Grade II listed Victorian Chapel House renovation and extension incorporating an air source heat pump by Mark Wray Architects. Photo by The Modern House.
Heat pumps are increasingly common in new builds, but questions remain about their effectiveness in older buildings.
This article aims to provide an impartial assessment of their suitability for such properties. Drawing on my 20 years of experience as an architect specialising in the renovation of older homes, I will share insights on using heat pumps in various contexts, including terraces and listed buildings.
Heat pumps transfer heat from air, ground, or water using refrigerants and have been used in industrial manufacturing for decades, but are relatively new to residential buildings. They operate at much lower flow temperatures than gas boilers and are most effective in well-insulated, draught-free homes. Older buildings, however, often lack basic insulation.
Gas boilers, which run at high flow temperatures, can heat draughty or poorly insulated homes more easily, though they are less efficient and produce higher carbon emissions.
Retrofitting a heat pump, or any other heating system for that matter, in an older, poorly insulated building can pose challenges if the property's thermal performance isn’t improved.
READ: Retrofitting Heat Pumps Into Houses: A Comprehensive Guide To The Pros & Cons
It’s possible to install a heat pump in an uninsulated building, but it's usually impractical or undesirable.
For a heat pump to run efficiently, it needs to operate at a low flow temperature, which requires large radiators or underfloor heating to distribute heat effectively. As with any heating system, radiators must be sized to suit the size of the room and the property’s thermal efficiency.
In an uninsulated home, especially one with large rooms and high ceilings, very large radiators are required to maintain comfort. Alternatively, underfloor heating could be used, but it requires insulation underneath to function properly. In contrast, an insulated building can use smaller, more practical, and attractive radiators.
While claims that heat pumps require high levels of insulation are overstated, installing a heat pump in a completely uninsulated home isn’t sensible either. Before adding any new heating system, it’s wise to improve your property’s thermal efficiency to ensure it runs economically and keeps your home warm and comfortable.
A ground source heat pump serving underfloor heating and a wood burning stove helps maintain the character of this 300-year-old renovated cottage by Tuckey Design Studio. Photo by James Brittain.
One of the most important factors in any heating installation, especially with heat pumps, is the quality of the design, installation, and commissioning. Heat pumps aren’t a one-size-fits-all solution; each property must be carefully assessed, with the system tailored to its specific characteristics.
The design, installation, and commissioning are arguably more critical than the heat pump itself. For example, a poorly installed, top of the range heat pump won’t perform as well as a mid-range unit connected to a properly designed system. Each stage should be done by a highly trained person or, more commonly with larger installer, by three different specialists.
Unfortunately, there aren’t enough experienced heat pump engineers in the UK, leading to poorly designed systems that don’t run efficiently – a source for much of the bad press. At a minimum, your chosen engineer should be MCS certified, while Heat Geek Elite installers are among the best trained in the UK.
Accurate heat loss calculations are crucial for a successful heat pump installation. A qualified heating engineer should perform this assessment, determining how much energy your home needs to stay comfortable. This guides the system design, from the size of the heat pump to the number of radiators or underfloor heating circuits required.
They will also recommend home improvements to optimise the installation. At this stage, it’s advisable to consult an architect (or another design professional) with expertise in improving a building’s thermal performance. They’ll ensure a cohesive solution, making sure the system runs efficiently, is practical, and doesn’t cause defects or negatively impact the building’s function or design.
External wall insulation is the most efficient way to insulate a solid wall. Project by Prewett Bisley.
Preparing your home for a heat pump may seem daunting, but many of the necessary upgrades – such as insulation and draught proofing – can be done gradually and at a relatively low cost. These improvements not only boost your home's energy efficiency but also ensure your heat pump system operates at its full potential.
Before starting any work, it’s important to plan the upgrades required to make your home "heat pump ready." This will help you prioritise the most impactful improvements and ensure that each stage of work contributes to the overall efficiency of your heating system. With careful planning, you’ll avoid unnecessary effort or wasted resources, setting your home up for a smooth transition to heat pump technology.
To ensure a heat pump is installed and operates efficiently, the first step is to enhance your home’s thermal performance. This is often referred to as a "fabric-first" approach, prioritising improvements to the building envelope (walls, floors, roof) before installing new heating systems.
Start by addressing the areas of highest heat loss:
READ: How to Insulate Your Home: An Architect's Expert Guide for Homeowners
Contrary to some claims, you don’t necessarily have to strip out small-bore pipework to accommodate a heat pump. In fact, heat pumps can often integrate with existing small-bore pipework, commonly using a buffer tank to help regulate the flow.
If you already have a gas central heating system, much of the existing pipework is likely reusable. However, you may need to upgrade some of your radiators and add new ones (or install underfloor heating) to ensure compatibility with the lower flow temperatures required by a heat pump.
A qualified heat pump engineer can assess the suitability of the existing plumbing and, through heat loss calculations, determine the extent of the changes required.
In this Victorian renovation by Bradley Van Der Straeten Architects, an ASHP serves underfloor heating at ground floor level and column radiators above. Photo by French + Tye.
Installing a heat pump in an old building sometimes comes with additional challenges.
Planning controls may limit the types of upgrades you can make, particularly for listed buildings and those in conservation areas, where preserving the building’s character is a priority.
If you live in a built-up area, noise restrictions may complicate the installation of air source heat pumps, making planning permission more likely to be required.
READ: Do Heat Pumps Need Planning Permission?
Locating an ASHP in the rear garden, away from neighbouring properties, is a good solution for meeting noise requirements in built-up areas. Photo by Bradley Van Der Straeten Architects.
Some manufacturers market their heat pumps as "high temperature," but this is often just clever branding. These units, typically using R290 refrigerant (a quality indicator), aren't meant to run at the high temperatures of gas boilers. The high temperature function is primarily for running a legionella cycle, with normal operation at 35-50°C.
Running a heat pump at high temperatures increases costs and wear, shortening its lifespan. Instead, focus on improving your home's thermal performance and hiring a qualified engineer for proper system design, installation, and commissioning.
To reduce heat loss install high efficiency double-glazed windows and thermal blinds. Project by Tuckey Design Studio. Photo by James Brittain.
Hybrid heat pumps, which combine a heat pump with a gas boiler, offer a potential low-carbon heating solution where a standalone heat pump is difficult or too expensive to install.
Systems like the Intergas Extend Split use an air source heat pump (ASHP) as the main heat source, supplemented by a gas boiler to enhance performance, making them more carbon efficient than other hybrid systems which use more gas. This type of hybrid system can provide cheaper installation costs and greater flexibility, particularly in homes that may struggle to meet heating demands with a heat pump alone.
However, it’s important to note that, because they still rely on fossil fuels, they do not qualify for government grants.
Many older buildings feature fireplaces and chimneys designed for burning solid fuel. Where a standalone heat pump is difficult or expensive to install, a high efficiency wood burning stove can serve as a supplementary heat source.
However, it's essential to consider the environmental concerns associated with wood-burning stoves, particularly regarding particle emissions. Stoves with a clearSkies certification is said to produce up to 90% lower emissions than an open fire and up to 80% lower than the average 10-year-old stove.
Using dry firewood is also crucial for minimising emissions. Legally, wood must now contain less than 20% moisture.
Grade II listed Victorian Chapel House renovation incorporating an air source heat pump and wood burning stove by Mark Wray Architects. Photo by The Modern House.
In summary, heat pumps can be an effective solution for older buildings.
While they can operate in poorly insulated properties, enhancing your home’s energy efficiency by improving insulation, draught proofing and upgrading radiators will ensure the optimal installation.
For properties with budget or heritage constraints, hybrid systems that supplement a heat pump with a gas boiler can provide a viable low-carbon heating alternative.
Above all else, the key to a successful heat pump installation lies in engaging a qualified heating engineer to design, install, and commission the system properly, ensuring it meets the specific needs of your home.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
