Many homes with gas boilers aren't efficient enough to simply swap the boiler for a heat pump. Heat pumps run at around 50°C where boilers run at 80°C, so the more efficient your home, the better a heat pump will work — with less noise, lower running costs and often a smaller, cheaper unit.
When you ask an installer to quote, they will assess your home for five things:
Before any of that, it makes sense to run your own assessment. This article shows you how — and, just as importantly, gives you the right questions to ask installers when they visit. Asking good questions is how you tell a well-designed system from a like-for-like boiler swap.
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"EPC is such a rubbish way of measuring anything! You should be able to use a heat pump in a house that is EPC C, but it's not that simple." " Colin Usher, Retired architect and low energy house designer |
An Energy Performance Certificate is mandatory when you sell or rent out a home. It lasts 10 years and involves a professional survey. It is not, however, a reliable guide to whether your home suits a heat pump.
Colin Usher, the low-carbon homes architect who has advised on this series, is direct about it:
"EPC is such a rubbish way of measuring anything! You should be able to use a heat pump in a house that is EPC C, but it's not that simple."
Colin Usher, low-carbon homes architect
There are three structural weaknesses in the current system:
A proper heat pump assessment is far more thorough, and it may find that a highly rated home is less suited to a heat pump than one with a lower rating.
The EPC system is being overhauled. The Home Energy Model (HEM) will replace the current Standard Assessment Procedure, and the familiar single A–G band will be replaced by four separate metrics.
| New metric | What it measures |
|---|---|
| Fabric performance | How well the building itself retains heat, independent of the heating system |
| Heating system | The type and efficiency of the heating installed |
| Smart readiness | Whether the home can shift energy use to cheaper, cleaner periods |
| Energy cost | Running cost, retained for information rather than as the headline rating |
The reforms were originally planned for October 2026 but the government confirmed in March 2026 that the launch has moved to the second half of 2027. Existing certificates remain valid, and the current rating is expected to run alongside the new one until at least October 2029.
Two points matter for anyone considering a heat pump. Under the proposals, a property heated by fossil fuels will not be able to achieve the top bands on the heating system metric. And because fabric performance becomes a metric in its own right, the insulation and draught-proofing work described in this series will be recognised directly rather than being blurred into a single score.
Correct as of August 2026. The precise band boundaries and launch date are still to be confirmed — check gov.uk for the current position.
The more energy efficient your home, the better everything downstream works:
In a poorly insulated home with dated windows, heat leaks out as draughts and through the roof, walls, windows and doors. Improve those first and your bills fall before the heat pump is even installed.
Draughts are usually the cheapest and easiest problem to fix. Your home needs ventilation, but uncontrolled air movement takes warm air with it. The usual culprits are:
How to find them: pick a windy day, close the internal doors, and hold a lit incense stick near each window and door in turn. Watch where the smoke is drawn. Under the windowsill is a very common finding, because installers often fail to seal beneath a replacement window.
Chimneys are the biggest single offender, since they are designed to draw large volumes of air out of the house. Our guide to draught proofing your home covers what to do about each of these.
A professional assessment looks at efficiency room by room. Before that, check three things yourself:
If any of these are missing or underperforming, it is worth upgrading before installing a heat pump. Wall insulation can be expensive, so as a first step make sure your loft insulation is up to standard and work outwards from there.
Because a heat pump runs at around 50°C rather than 80°C, a radiator needs a larger surface area to deliver the same heat into the room. If you have old cast-iron radiators or unusually large steel ones you may be fine. Most homes are not.
Radiators are rated on a delta (∆) scale, which describes the temperature difference between the water and the room:
Here is roughly what that means in practice:
| Room | Gas radiator (∆50) | Heat pump radiator (∆30) |
|---|---|---|
| Small bedroom | 500 x 500mm | 500 x 900mm |
| Double bedroom | 500 x 800mm | 600 x 1300mm |
| Large living room | 600 x 1200mm | Two at 600 x 1200mm |
The other thing to look at is the radiator type. A Type 11 radiator has a single panel and one set of fins; a Type 22 has two panels and two sets, giving far more surface area in the same wall space. Swapping Type 11 radiators for Type 22 at the same physical size is often the cheapest way to gain output.
Replacing radiators is not a small job. If budget is tight, many people do it as and when. Prioritising the rooms you actually spend time in — and putting a large radiator in the hallway, where heat travels up the stairs — gets you most of the benefit for a fraction of the cost.
Windows lose energy two ways: by conduction through the glass, and through draughts around the frames. The smoke test above finds the second. For the first, look for condensation between the panes — that means the sealed unit has failed and the insulating gas has escaped.
Before replacing anything, note that on uPVC windows the glass unit can often be replaced while keeping the frame, which costs considerably less than a full replacement. If full replacement is necessary and budget is a constraint, prioritise the rooms you use most.
Modern external doors are tight-fitting and stop draughts immediately. When choosing one, ask whether you actually need a letterbox opening. Even draught-inhibiting letterboxes don't stop draughts entirely, and Colin favours the European approach instead:
"These can have the added advantage of a parcel shelf for your online shopping parcels."
Colin Usher, on external letterboxes
More detail in our guide to improving windows and doors.
Unlike a combi boiler, a heat pump cannot produce hot water on demand. It pre-heats and stores it, as older gas systems did. If your cylinder was removed when a combi was installed, you will need a new one.
| Home size | Cylinder size |
|---|---|
| 1–2 bedrooms | 150–180 litres |
| 3–4 bedrooms | 180–250 litres |
| 5 bedrooms or more | 250–300 litres |
That works out at roughly 50 to 60 litres per person per day. If you have en-suite showers throughout, size up.
An old airing cupboard is the obvious candidate, though you will lose the shelf space you used for bedding and towels. It isn't just the cylinder: the system also needs an expansion vessel (roughly the size of two footballs), a volumiser (about the size of a golf bag), plus pumps and valves.
If the airing cupboard is too small, consider a utility room, garage or understairs space. Identifying a workable location before installers visit saves a lot of back-and-forth.
To qualify for government support, your installer must be MCS or Flexi-Orb certified. Expect an initial walk-round and a ballpark cost.
Treat that first visit as an interview. Ask:
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A warning sign worth acting on If an installer overlooks obvious issues — undersized radiators, missing insulation, microbore pipework — get a second opinion. An installer designing a system for your home will raise these unprompted. One quoting a like-for-like boiler swap may not. |
Once you have chosen a company, they will carry out a full physical and paper assessment including a room-by-room heat loss calculation. This is normally charged as part of the installation package.
Yes. You can check insulation, draughts, window condition and space for a hot water cylinder before contacting installers. Only a professional heat loss assessment can determine the correct heat pump size and confirm whether radiators or pipework need upgrading, but doing your own first means you know what to ask.
Possibly. Heat pumps run at lower temperatures than gas boilers, so many homes need larger or double-panel radiators for the same comfort. Swapping Type 11 radiators for Type 22 at the same size is often the cheapest route. Your installer will calculate the requirement room by room.
Yes. Victorian, Edwardian and other older properties can use heat pumps successfully. The key factor is not the building's age but how well it retains heat. Improving insulation and reducing draughts may be necessary to get the best performance, but age alone is not a barrier.
Most homes replacing a combi boiler will need one, because heat pumps store hot water rather than heating it instantly. Existing cylinders can sometimes be reused, but many installations involve upgrading to a model designed for heat pump temperatures.
EPCs give a useful indication but are not a substitute for a heat pump assessment. They rely on standard assumptions and may not reflect modern technologies or how a household actually uses energy. A room-by-room heat loss calculation gives a far more accurate picture of suitability.
Under the reformed system expected in the second half of 2027, homes heated by fossil fuels will not be able to reach the top bands on the new heating system metric. A heat pump, combined with fabric improvements, should therefore score better under the new methodology than under the current one.
A successful heat pump installation starts long before the equipment arrives. By checking insulation, draughts, radiators, windows and cylinder space yourself, you identify what needs improving and can have a far more informed conversation with installers.
Most homes can accommodate a heat pump. The best results come when the property is prepared properly — investing in efficiency first reduces running costs, improves comfort, and may allow a smaller and less expensive system to be installed.
Your own assessment won't replace a professional heat loss survey. But it will help you ask the right questions, and make sure you choose an installer designing a system for your home rather than simply swapping one heat source for another.
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Next in this series 3. Draught Proofing Your Home Before Installing a Heat Pump — where the draughts are, how to find them, and how to seal them without making your home unhealthy. Previously: 1. How to make your home heat pump ready |
Produced with Colin Usher, low-carbon homes architect. Last updated: 3 August 2026.
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