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rad size and efficiency

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 6 months ago

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AdamK

(@adamk)

Reputable Member Member

Joined: 6 months ago

Posts: 82

Topic starter 28/02/2025 11:19 am

im going to have a Vaillant fitted and just see how it runs with the rads i have currently. how does having potentially undersized rads affect efficiency? or does it only affect comfort? assuming im controlling on weather comp?

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Brendon Uys

(@heacol)

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Joined: 4 years ago

Posts: 408

28/02/2025 11:40 am

It is just thermodynamics, the higher you move the flow temperature the more it will cost to run, existing radiators may need a higher flow temperature so the heat pump will cost more to run, your radiator upgrade, will usually be paid for in less than 2 heating seasons.

Director at Heacol | Expert Heat Pump Consultant | Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.

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AdamK

(@adamk)

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Joined: 6 months ago

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Topic starter 28/02/2025 12:00 pm

@heacol

what if i set the system so its controlling on flow temp only, surely it would then just be a case of seeing which rooms dont get to 20c and upgrade those?

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Brendon Uys

(@heacol)

Noble Member Contributor

Joined: 4 years ago

Posts: 408

28/02/2025 12:19 pm

@adamk Yes you can to that. I never use old radiators. If the customer insiste, I walk away, it is not worth it.

Director at Heacol | Expert Heat Pump Consultant | Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.

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JamesPa

(@jamespa)

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Posts: 2715

28/02/2025 5:08 pm

Posted by: @adamk

↑

what if i set the system so its controlling on flow temp only, surely it would then just be a case of seeing which rooms dont get to 20c and upgrade those?

Adia Thermal sort of advocate this approach and produce some hardware to aid the determination.

In many cases however it will be obvious that an upgrade is needed, best in my view to do the calculations up front. If any are marginal then you could adopt the suck it and see approach for those. You need an increase in area of a factor of 2 or more if you move from a system designed for a boiler at a flow temp of 70C to a system designed for a heat pump at 45C, but if you have added insulation since the original installation, or the rads were anyway oversized, some may be reusable (some of mine were)

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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