Real world running cost estimate for 20KW ASHP

Thebarn · 2024-03-07T09:20:08Z

Hello,I have recently been quoted for an ASHP and I am wondering if anyone has a similar set up and what the real running costs for it are. My property is 315sqm with underfloor heating upstairs and downstairs. I have been quoted a Warmflow Zeno 20kw AS03. The calcs predict a demand of 13.22kw, so an ASHP of 20kw has been proposed with 300 litre hot water and buffer tank. The house is well insulated but is a T shape, so many rooms have 3 external walls.I am currently on oil using about 3200 litres per year costing circa £2.1k per year. Some of my own calcs for running based on the quote are coming in at £5.5k a year to run!! Quoted heating demand of 54,156 kw. So with a COP of say 3.5 that would be 15,473 kw needed in electric, at £0.35 per kw, would be £5,415 per year to run? Is this correct or do I have my numbers in a muddle? Does anyone else have a similar property with a 20kw ASHP and what are your running costs? Many thanks in advance.

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Air Source Heat Pumps (ASHPs)

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cathodeRay

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08/03/2024 8:32 pm

Posted by: @thebarn

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Yes, it is very confusing to have been quoted 11kw, (16kw when I looked into it a few years ago), then 18kw and then 20kw! Go in too low and be cold and too high and be wasting electricity and money.

The ideal is to go for for something that just exceeds your heat loss at design temp (-2 or thereabouts depending on where you are) when running at those outside air temperatures. Any more, and you will get more cycling at warmer weather, as the heat pump can't reduce it's output below a certain level, and less and you will be likely to feel cold when it is very cold outside. That is what I ended up with: nominal 14kW unit, but only coughs up a little over 11kW at around zero outside, estimated heat loss 12.3 kW, result is actual indoor air temp falls a bit below desired indoor air temp during cold weather.

As others have said, heat loss calculations are not an exact science, and in some cases I suspect involve the use of random numbers. But a tolerably thorough assessment should usually give you a reasonable ball park figure. That said, there has been a lot of debate in other threads about empirical measurements (albeit rather crude, because of the surfeit of variables involved), and how these often do not agree with the heat loss calculator estimates.

Midea 14kW (for now...) ASHP heating both building and DHW

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