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A conundrum…

cathodeRay · 2023-03-07T19:25:31Z

This has had me baffled for a while. I have been logging weekly whole house energy use now I have a heat pump, to compare it with past non-heat pump use. I'm using total house energy use because that is the simplest like for like comparison. There are three relevant periods: Oct 2012 - Oct 2018: oil central heating, (mostly) immersion heater for DHW. House generally comfy throughout Oct 2018- Feb 2022: standard mains electricity, ie one or two room Argos type fan and convector heaters (oil tank on the blink). House freezing cold Feb 2022 to date: air source heat pump. House generally comfy throughout The data comes from: (1) for oil period (2012 - 2018) weekly use, annual oil usage distributed over a 30 week heating period in the same proportions as found in (2) below plus average weekly electricity use. I converted litres burnt to kWh burnt by multiplying litres by 10.34. (2) for standard mains electricity period (2018 - 2022) averaged weekly recorded whole house meter readings (3) for heat pump period (2022 to date) weekly recorded whole house meter readings oer the current heating season The values are therefore total house energy use per week, based on averages for pre-heat pump data, single week data for heat pump data. I then plotted the data: This shows that my total house energy use (in kWh, not GBP, that's a very different story) was generally higher using oil than it was during the standard mains electricity period and the current heat pump period. So far so good, but here's the conundrum. Let's convert the energy used to energy produced - ie energy actually delivered to the house - using reasonable efficiencies of 80% for oil, 100% for standard mains electric heaters and 320% for the heat pump: How can can it be that the heat pump needs to deliver vastly more kWh to the house to achieve broadly the same levels of heating and comfort that I got before with oil CH? Put another way, in effect, a heat pump doubles and trebles my heat loss. The raw data is pretty robust: they are simple records of oil consumed (based on tank refill dates and volumes) and metered kWh of electricity. I've said this before, either I am being very thick, or there is something seriously astray with the numbers...

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Air Source Heat Pumps (ASHPs)

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cathodeRay

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Topic starter 10/03/2023 11:40 am

Posted by: @batalto

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I believe our annual kw/m2 is quite good in the overall scheme of things

I'm sure that is right. Presumably your recent large extension is well insulated, and you can enjoy the benefits of that.

Posted by: @iaack

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One question if I may, where was your old oil boiler located; within you home or in an outbuilding?

Inside. That made it itself an internal heat emitter, but it also sucked in cold air for combustion, so what the net heat contribution was after allowing for both factors is unclear.

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

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Derek M

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10/03/2023 12:17 pm

Posted by: @batalto

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@cathoderay I believe our annual kw/m2 is quite good in the overall scheme of things

Things do stay the same. My wife still hates the cold. 🙄

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IaAck

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10/03/2023 12:28 pm

You suggested a figure of 80% for the efficiency of you oil boiler for converting the fuel to hot water, where does the other 20% of the energy goes, not all up the flue surely. I would bet another 4 to 6% goes to heating the surrounding area. I would also suggest that 4 to 6% of the heat produced by your heat pump and external pipework is also lost to its surroundings, however as its running 24 hours a day, that is not insignificant. Additionally, the heat lost during defrost cycles needs to be accounted for.

Not switching off the heating at night will mean the building fabric will continue to loose heat at a higher rate and in one direction (outwards) whereas if you switch your heating off the overall heat lost is reduced as heat will now also flow from the building fabric back into your home, the building fabric will cool, the delta t with the outside will reduce and the heat lost to the surroundings reduces, thus improving overall running costs. I don't think its so much of a conundrum, just a few things that all add up to be significant at the end of the day.

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batalto

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10/03/2023 12:49 pm

@iaack my boiler actually told me the efficiency on the screen - it was 68% and it was a new boiler (only a few years old). Boiler efficiency stats are lies in my view. The packaging claimed over 90%

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE

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cathodeRay

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Topic starter 10/03/2023 1:04 pm

Posted by: @iaack

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I don't think its so much of a conundrum, just a few things that all add up to be significant at the end of the day.

It still was a conundrum when I wrote the original post, but now we may be getting nearer to a solution, which was the whole point of writing the post!

You may well be right (that just a few things do add up), but it is quite a big difference, we are mostly looking 100%+ differences, not just a few percent here or there. Assuming very level conditions, which of course usually don't happen, 24 hour running might be expected to use 50% more total energy than running with an 8 hour overnight setback. But it's complicated by any boost added after the setback, needed to achieve the recovery after the setback. But 50% extra is not the same as 100%+ extra.

The night setback or rather the lack of it because the heat pump can't recover in a reasonable time is also a running sore which has come up many times. No one appears to be absolutely sure, but in my own mind I have persuaded myself that a boost after a setback will always use less total energy than keeping the heating on 24 hours a day, but there is no easy way to set up that boost on a Midea unit, so I haven't been able to test my theory.

@batalto - indeed. Instead of it being the snake oil salesmen at work, it is the snake gas salesmen at work, and we all know snake gas is even worse than snake oil.

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

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IaAck

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10/03/2023 1:04 pm

@batalto was your boiler located in the middle of your garden, or in your kitchen, utility room or loft. Your missing my point, whatever the boiler efficiency is, its about coversion of fuel energy to hot water + lost energy to surroundings. Where the surroundings are does matter in these circumstances.

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