Architect’s Advice On Heat Pumps

Earlier this week the Government announced their plans to offer grants for homeowners to replace Gas Boilers to Heat Pumps. This is the first part of what are expected to be a series of measures introduced in the lead up to, during and after, COP 26.

Over the years, HPA Chartered Architects have worked on a number or projects which use Heat Pumps. Here’s a brief explanation of what they are, and a few do’s and don’ts based upon our own experience over the years:

Heat Pumps aren’t new technology, in fact you’ll have one in your home already; as Heat Pumps are effectively fridges in reverse. They extract latent heat from a source (air, water or the ground), which is amplified and used to heat water. The heated water from the Heat Pump, heats your water tank and warms your house via radiators or underfloor heating.

The Ground Source Heat Pumps (GSHP) are the most efficient type of heat pump, as they extract latent heat from the ground, which is a constant temperature a metre or so below the surface. The heat is usually collected via underground coils, which require a large surface area. This is usually ideal for houses with lots of land. For tighter sites a vertical borehole can be used, but this is more expensive, and does require favourable ground conditions. We’ve often struggled to use GSHP units on rocky sites.

Water Source Heat Pumps work well if you live next to a large area of relatively deep and constant supply of water which you own, or can use. Unfortunately, this rules out most people! If you’re lucky enough to have some water next to you, then the coils on the base of the watercourse need to be deep enough to avoid freezing ice, and placed in an area which is not susceptible to drought.

St John’s Church, Yealand Conyers which uses a heat pump

Air Source Heat Pumps (ASHP) are the most commonly used type of Heat Pump. They extract Heat from the Air, and are located immediately outside the building. They are powered by electricity, so 1kw of electricity into the system produces an output of around 3 to 4 kw of Heat. We’ve been involved in many projects which use them, including a Grade II Listed Church. The system is even cheaper to run if you have Photo-Voltaic Panels and a battery at the property as well.

Heat Pumps don’t heat water as hot as a Fossil Fueled Boiler, and as a result of this they work best in well insulated houses which ideally have underfloor heating. We’ve found that it’s quite straightforward to design Heat Pumps into new build houses, which have to be well insulated and also generally have a floor construction which will accept underfloor heating.

It’s a bit trickier installing a Heat Pump into an existing building without having to dig the floor up to install underfloor heating; but it is possible. The most obvious difference is that you’ll need larger radiators 50% to 100% larger than the ones you’d see on a Fossil Fuel system. Alternatively, you can install fan assisted radiators, but these can create a distracting background noise.

Heat Pumps operate differently to fossil fuel boilers, as they can’t deliver a burst of heat; they work by providing medium temperature warmth over a prolonged period of time. Therefore, you need to leave them on pretty much all the time in order to maintain a constant temperature and operate at maximum efficiency. This approach can work in your favour if you have a building of high thermal mass such as a stone cottage.

Naturally Heat Pumps, like all boilers, work most efficiently in draught-free insulated buildings; so, it’s always best to invest in the fabric of the building before you install the boiler to keep the running costs as low as possible. For older properties this should go hand in hand with an understanding of how the property works in order to ensure how the breathability and thermal bridging occurs. We always advise our clients to retrofit with a fabric first approach, if they get the opportunity; it makes sense both environmentally and economically!

For more information on understanding your how your old house works we’d recommend the ‘Old House Handbook’ published by the SPAB https://www.spab.org.uk/shop/product/old-house-handbook and the publication of the LETI Retrofit Guide which was launched this week which gives practical opportunities of where you can start that isn’t just limited to the installation of a heat pump. https://www.leti.london/retrofit

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