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World’s Most Expensive Kettle: Undersized Heat Pump

Air Source Heat Pumps (ASHPs)

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Graham Hendra

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Topic starter 07/04/2024 7:57 pm

This week, I was contacted by someone living in a 200m^2 new build house, equipped with a 4 kW Mitsubishi CO2 heat pump. Despite the advanced setup, he was uncomfortably cold, the running costs were sky-high and he was understandably not happy.

Assuming the house's heat loss at 4,000 Watts and given its size, we're talking about a heat loss of 20 Watts per square meter. In my extensive experience with heat loss calculations, I've seldom come across a figure that low—it's almost unheard of. Clearly, the heat pump is too small for the job.

With outdoor temperatures at 0°C, they struggle to warm the house above 17°C, even with the system running non-stop. Complicating matters, the heat pump is burdened with heating a built-in 200-litre hot water cylinder, an inadequate capacity for a house this size.

Consequently, it must complete two hot water heating cycles daily, dedicating around four hours—or one-sixth of the day—solely to this task. Factoring in hot water heating alongside space heating stretches the system beyond its limits.

To compensate, the unit often relies on the immersion heater for assistance, which, during colder days or when the homeowner showers, operates continuously. This reliance inflates the running costs significantly, effectively turning their heating system into the world's most expensive kettle. Ironically, while the homeowner is frustrated with heat pumps, the real issue lies with the poor design and the inaccurate sizing of the system by the designer or installer.

It's important to note that there's inherently nothing wrong with using a heat pump in a 200m^2 new build; countless successful installations across the UK and Europe attest to this. However, these systems shine when correctly sized. The missteps seen here harken back to an era when industry knowledge was more hit or miss.

One would hope such issues are behind us, yet it's baffling how often people undertake tasks without fully understanding the requirements. It's an odd and regrettable practice.

This topic was modified 4 weeks ago by Mars

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Mars

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07/04/2024 8:03 pm

It’s ridiculous how much of this is still going on and how installers are getting away with it. It’s bordering on criminal.

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JamesPa

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07/04/2024 8:16 pm

I think the passivhaus standard is 10W/sq m and, if I'm right, 20W/sq m isn't impossible.

I'm not for a moment suggesting the design is right, if the house is cold then obviously it isn't, but if it were only 10kW/sq m and had a sufficiently large dhw tank 4kW might be about right I would think, unless I'm missing something.

Just to stress I'm not saying the design is right, just that it could be in the right house (of the same size), which obviously this isn't. Is it possible that the heating designer was given incorrect info by the architect, or that the fabric spec was downgraded during the build but nobody thought to rework the M&E. Of course either is equally inexcusable.

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Graham

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07/04/2024 9:20 pm

I have a 10kw grant system, which struggles in below 0c.
But we have to rely on the people selling the systems to be honest.which most are not

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Graham Hendra

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08/04/2024 7:19 am

im not sure its done on purpose, I just think that people dont know how to do it properly and the support rom supplies is nowhere near good enough.

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Graham Hendra

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Topic starter 08/04/2024 7:24 am

i agree pasiv houses are 10W/m^2 but by definition a passiv house wouldn't need a heating system. so i never come across them. They are also incredibly rare.

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Graham Hendra

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Topic starter 08/04/2024 7:26 am

i dont think they are dishonest, they just cant do the maths. back in the old days the supplier did the heat loss so this dindt happen. now any idiot can do it and get it wrong. no one would undersize as a choice, unless they wanted to go bust.

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JamesPa

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08/04/2024 7:50 am

Posted by: @grahamh

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i dont think they are dishonest, they just cant do the maths. back in the old days the supplier did the heat loss so this dindt happen. now any idiot can do it and get it wrong. no one would undersize as a choice, unless they wanted to go bust.

In a new build, which I presume this is, the supplier is reliant on what the architect tells them about the fabric, which is why I speculated that maybe something had gone wrong in this interaction.
As a matter of interest can the situation be rescued without swapping out the ashp (a question which amounts to: is there any behaviour change in the way the system is used that would solve it, or is it already used in the most effective way possible). If this is south of England the shortfall is (apparently) only about 25% and thats on days that don't occur very often. Obviously I understand you might not know the answer!
I'm still not trying to defend the indefensible, just exploring out of interest.

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Derek M

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08/04/2024 9:02 am

Posted by: @grahamh

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i dont think they are dishonest, they just cant do the maths. back in the old days the supplier did the heat loss so this dindt happen. now any idiot can do it and get it wrong. no one would undersize as a choice, unless they wanted to go bust.

I'm afraid saying 'they just can't do the maths' is not an adequate reason. The system designers should either learn how to do the maths or find someone who can do the maths. After-all it is not rocket science and there are a number of tools available to help.

Even just applying common sense should indicate that a 4kW heat pump in a 200 m2 home is not the norm and should be double checked.

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Dave

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08/04/2024 11:28 am

Passive houses do need a heating system; nobody ever said they didn't. The limit is chosen such that they can be heated by heating the normal ventilation air. I live in a certified PH so I know of what I speak.

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