How to undersize a heating system.
Graham Hendra
Heat pump product development engineer. Author of heat pump text books. Heat pump builder.
This week I was called by a fella who had a 200m^2 new build 2023 house with a 4 kW Mitsi CO2 heat pump. He was cold, the run costs were huge he was unhappy.
If we assumed that the heat loss was correct at 4000 Watts and the house is 200m^" the heat loss is 20 Watts a metre squared. I've done a few thousand heat losses in my time and I have never seen any house with a heat loss that low, even with heat recovery that's a pipe dream.
The heat pump is definitely undersized, if its 0 C outside they cant get the temperature in the house about 17C inside in any room, even when running it 24/7. It doesn't help that the unit comes with a built in hot water cylinder of 200 litres and the poor unit is trying to do hot water as well.
The heat pump could heat the cylinder from cold to 50C in 129 minutes. A 200 litre tank is too small for a 200m^2 house so it has to do 2 hot water cycles a day. That's 4 hours or 1/6 of the day just trying to heat the hot water. When you size a heating system you have to allow time for hot water on top of the heating demand. If it spends 4 hours a day in hot water mode its only got 20 hours to heat the house.
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So guess what the unit does? it turns on the immersion heater to help. In this case the immersion heater runs constantly if its below a couple of degrees C. And it runs every day when the homeowner takes a shower, even in this weather (its 16C today) So the run cost is huge because they dont have a heat pump, the have a direct electric heating system. The worlds most expensive kettle.
Funnily enough the customer hates heat pumps, but he should really hate the designer or the idiot who guessed the size of unit required, oh and the installer.
There is nothing wrong with a heat pump in a 200m^2 new build house, in fact there are thousands of these sort of installations working brilliantly across the UK and Europe, they just work way better if its the right size.
Back in the bad old days when none of knew what we were doing this sort of thing was quite common, I though we were over this now. I've never understood why people take on jobs they dont have a clue about. Odd behaviour.
Energy Everywhere - facilitating use of environmental or discarded heat. Product development, entrepreneurial aspirations, embracing innovative technological solutions to fix old problems.
11 个月That QUHZ CO2 heat pump is also rubbish at doing low temperature heating. It only operates effectively when the return temperature is cold, hence why it's SO good at doing hot water with a stratified cylinder. I had one of those systems and I closely monitored its operation. And yes, not only the wrong heat pump but massively undersized. There's plenty good sizing guidance and tools, so it's unforgivable to get it wrong.
Senior Finance | financial modelling | PFI/PPP specialist | SME's & Entrepreneur support for business growth | non-exec
11 个月As a consumer, with a Heat Pump fitted a few years ago now, it's really refreshing to find someone in the trade calling out bad design and/or poor workmanship. We had an absolute nightmare with our installer, who badly installed the system, flooding my house and damaging newly fitted oak doors and door frames in the process, failed to commission the system correctly, installed the wrong type of shower (electric shower!! just why!!), the list goes on and on. Thankfully, we have now found an excellent company who undertakes our servicing and will do any future replacement work too, but it is really concerning how many "installers" hold MCS accreditation and yet are unable to meet the minimum standards required for the work.
EVs │ Sustainability │Business Development │ Partnerships │ Sales Enablement │ ? All opinions are my own ?
11 个月I'm not a big fan of too much regulation, but surely there is a standard, app/spreadsheet, approvals process and sign-off process for low carbon heating in new builds?
Principal of Twinn Sustainability Innovation - for a built environment that do not cost the Earth
11 个月So I'm thinking that the fella's undersized heat pump is because something else has been cocked up and needs checking (controls, balancing, rads, fabric not to Part L, etc). I note it is a CO2 heat pump and hence very high temperature type, which probably means different temperature difference across radiators, quick hot water reheat, etc. So fertile for someone overlooking something.... Keep up the excellent discussions pieces Graham.
Principal of Twinn Sustainability Innovation - for a built environment that do not cost the Earth
11 个月Good stuff Graham, however we must be careful of catch-all statments like "done a few thousand homes" thus it must be wrong. A new Passivhaus home has a heat loss target of 10 W/m2 and most are then measured at this (unfortunately we in the UK do not actually measure most homes, we just do uncorroborated predictions). A new Part L home could well be 20 W/m2 (x2 Passivhaus). The average UK home size is 85m2 (England House Survey) and new-build average 100m2. Hence it is quite possible to have an average UK new home to have a heat loss that is less than 2kW, and a 200m2 at 4kW or less! Also good point on hot water reheat. But, the presumption on bigger than 200 litres storage is a legacy of header tank poor pressure days. Having switched over to pressurised hot water as the Europeans have been doing for years, we can likewise be switching to high performance low-flow showers (https://www.europeanwaterlabel.eu/thelabel.asp) at ￡50 a time, with a reduced stored volume and reheat by as much as a third - saving our clients costs and energy bills.