Low flow temperature radiators

Reed_Richards

There is a maximum theoretical COP at which a heat pump can operate.  This would be determined by the leaving water temperature (measured in Kelvin) divided by the difference between that temperature and the cold temperature from which the heat is being pumped.

Lets suppose that the  leaving water temperature is 50 C, that's 323.15 K.  And let's suppose it's 0 C outside so the temperature difference is 50 C = 50 K.  So the maximum possible COP a perfect heat pump could achieve at those temperature would be 6.463.

Now suppose the same perfect heat pump takes the leaving water up to 70 C =343.15 K.   The temperature difference is now 70 C = 70 K.  The maximum possible COP now is 4.902.   So it is going to cost you 31.8% more to run your perfect 70 C HT heat pump that a perfect 50 C heat pump.  And because you have kept your old radiators and these need to be run hotter throughout the year the HT system never catches up.  

mnbvcxz

I don't know much about radiators but possibly consider,

the size of pipes running to the radiators. They may be too small to deliver the higher flow of lower temperature water. If they are cemented into the floor then you are pretty much into an entirely new central heating system which would be best to put off... This is a big reason why most people can't just cheaply attach a heat pump to their existing central heating system.

Radiators sized for different temperatures on one system might lead to interesting balancing issues, possibly adjusting the trvs would compensate but maybe not.

Yes you can adjust a radiator for lower temperatures by just buying it bigger and or with more walls. You can also get them made out of aluminium and other materials more efficient than steel. There are also expensive ones designed purely to be run at a low temperatures with built in electronics and mechanical fans to increase ventilation. They are generally sold to people along with low temperature heat pumps.

So the theory is good, and if you were putting in central heating from scratch using large pipes and sizing the system to run at low temperatures in anticipation of a future heat pump would be a reasonable idea. But retrofitting a bit at a time sounds tricky. Probably better to save the money and see what available in ten years or so. Perhaps we will all be buying cheap laminate flooring with built in heating pipes to form giant underfloor radiator on the cheap. Who knows.

waqasahmed

1961Nick said:

Even with a COP of 3, a HT heat pump would be at parity with the current cost of gas heating. A TOU tariff could make it cheaper to run than a gas boiler?

A modulating HT heat pump with weather compensation & a sophisticated control system would achieve a COP in excess of 4 most of the time.

Regarding cost, the domestic ASHP boiler is still very much a cottage industry & has a long way to go to see the benefits of scale. Achieving a flow temperature capability of 60C, would enable it to be plumbed into an existing wet system without any costly property modifications.

Sorry. Cottage industry? 

QrizB

waqasahmed said:

Regarding cost, the domestic ASHP boiler is still very much a cottage industry ...

Sorry. Cottage industry? 

Definition here.

Historically, a business run from home by family members. The pre-industrial textile industry, for example.

The term is applied more widely these days to refer to a sector that's dominated by small businesses. Domestic solar PV and ASHP fit this definition.