Aim for 15+ l/kW System Volume

Is that heat loss, or maximum output of the ASHP?  Understand they should be fairly close in a good system, but both vary by outside temperatures.

I think you’ll find it depends on the room heat loss. A low loss room can be heated by a much smaller difference between mean water temperature and required room temperatures with a moderate sized radiator. Of course we usually all want the smallest radiator that will work in the room. They’re not works of art despite the marketing of some designs.

Sufficient system volume is also essential for ensuring there is enough energy in the system to perform effective defrost cycles (unless there is some form of additional heater present). Without sufficient volume, constant defrost cycles will quickly kill efficiency in colder weather, and leave the house cold.

We run a standard flow temp of around 32C all year round (certainly between 30-35C) and achieve a comfortable (for us) 20C room temp. Our radiators are all of standard K2 types. Other than on the very coldest of nights, even running a flow temp of 30C overnight puts out more heat than required as we like a setback of 18C for sleeping.

As long as your emitters are large enough to dissipate the required amount of heat at reduced flow temps, very low flow temps are possible. The limiting factor for us is actually dissipating the ~3.5kW of heat produced by the heat pump when running at minimum during milder weather, hence the required 32C flow temps. Any lower and the radiators cannot dissipate that 3.5kW output flowing from the heat pump.