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Pete W.:
Hi there, Ade,

May I offer a few observations regarding heat pumps.  Please forgive me if I'm 'teaching Grannie to suck eggs'!

The figure of merit for a heat pump is called the 'Coefficient of Performance' ('CoP').  CoP equates to the ratio of the heat you move divided by the energy your pump needs to achieve the moving, so big is better.  It's the absolute temperature (i.e. degrees Kelvin) at the heat source divided by the difference between the source and output temperatures.  (It might be the output temperature rather than the source temperature, I forget.  It doesn't alter what I'm going to say next because, with absolute zero being -273° C, they're almost the same thing!)

The point is, to achieve a high CoP you need to keep the difference between source and output temperatures small.

So, if you're using the heat pump for heating, you need to prevent/avoid the heat gathering system from icing up.  Ice is a very poor conductor of heat so the pump has to lower the temperature of the gathering system to 'suck' the heat in through the ice.  The temperature that is actually effective in the CoP calculation is the temperature at the coldest point, if that falls, the operating differential gets correspondingly bigger, reducing the CoP.

In my view, that means steer clear of air source systems and give preference to ground-water or running water sources.  Put enough tubing in the heat gathering system to avoid the source water freezing.  Can you chuck a couple of hundred feet or so of copper pipe into the Mersey?   

beeshed:
watched a bit of the video - nice lathe in the background at one point. Could only tolerate a few seconds of the verbal - seemed like he thought heat bumps had only just been invented. As with solar water heating, subsidised PV etc the thing to look at is not COP stuff but reliability and lifetime. Although it seems like you are saving money on  heat at a fraction of the cost of the electricity you put in you need to work out cost of any mandatory maintenance, planning permission, service charges and how long it will last. Suddenly the savings plummet.

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