Probably worth noting from Mike's comments above that it's not just the COP that falls with large system temperature differentials - it's also the output kW to the system - to the point that it's basically running as a direct acting electric heater.


Regards


OMS

Dear mapj1, 

I won't get into an argument with you.  

I wrote down and said again: Our world is made up of a lot of evolving engineering technology and we are the engineers of a time race that says with you don't know physics built from math base then unfortunately you seem out of the question. Everything you listed in the post is beautiful but there is no scientific proof of what you wrote down.

Isn't that the same thing? I'm also wondering if even the best installations really can achive much above performaing as a direct heater in very cold UK winter conditions, irrespective of what the manufacturers might claim, or am I wrong?

Well, if you read the manufacturers data correctly, then all is revealed - COP and rated output aren't strictly the same thing, no.


However, most of it is wrapped up in standardised test methods that assume far more benign winter conditions that the UK experiences - couple that with a lot of UK heating systems that can't operate correctly on low flow and return temperatures and quite close differences between flow and return so as to render the heatpump unsuitable to meet the claims in the sales leaflet.


Typically HP performance data is given for an external temperature of +7C and a LTHW temperature of 45C flow and 40C return - so a mean water temperature of 42.5C against a room temperature of 20.5C - ie a delta T to the space of 22C. A typical radiator has a design output with a delta T to the room of 50C by comparison.


The last one I did for friend based on UFH was designed for a flow temperature of 28C and a return of 23C to his UFH which controls on slab temperature of circa 21C - ie when the room is warmer than 21C, basically the slab can't deliver heat (as we obey the laws of thermodynamics). The system works well with an ASHP and about 3000litres of buffer store (which is actually a large circular tube up the centre of his spiral staircase)


Regards


OMS