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Question regarding Boiler Upgrade Scheme eligibility for ASHP with gas boiler replacement and existing A2A

Air Source Heat Pumps (ASHPs)

Last Post by Rob of York 35 minutes ago

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Rob of York

(@rob-of-york)

Trusted Member Member

Joined: 2 weeks ago

Posts: 54

Topic starter 11/09/2026 12:14 pm [#3161]

I am looking to replace my gas boiler with an ASHP, although the journey will be a slow and cautious one.

I've done heat loss calculations based on both a detailed theoretical model and detailed practical measurements, see this thread: https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/adventures-in-heat-loss-calculations/

The conclusion is that the ASHP will need to provide 6kW for spacing heating (-3C up to 21C) and hot water. It is likely, possibly inevitable that MCS calculations will give a higher figure.

I already have an air-to-air system installed in two rooms, Daikin 2.5kW and 3.5kW units that are very efficient on heating (SCOP 5.2).

In sizing an ASHP, can the heat loss from those rooms be omitted from the total, and still mean that I am eligible for the BUS grant, with the ASHP sized to meet the heat loss for the remainder of the house?

In practice, I want the ASHP to be able to cover the space heating and hot water requirements for the whole house under normal winter conditions, for which 6kW should suffice; however, if we get extreme weather, below the design temperature of -3C, then I have no issue with having to turn on the air-to-air to supplement the ASHP.

My preference would be for a correct sized ASHP, rather than an oversize one, with all the extra costs and issues that can entail. Being able to specify a 6kW or 7kW ASHP would give me a lot of choice.

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