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by cheradenine_uk 1060 days ago
Air-Air should be more popular in the UK - modern units have very attractive COP (gain) - nearly 5.0 - without complexities of right-sizing pipework and emitters.

I think it's a tragedy that they aren't more popular. One of the reasons is that they don't qualify for installation grants. This in turn means the installers (ahem, grant-harvesters) are bad-mouthing them vs air-water to potential customers.
1 comments
the_third_wave 1060 days ago
From what I gather those high COPs are not actually met in real life since they occur when the temperature difference between intake and outlet are too low to be useful. In practice the SPF (Seasonal Performance Factor, the average COP for a heat pump used for heating and hot water through all seasons) for air-source heat pumps in Germany lies around 3 (value given is 2.99), water-source heat pumps end up around 3.7 (value given is 3.72) [1] which makes them preferable over resistance heating but not necessarily economically effective given the discrepancy between electricity and gas prices and higher installation costs. These results are from 2009, it is likely that more recent heat pumps can achieve better conversion factors.

[1] https://wiki.ucl.ac.uk/download/attachments/11962823/staffel...

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cheradenine_uk 1060 days ago
I'm talking about the emitter rather than the source. (ductless) Air/Air is relatively unused in the UK (except for aircon) for the (stupid) reasons described, work perfectly well even in very cold climates) and offer extremely good COP values (4.68@ 8C, 2.46@ -8C) which is likely better than retro-fit of Air/Water, given the difficulty of right-sizing such a scheme in an existing, older building. If the government really wants uptake of heat pumps, they should be pushing for these to be installed in flats (like they do in most of the rest of the world).

As you say (and the original article points out) - whether it's economically a good idea depends a lot on the electric:gas price ratio, which currently is around 3.2, so the COP needs to exceed 2.95 or so for the OPEX to be lower (let alone the CAPEX of having the thing installed in the first place). And historically the gap has been even larger - nearly 5:1, meaning the gain would have to be 4.56 even to break even - "somewhat ambitious"!

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