Understanding Heat Pumps Before You Install One

Why a heat pump behaves differently from a boiler

A gas boiler burns fuel and delivers heat at 70–80°C. A heat pump moves existing heat from the air or ground into your home, and it does that most efficiently at much lower temperatures — typically 35–50°C. That single difference explains almost everything else about the technology, from why radiators sometimes need changing to why the system runs for longer, gentler periods rather than short, fierce bursts.

None of that is a problem in itself. Heat pumps are proven, reliable and increasingly common across the UK. But they reward preparation. A well-designed installation in a reasonably insulated home is quiet, comfortable and cheap to run. A hurried one bolted onto a leaky house with undersized radiators will disappoint. The difference is nearly always the survey and the design work behind it.

Fabric first: get the building ready

Before anyone sizes a unit, it is worth dealing with the obvious heat losses. This is not box-ticking; every improvement lowers the flow temperature your system needs, which directly improves efficiency and reduces running costs.

• Loft insulation — top it up to at least 270mm if it is thin or patchy.
• Wall insulation — cavity walls are usually straightforward; solid walls are more involved but often still worthwhile.
• Draught-proofing — around doors, letterboxes, pipework entries, loft hatches and suspended floors.
• Glazing — double glazing is a baseline; older units with failed seals are worth replacing.
• Floor insulation — a bigger job, but sensible if you are renovating anyway.

An Energy Performance Certificate gives a rough picture, though it is no substitute for someone actually measuring your rooms and radiators. Treat it as a starting point for a conversation, not a verdict.

Sizing: the most common and most costly mistake

A bigger heat pump is not a better heat pump. Oversizing leads to short cycling — the unit repeatedly switching on and off — which wastes energy, adds wear and can make the system noisier. What matters is matching output to the genuine heat loss of your home on the coldest design day.

A proper calculation works room by room, in watts, using measured dimensions, wall construction and insulation levels. It should also account for the real-world output of your existing radiators at lower flow temperatures, because a radiator rated for 70°C water gives out far less heat at 45°C. Many homes need some radiators enlarged or replaced — often in the living room, hallway or bathroom.

• Heat loss figure — expressed in kilowatts at your design outdoor temperature.
• Emitter output — the total heat your radiators or underfloor loops can deliver at the chosen flow temperature.
• Hot water demand — heat pumps normally work with a cylinder, commonly 150–250 litres for a family, and reheating takes longer than a combi.
• Pipework and controls — pipe sizes, zoning and whether weather compensation is included.

What a thorough survey looks like

Expect a surveyor to spend an hour or two in your home, and to ask questions rather than simply measure. If you receive a quote without a visit, treat it as a rough guide rather than a firm proposal.

• Room-by-room measurements, including ceiling heights and window sizes.
• Wall type, cavity presence and insulation depth.
• Existing radiator dimensions, types and pipe connections.
• Where the cylinder can go, and whether the airing cupboard or garage is suitable.
• Outdoor unit position — airflow, distance from boundaries, noise sensitivity and drainage for condensate.
• Electrical supply, consumer unit capacity and earthing.
• Your actual hot water habits: how many showers, how often, at what times.

A good surveyor will also talk you out of things. If your home needs insulation first, they should say so.

Living with one day to day

Heat pumps suit a steady approach. Set a comfortable temperature and leave it, using modest setbacks overnight rather than switching the system off entirely. Weather compensation — where the flow temperature adjusts automatically with outdoor conditions — does much of the work for you, so ask for it to be set up and explained properly at handover.

Expect longer run times at lower output; that is normal and efficient. Defrost cycles in cold, damp weather are also normal. Keep external units clear of leaves and furniture, and avoid blocking airflow with screens or planting. If the system feels wrong — cold rooms, constant cycling, noisy operation — raise it with your installer rather than living with it.

Questions worth asking before you commit

• What flow temperature is the system designed to run at?
• What is my calculated heat loss, and what is the unit's output at design conditions?
• Which radiators are being changed, and what are their new outputs?
• What cylinder size is proposed, and how long will it take to reheat?
• Is weather compensation included, and who will set it up?
• What does commissioning and handover include, and what warranties apply?

Ask for the heat loss calculation in writing. A confident installer will share it happily, and it gives you something concrete to compare between quotes. Get the survey right and the rest of the project tends to follow.