Calculate heat output and COP for a system with a counterflow plate heat exchanger between the heat pump and the radiator system. Calculation based on effectiveness NTU method [1].
Heat pump side (Primary)  Radiator side (Secondary)  

Flow rate 
L/min

L/min

Heat capacity 
J/kg.K

J/kg.K

Flow temperature 
°C

°C

Return temperature 
°C

°C

Mean water temperature 
°C

°C

Comparison with opencircuit system without a heat exchanger:
[1] Effectiveness NTU method, implementation here ported from python CalebBell/ht library.
[2] 15000 W @ 50K = 7x 1200x600mm K2 double panel radiators, each with an output of 2145W @ 50K.
[3] Carnot COP equation based on +4K offset on flow temperature, 6K offset on outside temperature and a practical efficiency factor of 50%.