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Next steps on my ASHP journey: Radiators and Pipework

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Air Source Heat Pumps (ASHPs)

Last Post by Rob of York 27 minutes ago

26 Posts

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Rob of York

(@rob-of-york)

Eminent Member Member

Joined: 2 weeks ago

Posts: 20

Topic starter 05/09/2026 4:57 pm

Changing the 15mm pipe would be very disruptive and no doubt expensive, so I don't want to go there.

As an alternative, I would be happy to compromise on not running at DT5, but using DT10 instead. That would halve the flow rate needed to transport any given amount of power, and may bring my 15mm pipework back into the workable and quiet range. Noise is a serious issue if the heat pump is going to run 24/7.

A system working at DT10 would give a flow temperature increase of 2.5C over DT5 to get the same radiator temperature, since radiator temperature is the average of flow and return.

If I look at the SCOP values for a Vaillant Aerotherm 7kW, they are 4.13 at 40C flow and 3.91 at 45C, hence about 4.02 at 42.5C flow. So for me, with my planned radiator upgrades, running at DT10 instead of DT5 likely gives a reduction in SCOP from 4.13 to 4.02. Currently, I use about 13,000 kWh for heating and hot water. The reduction in SCOP equates to less than £23 per year at standard electricity prices, and much less in practice with cheap rate overnight electricity and solar. Hence, I can't see any economic argument for changing the pipework - the payback period would not be in my lifetime!

Question then is would my pipework be fine with the flow rate back at about 0.167l/s and DT10 to deliver 7kW?

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Rob of York

(@rob-of-york)

Eminent Member Member

Joined: 2 weeks ago

Posts: 20

Topic starter 05/09/2026 5:12 pm

@bobflux

The hydraulic simulator is very interesting. I don't know if I can give you enough information to do a reasonable model of our heating system, particularly as I don't know which routes the pipes actually take. The only other information that I have right now is that there are 6 radiators upstairs and 7 downstairs, which will become 8 as I plan on adding one.

I have a thermal camera, so I can take a look however, the heat signature may not be visible if the pipe is not up against the floor or ceiling but rather runs along a joist part way up. When I've walked around looking for cold spots on really cold days I've never noticed any pipes except at the radiators themselves. This is in stark contrast to my parent's bungalow which has micro-bore pipe in the internal walls and is really obvious on the thermal camera.

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