ReedRichards

Okay, @JohnMo, you're specifically considering trying to heat up a cold floor with a huge thermal mass. But, I presume, enough thermal conductivity that all the floor remains at much the same temperature. Does concrete/screed have a high thermal conductivity? I've always assumed that it doesn't, like stone. But if it manages to extract a lot of heat from the pipes then I suppose it must. Sorry, I can't work out your picture as it has no labels.

But I don't have a big delta T, why should I? When the heat pump starts up my radiators don't give out much heat so the return water temperature is only slightly different to the flow temperature. The gap widens as the flow temperature increases but it never gets more than the about 5 C or so that the radiators are balanced at. @JohnMo, you'll have to explain the mechanism by which you could get such a large heat loss as to give you a big delta T.

You're never going to have a fixed 5.8 kW hot loss, though.

That's not at all the way my LG heat pump works; it will ramp up to the set flow temperature without caring about the return temperature. When it gets to the set temperature it will continue at a fixed pump speed or an "optimum" pump speed or it will target a specified delta T between flow and return temperatures. And it does all this at a much much better COP than the 1.5 suggested above (unless it was very very cold outside).

Hopefully no one is disputing this. Except possibly @nodwhose earlier comment displayed a lack of understanding.

Either you have a new installation with a low-temperature gas boiler. Or you replaced your rads with larger ones at some time in the past. Or the rads were massively over-specified when they were installed to work with a much higher flow temperature than the 30-40 C you now use

Well if the system was set-up to operate with third party thermostats you most probably DO need them. You might ultimately be able to disconnect them when you have found your way around the controls but it's too ambitious to do this in the first instance. My advice would be to disable use of weather compensation and set a fixed water temperature near the high end of what your heat pump is capable of. This is not the most economical way of operating your heat pump but it removes a level of complexity and potential for error and therefore should give you a better chance of telling if there is a fundamental problem or not.

Would you set the heat pump at its maximum fixed leaving water temperature to do this?

A fact which I acknowledged in my reply. I've had a heat pump for 4 years now and warm radiators can provide all the heat you need.

My Atag gas boiler was installed in the spring of 1998 and it had both Weather Compensation and Load Compensation. From memory it was a 24 kW system.

This is entirely dependent on the size (I.e. surface area) of the radiators. Make that surface area big enough and they will be fast to heat a room, even though they don't get very hot. Make that surface area big enough and they will heat a poorly insulated house perfectly well. Or if you haven't got room for a radiator with a suitably large surface area, get one that is fan-assisted.

Your room heat loss figures must apply to a specific outside temperature. What happens when the outside temperature differs from the specified value?

And my point is that you are wrong.

You are thinking of radiative energy and in the context of atoms and small molecules. But the difference between cold air and warm air is that the molecules have more kinetic energy and are moving around faster. The molecules with more kinetic energy interact with your body by taking away less extra kinetic energy when the bounce off you so you don't cool so fast, assuming that the air temperature is less than your body temperature.

One thing I have never been clear about is how the temperature drop across your radiator interacts with weather compensation. For example, if I set up a 5 C drop across the radiator when the input water temperature is 50 C, what should that be when the input water temperature is 40 C or 30 C. And presumably that differs if my heat pump is trying to maintain a fixed flow or maintain a fixed temperature differential between the leaving and return water temperatures?