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If a heat pump can only heat DHW to 50C then what? Issue with 18KW Heliotherm heat pump

errmm · 2026-01-28T23:35:42Z

Hi, I'm helping someone who has a big 18KW Heliotherm heat pump with a DHW heating issue. I don't fully understand all the tech myself (it's two massive cylinders (like 6ft)) in a plant room. After a couple of showers the water is has dropped 10 degrees so is down to around 39C and so he's having to reheat the water again as without this the warm but not hot. When he spoke with his heatpump installer, they said this is how it just is. I think the point they were making is that this heatpump can't heat DHW higher than 50C and so after two showers this is normal. Whereas in my house with gas and a boiler half the size, we can get 3 showers no problem. So is this the norm for heatpumps that can only heat DHW to 50. I know others with heatpumps who heat DHW once, presumably that system can heat to 55C and so even after two showers the water is acceptable? Grateful for any of your thoughts, Mike

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Domestic Hot Water & Cylinders

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errmm

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Topic starter 29/01/2026 9:38 pm

Hi,

Yes it's a big new house 2022, grand design ish. Probably almost passivhaus standard, EPC A 99.. So there's a circulation pump as well going on.

Mike

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Mars

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29/01/2026 10:03 pm

@errmm, I suspected as much. This is high-end, expensive kit, so an installer shrugging and saying ‘this is how it just is’ is frankly enormous BS. Systems at this level should not be running out of usable hot water after a couple of showers.

It would really help to see more photos of the plant room, as @jamespa suggested, because on the face of it something is seriously wrong here.

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Mars

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30/01/2026 4:22 pm

@errmm, I reached out to the distributors of Heliotherm in the UK to get more insights and they have informed me that they are now in touch with you and "on the case". Please let us know what they say.

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errmm

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Topic starter 30/01/2026 8:06 pm

@jamespa

Attached are a few photos I took from today. I also have a video if that might help.

Mike

This post was modified 1 week ago by Mars

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JamesPa

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31/01/2026 1:29 pm

Posted by: @errmm

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@jamespa

Attached are a few photos I took from today. I also have a video if that might help.

As you now have a Heliotherm engineer on the case I wont comment further. As @mars says this is high end stuff, the suppliers need to be made to work for their money!

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Mars

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31/01/2026 4:02 pm

@jamespa I'm still very curious why the installer said "this is how it just is"! Quite the statement that given the kit they've fitted.

I'm not sure I'll get an update from the UK distributors of Heliotherm on this, so would appreciate an update from @errmm when changes and tweaks have been made.

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Mars

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06/02/2026 4:59 pm

@errmm, curious to see if there have been any updates on this project. Please let us know.

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errmm

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Topic starter 07/02/2026 12:23 am

Hi,

Nothing yet. I've asked customer to measure flow and asked Heliotherm to explain exactly what that other cylinder does because it suggests instant hot water. Hopefully I'll have more information in a few days time.

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