Ask most people what a heat pump installation involves and they picture the box outside. In practice, a large share of the work, and of the variation between quotes, happens indoors: in the airing cupboard, behind the radiators and along the pipe runs. This page follows that work room by room.
Why the inside of the house changes
A gas boiler can send water to the radiators very hot, so even a small radiator gives out plenty of heat. A heat pump works best sending water out cooler, and it runs for longer at a steadier output. The temperature it is designed to send out on the coldest day is the design flow temperature, set during the heat-loss survey and design.
Cooler water means each radiator gives out less heat than it did before. Where a room's radiator was already generous, that is fine. Where it was only just big enough with boiler-hot water, it now falls short. That is the whole logic of radiator changes: it is a room-by-room comparison, not a blanket rule.
- Outdoor unit takes heat from outside air and warms water.
- Hot water cylinder stores heated water for taps and showers.
- Radiators (or underfloor heating) release the heat into rooms.
- Flow, warm Return, cooler
Radiators: which change and why
For each room, the design compares two numbers: the heat the room loses on a cold day, and the heat its radiator can give out at the design flow temperature. If the radiator falls short, it is replaced with one that has more surface: a larger panel, a double instead of a single, or a different type. Sometimes a radiator is moved to a better wall at the same time.
The list of changes should be in the design as a radiator schedule: room, existing radiator, its output at the new flow temperature, and what replaces it. Two quotes that differ a lot on price often differ here first, because one installer has chosen a lower flow temperature and more radiator changes, the other a higher one and fewer. Neither is automatically wrong; you just need to see which you are being offered.
The size of the job also tracks the size of the heat pump. The official median price for a 6 to 8 kW installation in April 2025 to March 2026 was £12,164, against £15,366 at 12 to 14 kW. Bigger homes need bigger units and, usually, more emitter work too.
Underfloor heating
Underfloor heating gives out heat over a whole floor, so it works well with low flow temperatures. Because it means lifting and relaying floors, it is usually planned where floors are coming up anyway, such as an extension, rather than added purely for the heat pump.
The hot water cylinder
A combi boiler heats water on demand as you run a tap. A heat pump is not designed to do that: it heats a store of water in a cylinder, which then supplies the taps and showers. So a home moving from a combi gains a cylinder; a home that already has one may keep its space, though the cylinder itself is usually changed for one made to work with a heat pump's lower temperatures.
The design should state the cylinder's size and where it goes. Typical places are an airing cupboard, a utility room, a loft with a floor that can carry the weight, or a garage. Space is the most common worry, so settle it at the survey rather than on the first day of the fitting.
A cylinder can be vented, fed from a cold water tank in the loft, or unvented, fed straight from the mains. Unvented hot water storage is work that falls under building regulations, and the competent person schemes list it as its own area of competence. Ask the installer how the cylinder will be signed off.
Buffer tanks and pipework
Some designs add a small buffer tank between the heat pump and the radiators, to smooth out how the system runs; others do without. It is a design choice with a space cost, so if one is on your quote, ask why it is needed in your system.
Pipework can change too. A heat pump moves more water at a lower temperature, so some existing pipes may be too narrow for the new flow, and the run from the outdoor unit to the heating needs insulating outside. Any pipe changes should appear on the quote, not as a surprise later.
Controls
A heat pump is set up to run steadily, often using weather compensation, which adjusts the flow temperature to the weather outside. The controls replace the boiler's timer and thermostat, and how they are set at commissioning matters for how the system behaves. Ask for them to be explained at handover, and keep the settings in the paperwork. The commissioning and handover page covers what to ask for.
Building regulations and sign-off
Fitting a heating or hot water system is building work. Installers registered with a competent person scheme can self-certify that it meets building regulations instead of applying to building control; GOV.UK lists the authorised schemes for heating and hot water and for microgeneration. If your installer is not in such a scheme, the work needs building control approval instead. Either way, you should end up with paperwork that shows the job was signed off. One thing never changes: keeping a gas boiler running alongside the heat pump makes it a hybrid system, which the grant does not fund.