a Grant HP please share if they have experienced similar? (Particularly if the HP is oversized).
3. Are my thoughts on flow temps. for the radiators and UFH reasonable or might there be a better way?                                             For example, would it be feasible / better to run all the UFH zones closer to an “open loop” at the same slightly higher temp. as the first-floor radiators (29-33 deg.) if I can get the individual room circuits better balanced/restricted to avoid overheating?