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Heat Pump Pipe Sizing Question

trebor12345 · 2026-05-07T15:55:28Z

I watched Mars' video yesterday on his heat pump install "What Does a Sub-Par Heat Pump Installation Actually Look Like?" Got me thinking about pipe sizing from, the outdoor unit to the cylinder inside, for my new 3.5kw vaillant arotherm being installed in a few weeks time. The proposal was for 32mm PEX/PE-RT/Multilayer Pipe, has oxygen barrier. The run is 10m in total length. In this case 32mm (26 mm ID) might be too large, and 25mm (20 mm ID) might be a better size. Can I have your thoughts please?

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Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 4 months ago

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Batpred

(@batpred)

Noble Member Member

Joined: 2 years ago

Posts: 1145

09/05/2026 4:22 pm

Posted by: @jamespa

↑

I think I mentioned that I am on the Hertfordshire Retrofit Strategy group (as a consumer, I have no 'professional interest) and yesterday the team set itself the task of coming up with an offer for 'boiler failed in winter' scenario. I must say I thought this was brave, clearly this scenario needs to be tackled eventually, but I wasn't personally advocating this task at such an early stage.

One person, obviously from the industry, made the comment 'our heat pump lead times are 6 weeks, I doubt many will wait that long' to which my response, only partially tongue in cheek, was 'well that's a problem then, the local plumber will fit a new gas boiler next week - something needs to change, and its not the local plumber!

Good point, I polled locally (interestingly in Herts) about what our neighbours plans would be to deal with a boiler failure. The vast majority answered "get a plumber to install a new boiler". Hope that it was before the Strategy!

The tougher question would be "how many combi boiler failures have ever been followed by an ASHP install". I know how though it has been to manage expectations at home with our failing combi boiler..

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5404

09/05/2026 4:25 pm

Posted by: @batpred

↑

. Hope that it was before the Strategy!

The aim to have a solution is currently an aim the group has set. It hasn't started working on it yet!

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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Batpred

(@batpred)

Noble Member Member

Joined: 2 years ago

Posts: 1145

09/05/2026 4:26 pm

@jamespa

Is this the input to the retrofit strategy?

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5404