What to ask: whether they will tell you if your property is unsuitable. It sounds like a soft question. It isn't. An installer who has never turned a job down is not assessing anything.
Stage 2: The heat loss survey — the stage that matters most
This is the one to pay attention to, because everything downstream depends on it and it is where corners get cut.
A proper survey is a room-by-room heat loss calculation. The surveyor measures each room, notes wall construction, window type and glazing, ceiling heights, insulation levels, and works out how much heat each room loses on a cold day. Add those up and you have the property's heat loss in kilowatts. That number determines the size of the heat pump.
It takes a couple of hours minimum. In an older or awkward property, longer.
What a bad survey looks like: someone walks round, counts your radiators, asks how many bedrooms you have, and quotes you off a rule of thumb — so many kW per square metre. This is the single most common cause of heat pump installations that disappoint. Oversize the unit and it short-cycles, runs inefficiently and costs more to buy. Undersize it and you are cold in February.
What to ask:
- "Will I get the room-by-room heat loss figures in writing?" — the answer should be yes, without hesitation
- "What flow temperature are you designing to?" — lower is better for efficiency; a design around 45°C or below is a good sign, 55°C+ suggests they are avoiding radiator work
- "Which radiators need changing, and why?"
If an installer is reluctant to hand over the heat loss calculation, that tells you something important before you have spent a penny.
Stage 3: The design
Behind the scenes, the survey turns into a specification. This covers:
- Heat pump size and model — matched to your calculated heat loss, not rounded up for comfort
- Flow temperature — the temperature of the water going to your radiators. Heat pumps get their efficiency from running cooler than a boiler, so the design aims low.
- Radiator schedule — which stay, which get upsized. Lower flow temperature means each radiator needs more surface area to deliver the same heat.
- Hot water cylinder — if you currently have a combi boiler you almost certainly need one, and it needs somewhere to live. This surprises people.
- Outdoor unit siting — where the unit goes, considering noise, airflow, pipe runs, and your neighbours
- Pipework — some older systems have pipework too narrow to carry the required flow
The cylinder question
Worth flagging separately because it catches so many people out. A combi boiler heats water on demand and needs no tank. A heat pump normally works with a hot water cylinder — typically 150 to 250 litres, roughly the size of a large fridge, needing airing cupboard space or similar.
If you had your airing cupboard converted when you got the combi, that space has to come from somewhere. Better to know at design stage than on install day.
Stage 4: The quote
A good quote is boring and specific. It should name the actual heat pump model, list every radiator being changed, specify the cylinder, state the flow temperature the system is designed to, and show the grant and VAT already deducted so you see the real number you pay.
On VAT — one thing worth understanding, because it is widely misreported. Qualifying heat pump installations carry 0% VAT under HMRC VAT Notice 708/6 until 31 March 2027. But energy-saving materials were never charged at the 20% standard rate; they sat at the reduced rate of 5%. So the relief saves you 5%, and from 1 April 2027 the rate goes back to 5% — not 20%. Anyone telling you that you will pay 20% more after March 2027 has got it wrong.
What to ask: "What isn't included?" Scaffolding, making good, electrical upgrades, and cylinder relocation are the usual suspects. A fixed price with a list of exclusions is honest. A fixed price with no exclusions is often a fixed price with surprises.
Stage 5: The grant
The Boiler Upgrade Scheme is the main grant, and the important thing to understand is that you don't apply for it — your installer does, on your behalf, to Ofgem. The grant is deducted from your invoice. The money never passes through your hands.
Current amounts:
- £7,500 — air-to-water or ground source heat pump
- £9,000 — the same, for off-gas-grid homes currently on oil or LPG, from 21 July 2026 to 31 March 2027
It is not means-tested. Your income is irrelevant. What matters is the property and what you are replacing. The standard £7,500 is confirmed through to 2030.
Two conditions worth knowing. The installation must be carried out under MCS certification — that is a scheme requirement, not a preference. And once the voucher is issued there is a three-month window to complete an air source install (six months for ground source, because of the groundworks).
What to ask: "Whose MCS registration is my installation being certified under, and will the certificate be in your name?" Some installers work under an umbrella company's MCS registration rather than holding their own. That is a legitimate and common arrangement — but you are entitled to know, because it determines whose name appears on your certificate and who carries the compliance responsibility.
Stage 6: Installation
The on-site work. Typically 2 to 5 days depending on how much radiator and pipework changes.
1. Day one, strip out. The old boiler comes out. If you are replacing an oil system, the tank usually goes too. Expect the heating to be off, so plan for this not to be the coldest week of the year. 2. Outdoor unit. A base or brackets go in, the unit is positioned, and pipework is run through the wall to the indoor components. 3. Indoor works. Cylinder in, controls fitted, pipework connected, electrical supply run to the outdoor unit. 4. Radiators. The ones flagged in the design get swapped. This is usually the messiest part — draining down, lifting boards if pipework needs upsizing. 5. Fill, pressurise, test. The system is filled, checked for leaks, and the electrics tested.
What to expect practically: several people in your house, some dust, at least one day without heating or hot water, and probably a hole drilled through an external wall. Reputable installers dust-sheet and tidy up. It is still a building job.
Stage 7: Commissioning and handover
Commissioning is where the system is actually set up to run well — flow temperature set, weather compensation curve configured, hot water schedule programmed, controls balanced. It is not a formality, and rushing it is why some heat pumps disappoint despite a decent design.
Weather compensation deserves a mention. It is a setting that automatically varies your flow temperature based on the outside temperature — cooler water on mild days, warmer when it is genuinely cold. It is one of the biggest single influences on running cost and it is sometimes left switched off. Ask whether yours is enabled and how it has been configured.
The paperwork you should walk away with
Do not treat this as admin. Some of it is worth money and some of it you will need years from now.
- Your MCS certificate. This is the important one. It is lodged on the MCS Installation Database and it proves the installation was certified. You need it for grant compliance, you need it if you ever add solar and want export payments, and buyers' solicitors ask for it when you sell. If you don't receive one, chase it.
- The heat loss calculation and system design — so any future installer or engineer knows what they are working with
- Commissioning certificate — the settings the system was handed over on
- Manufacturer warranty registration — often needs doing within a set window
- Building control notification where applicable
- Your workmanship guarantee — separate from the manufacturer's warranty, and worth reading
What to ask: "When will I get my MCS certificate, and whose name will be on it?"
Stage 8: The first winter
Nobody tells you this part, so here it is: a heat pump does not behave like a boiler and the first few weeks feel odd.
A boiler blasts hot water round quickly, gets the room warm, and switches off. A heat pump runs for long periods at a lower temperature, keeping the house at a steady temperature rather than swinging up and down. Radiators feel warm rather than hot. That is correct, not a fault — it is where the efficiency comes from.
Two habits to unlearn:
- Don't turn it off when you go out. With a boiler that saves money. With a heat pump, letting the house cool and then hauling it back up is usually less efficient than holding it steady.
- Don't crank the thermostat to heat the room faster. It won't. It will just overshoot.
Expect some tweaking. A good installer will adjust the curve after a few cold weeks once there is real data. Ask whether that follow-up is included before you sign.