Vilks

It is complex combination of thermodynamics (Pressure vs enthalpy) of the refrigerants and mechanics of the compressor/-s and its type/amount of them. Most of that flies well above my head currently. In short, from what I understand so far is that at lower temperature compressor has to work harder to compress the refrigerant to achieve desired temperature at a higher pressure. That’s where COP comes from. Work (electricity in) vs energy you get out (heat in kW). So when minimum designed compressor operation meets with amount of energy extracted from air via refrigerant at certain temperature is your minimum output. I could be way off, so take all this with a spoonful of salt.

What is a definition of a full cycle in these circumstances? Refrigerant cycle? Refrigerant cycle minus heat loss? What controls the heat pump to come on? What controls the delay between compressor start-ups? No manufacturer clearly states these things. I made calculations for @Ewan’s numbers. Mine are closer to 12min runtimes @15C outdoors Questions. 1. I don't have TRVs on 6 out of 15 heating emitters in our house, but all turn to 5 (13 radiators, 3 towel radiators= water volume around 52.35 litres without pipework calculations) 2. Originally a two zone system, but I keep both valves open. Overall heat supply is adequately balanced (some imbalance between rooms, doors opened or closed, solar gain in half of the house. To make it perfect I would have to rip open walls, floors to replace pipes to 15mm to radiators (microbore throughout). Second best option is to install pressure independent valves on all heating emitters (there might be some limitations with minimum flow – don’t always need the min specified by Danfoss one I looked at). Third just tedious individual balancing, but then emitter order is a total guesswork and with these cheap balancing valves, there isn’t much authority (min adjustment has great change and usually starts hissing at these almost closed positions). 3. I have one downstairs, in the hallway, keeps the whole house within limits. Pointless to do much more as solar gains are unpredictable and unmeasurable. I would run Heat pump with weather comp and open widows when room gets too warm if necessary ( almost a bonus with fairly good air permeability rating of 3.6m^3/h.m^2 (to be honest, I haven't looked into how much more this impacts heat loss)).

I have been going down the same rabbit hole. Trying to find the smallest unit that can satisfy at -2C and then be efficient at warmer temperatures. I can give you what I have found. Page 92 - min output of 2.1 kW for both 3 kW and 5 kW Vaillant units. Not the most informative table but then again, I have only seen Mitsubishi have better table than this one. LG Therma V 3 kW unit - does not specify lowest output and jumps around with flow temperatures at different outdoor temperatures. You could guestimate around 1 kW (4.18 x min water flow of 0.25l/s). Although, according to Vaillant, you should spec heat pump for most of the year rather then couple of cold days when backup heating could be used and even though getting lower COP for those days, overall SCOP should be better when heat pump could run more efficiently rest of the year. Research paper talks about 10 min being the minimum runtime: “…due to the need for a minimum run time to ensure good lubrication and negative impacts on compressor reliability” (Page 62). Other users have found ways to calculate minimum system volume or buffer size from their own or other sources (I found this, but they even go as far as 20 min minimum runtimes). So, if 2.1 kW is the minimum HP output and you suspect 1 kW is your lowest demand for warm temperatures (worst case for the heat pump) then if @JamesPa formula is used. You should get 31.58 Litres of minimum system volume to prevent Heat pump from running less than 10minutes, or 63.16litres for 20minutes (at delta5).