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15kw pump for 225m2 1990's house - over kill?

Air Source Heat Pumps (ASHPs)

Last Post by seaviews 11 hours ago

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seaviews

(@seaviews)

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Joined: 20 hours ago

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Topic starter 19/10/2026 10:32 pm [#3198]

Good evening. I am looking at switching from an oil-based heating system to an air-source heat pump. The house, built of brick and concrete in 1990, is located on the coast - think Cornwall/ mild winters. It has two floors with four bedrooms and three living areas distributed between them: the upper floor measures 135 m², and the ground floor—which is partially subterrainan and reinforced with stone, making it more temperate than the upper level—measures 90 m². The house has double-glazed windows (installed in 1990), but there is virtually no insulation on the upper floor or in the attic (although we do plan to resolve this). The original cast-iron radiators and piping are still in place (and in time we plan to replace as necessary with new piping and probably fancoils upstairs when funds allow). Our annual oil consumption has been approximately 3,000 liters. A big reason for replacing the oil boiler now is that it is from 1990, and thus very old, noisy and extremely smelly and dirty plus v difficult to get parts for. Winter outdoor temperatures typically range from 5°C to 10°C (from early November to the end of March), often accompanied by strong winds. The house is situated in an open area with an orientation that allows for direct sunlight from all directions throughout the year. As we are located 200 meters from the coast, any system installed will require protection against the saltwater/ coastal environment. We have received 3 quotes and all have recommended a 15 kW heat pump, paired with a 300-liter domestic hot water tank and a 100-liter buffer tank. This represents a significant investment, and we are concerned about whether the system is appropriately sized. We have read comments about equipment being over sized and that it is likely to be starting and stopping alot which creates wear and tear on the equipment, as well as concerns re buffer tanks. We are retired and spend the entire day at home. We would appreciate your thoughts and advice as we feel confused and overwhelmed....

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