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Retrofitting: moving to ASHP old oil boiler in large old property - experience/advice please on MSC calcs

Katcesca · 2023-01-30T15:13:01Z

Hi there knowledgeable people So we are in the process of slowly upgrading/retrofitting a large, unusual Victorian house. We are trying to work out if an ASHP is a viable proposal in terms of long term running costs. TLDR: current MSC calcs put ASHP running costs at 50% more than we currently spend on oil. It's the central portion of a large country house, but in terms of construction/build if you imagine a very large 3 storey 5 bed Victorian terraced house with 2ft thick granite walls you are pretty much there. 240 sq m of heated living space, some with very high ceilings so a large volume of air to heat. We've done the basic structural stuff, done heat loss survey, insulated roof spaces and basement floor, all windows double glazed, chasing down drafts, thermal curtains etc. There's not much more we can do in terms of insulating without massive cost. We've installed solar and a 10.5 kw battery, which at current rates will cut our elec bill by about 60%. Currently we have a large and old oil fired boiler which heats the house well but has very basic controls - on or off on a timer, no thermostat, no TRVs etc. We have to make a plan for its replacement as it's at least 20 yrs old. We run our heating in 2 or three blasts, maybe 8 hrs a day tops if it's really cold but otherwise more like 6 hrs. We also run an oil fired AGA for a couple of months in winter which helps maintain the base temp of the house but drinks oil like a fish. Currently we use about 3000l of oil a year, supplemented with a woodburner. The house isn't heated to a uniform temperature of eg 20 degrees - we migrate to the warm bits and add jumpers in winter. We are in a mild climate (Cornwall) and the house seems to maintain its ambient temperature pretty well due to thick walls. It's mostly between 16-19.5 degrees in winter. We don't tend to need any heating between May - late September when the house will be 18-22 degrees. Here's my Q: we've had an MSC survey by a company done, which calculated that with some pretty extensive radiator upgrades/additions (9) we could make it work with a 14kw Mitsubishi pump. However, the running costs they have suggested would see us spending quite a bit more than we currently do on oil. I guess it's because they are designing towards a temperature that we don't currently heat the entire house to all the time. (Design temps in calc are 18 degrees in some rooms and 21 degrees in main downstairs spaces.) They've calculated annual energy heat loss of 39495Kwh, plus hot water need 4900, total energy demand 44386. Currently, with our oil usage (at 10kw per l of oil) we are using about 30000Kwh for heating and hot water, so their calculation is 50% above our real-life usage. Their calc for annual Kwh heating consumption at 13155 kwh - at the price cap of 34p/Kwh that's £4472/yr which is £1770 more than we spend on oil currently. I guess the short answer is that we are currently under-heating the property and their proposal is costed for a more regular temperature all round. It would be lovely to have more constant and comfortable daytime heat if that's an affordable prospect. We are really keen to make the switch, and could find the renovation budget - but we can't risk being stuck with very high bills long-term if a new system required us to heat the house more uniformly and for longer resulting in high electricity usage. Given the large space heating demands, the margin for error here could be quite expensive! Currently we are thinking it would only make sense for us to switch if a) electricity prices for renewables were guaranteed to be lower than they are now and b) there was a more generous subsidy for switching (we'd be spending north of £15k even with the £5k grant) Can anyone bring any other perspectives or experience to the table?

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Air Source Heat Pumps (ASHPs)

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cathodeRay

(@cathoderay)

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24/09/2025 2:44 pm

@peanutbutter4 - welcome to the forum. Running costs: I think the fundamental thing, which is either mentioned previously in this thread, and certainly elsewhere, is that by going from an oil boiler to a heat pump, as I did as well, you very likely change how you run the system, typically going from morning and evening and possibly something during the day hots blasts with oil, to close to 24 hour running, on WC, with a heat pump. This alone is enough to bump up the energy use and so costs, because, on average, over a 24 hour period, the building is warmer. The trouble with a heat pump is that by and large you have to run it this way, because it can't do hot blasts. It is an awkward fact that isn't nearly well enough explained up front. That said, the house is more comfortable with the heat pump, and condensation, which happened with oil during the night time off period, is largely a thing of the past.

Midea 14kW (for now...) ASHP heating both building and DHW

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