Search with Wattson

Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

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

seaviews · 2026-10-19T21:32:47Z

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....

Page 3 / 3 Prev

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 1 hour ago

25 Posts

10 Users

29 Reactions

460 Views

RSS

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 4 years ago

Posts: 5562

25/10/2026 10:02 pm

@seaviews

I have read you latest response re buffer tanks and I must say that, based on that, I would not trust your installer. It's not that the answer is wrong, just that there are other ways to solve the problems he alludes to if indeed they exist.

For me this remains essentially simple. 15kW is almost certainly too big, and very definitely way too big after you have insulated which you plan to do. There is no identified reason for a buffer.

Thus, unless it's a desperate situation and based on the information you have shared I simply would not proceed until these issues are resolved. Thus I would either insulate, measure, and seek alternative quotes, or seek a bufferless quote with a heat pump not more than 12kW (which will likely be tolerably oversized after insulation) if it must be done now.

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.

👍

ReplyQuote

Page 3 / 3 Prev