But why would someone heat a boiler to 15°C only?
He is speaking about boiler temp an not room temps.
Water has about 13-15°C when it enters the house, either drinking water from the street or ground water.
So there would be no need for heating it if you dont want it warmer than 15.
I realy dont understand what he is doing. 
No complaints at all – only want to know what it is good for to learn other situations.
I have a heating pump too, but there is no sense for any boiler for it, it is working with ground water 13° in and 10° out.
If you dont oversize the pump it wont need any boiler with at least 1h working cycles using underfloor heating.
If you want to heat only one room or only parts you would need a boiler, but then you should heat it up to 25°C at least for longer pump cycles.
And for warm water we use 400l boiler with 50°C enough for 4 persons.
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