I’d be inclined to push back fairly hard on those heat loss calculations, if they result in a much larger ASHP than you really need.
- There are countless tales of woe on this forum about oversized HPs and the consequent poor efficiencies due to excessive cycling. Many users have ended up negotiating a replacement smaller unit with their installers as a result.
- Unless your house is very large and/or very poorly insulated (or you live on top of Bodmin Moor
), I’d be surprised if you actually need more than about 8kW. The MCS “rules” for calculating heat loss (and thus qualifying for the all-important BUS grant) are - or at least were (I believe that they are currently under review) - based on pessimistic ventilation flows. If you (or your installer) can demonstrate a lower ventilation component, you may end up with a HP better matched to your real requirement. - The only downside of a well-matched HP size (to max heat loss) is the potential restriction on setback (see House Thermal Inertia and Roomstat Setback (some cautionary notes)).