The nine questions to ask your heat pump installer.

Don't recite all nine of these at a surveyor on the doorstep. Let the installer look at your home and put a written proposal together, then use this list to read it. Almost everything that decides whether a heat pump is warm and cheap to run is set in the design, and the design is on paper before anyone lifts a tool. If you are a step earlier than that and not sure your home is even a sensible fit, the HeatPass check reads your EPC record and gives a verdict in about two minutes.

These questions are not a way to catch an installer out, or to play one quote against another on price. A good installer will be glad you asked, because they are the questions a good design has already answered. You are trying to tell a careful design from a quick one, on a purchase you live with for fifteen years.

Short on time? The five marked Priority below are the ones that matter most. If you read nothing else, read those rows and the sections behind them.

The nine questions to ask a heat-pump installer, each with a good sign and a red flag. Five are marked Priority as the most important.

Ask about	Good sign	Red flag
1. Design temperature Priority	Location-specific, about -2 to -5°C, with standard room targets	"21°C in every room"
2. Heat loss vs your bills Priority	Cross-checked against your last 12 months of fuel	"Three installers agreed, so it's right"
3. Flow temperature Priority	35 to 45°C, committed to on paper	55°C, or "we'll confirm at survey"
4. Projected SCOP Priority	3.5 to 4.0, from the datasheet at your flow temp	"4.5+" or "360%", no conditions given
5. Pipework limits	Per-room flag of microbore runs over ~1 kW	"Re-pipe the whole house", no analysis
6. Cylinder size Priority	200L for 1-2 people, 250L for 3-4, 300L+ for 5+	One stock size for every household
7. Buffer or volumiser	Volumiser, inline on the loop	A four-port buffer tank by default
8. Backup heat	An add-on option, or an honest fan-heater fallback	None, and no mention of the cusp case
9. The coldest day	Sized to the design temp with defrost margin	"It will always keep up"

1. Design temperature Priority

Good sign Location-specific, about -2 to -5°C, with standard room targets

Red flag "21°C in every room"

2. Heat loss vs your bills Priority

Good sign Cross-checked against your last 12 months of fuel

Red flag "Three installers agreed, so it's right"

3. Flow temperature Priority

Good sign 35 to 45°C, committed to on paper

Red flag 55°C, or "we'll confirm at survey"

4. Projected SCOP Priority

Good sign 3.5 to 4.0, from the datasheet at your flow temp

Red flag "4.5+" or "360%", no conditions given

5. Pipework limits

Good sign Per-room flag of microbore runs over ~1 kW

Red flag "Re-pipe the whole house", no analysis

6. Cylinder size Priority

Good sign 200L for 1-2 people, 250L for 3-4, 300L+ for 5+

Red flag One stock size for every household

7. Buffer or volumiser

Good sign Volumiser, inline on the loop

Red flag A four-port buffer tank by default

8. Backup heat

Good sign An add-on option, or an honest fan-heater fallback

Red flag None, and no mention of the cusp case

9. The coldest day

Good sign Sized to the design temp with defrost margin

Red flag "It will always keep up"

Every heat-loss calculation starts from a design outside temperature, the coldest condition the system is sized to cope with. In the UK that is a location-specific figure, roughly -2 to -5°C depending on where you live, taken from the CIBSE tables the MCS standard uses. It is the foundation every other number rests on, so it is worth checking first.

A good answer. A location-specific design temperature, paired with the standard MCS room targets: 21°C in the living room, 18°C in bedrooms, the hall and the kitchen, and 22°C in the bathroom. Different rooms get different targets because that is how people actually use them.

A weak answer. "21°C in every room." This is the classic hedge against a future "it feels cold" call-back, and it quietly inflates the heat loss for every room that should be 18°C, by something like an eighth. That pushes you toward a bigger, more expensive unit and more radiator swaps than the house actually needs.

Why it matters. The design temperature feeds the heat-loss number, which sets the unit size, which sets how many radiators have to change and what the job costs. Get the assumption wrong at the top and everything downstream is wrong with it.

The heat-loss survey produces a single number in kilowatts, your home's demand on the coldest design day. The most useful thing you can do with it is sense-check it against what your home has really used. Your last twelve months of gas or oil tell you, roughly, whether the survey is in the right ballpark.

A good answer. The installer shows the cross-check, or welcomes yours: annual gas or oil in kilowatt-hours, adjusted for how many hours the heating ran, lands close to the surveyed peak demand. A designer confident in their number is happy to see it lined up against reality.

A weak answer. "Three other installers got the same figure, so it must be right." Three matching surveys usually mean three surveyors reached for the same cautious assumptions, the 21°C-everywhere default and a safety margin on top, not that they each measured your house. Agreement between quotes is not the same as accuracy.

Why it matters. Surveys lean conservative because an undersized system generates complaints and an oversized one rarely does, so the bias runs one way. Your own fuel history is the single independent check you hold. If you are coming off oil or LPG, the cost-and-grants guide covers how that changes the grant and the sums.

Flow temperature is the temperature of the water the heat pump sends to your radiators. It is the single biggest lever on running cost, because the lower the flow temperature, the more efficient the heat pump. A good design targets 35 to 45°C at the design outside temperature. The trade-off is that a lower flow temperature needs bigger emitters.

A good answer. A design flow temperature of 35 to 45°C, stated in the proposal, with radiators sized to give out enough heat at that temperature. The lower the better for efficiency, as long as the emitters keep up. On existing radiators a good retrofit often lands nearer 50°C, which is still fine. It is up around 55°C that the efficiency really starts to drop.

A weak answer. "55°C," which throws away a meaningful slice of efficiency, or "we'll confirm at survey," or "we guarantee the efficiency, not the flow temperature." An installer who won't put a design flow temperature on paper hasn't designed for one, and the efficiency guarantee without a flow temperature attached is a tell that the heating community flags repeatedly.

Why it matters. A radiator's rated output is quoted at a high flow temperature, around 75°C. Drop the flow to 45°C and the same radiator gives out only about a third of that, and at 35°C closer to a sixth. Size the radiators for 49°C, then run them at 35°C, and they physically cannot deliver the heat, and no amount of turning the curve up fixes an emitter that is too small. The property-suitability guide walks through radiator sizing at heat-pump temperatures.

SCOP, the seasonal coefficient of performance, is the efficiency figure that becomes your running cost. A SCOP of 3.5 means the heat pump delivers 3.5 units of heat for every unit of electricity, averaged across a year. For a competent UK design at a sensible flow temperature, 3.5 to 4.0 is the honest range.

A good answer. A SCOP of 3.5 to 4.0, taken from the unit's datasheet at the flow temperature the design actually uses, with an allowance for defrost cycles. Above 4.0 is the top of the range, and it takes low flow temperatures, an R290 unit and careful commissioning to get there.

A weak answer. "4.5 or more" with no conditions, "360% efficient" with no flow temperature attached, or "we guarantee X% efficiency" with no datasheet behind it. One specific trap: some manufacturers' headline datasheet figures, Vaillant's among them, don't include defrost cycles, so a figure lifted straight off the sheet flatters the proposal in cold weather.

Why it matters. The SCOP is the running-cost projection in disguise. An inflated SCOP is an optimistic bill the house will never see. For reference, the Boiler Upgrade Scheme sets a floor of 2.8, the Energy Saving Trust's real-world trial recorded a median of 2.80, and the well-commissioned volunteer fleet on HeatpumpMonitor.org averages near 3.9. The running-costs guide turns SCOP into pounds.

If your home has microbore pipework, the narrow 10mm pipe found in a lot of UK homes plumbed from the 1970s on, and in some modern builds too, the design has to account for it. A 10mm run carries roughly 1 kW of heat comfortably. Rooms that need more than that down a single microbore run need a plan, but the plan is per-room, not the whole house.

A good answer. A room-by-room flag of which runs exceed about 1 kW, with a specific fix for each: re-pipe that one run, pair two radiators, or accept a slightly higher flow temperature on that single zone.

A weak answer. "We'll need to re-pipe the whole house," assuming the worst case across every room, or "microbore is fine," with no per-run analysis at all. The same large installer has been documented giving opposite answers on similar microbore homes depending on which surveyor turned up, so you want a reasoned answer, not a blanket policy.

Why it matters. A whole-house re-pipe is one of the bigger line items a quote can carry, and it is often unnecessary. The cost decision belongs on the handful of rooms that need it, not on a headline number that treats every run as the worst one.

A heat pump heats your hot water in a cylinder rather than on demand the way a combi boiler does. Size it for the household, not for the cupboard the old one fitted. The rough rule is 200 litres for one or two people, 250 litres for three or four, and 300 litres or more for five and up.

A good answer. A cylinder sized to how many people live there, on a heating coil big enough to charge it quickly at the heat pump's low flow temperature. A heat-pump cylinder needs a much larger coil than a boiler one, so a stock boiler-spec cylinder, or an old one kept to save money, can be the wrong cylinder even at the right number of litres. Ask too whether the bathroom towel rails run off the hot-water cylinder or the heating circuit, the former so they still warm through in summer with the heating off.

A weak answer. A stock cylinder size dropped into every job regardless of occupancy. "200 litres works for everyone" is the default a lot of owners quietly regret: it suits a couple, but a busy family of four or five can empty it and then wait on a slow heat-pump reheat.

Why it matters. The cylinder is the part you can't easily change later. Swapping it after the install means pulling plumbing back apart, so the size on the proposal is effectively the size you live with. It is worth getting right while it is still a line on a quote.

Some designs add a tank of water between the heat pump and the radiators. There are two kinds, and the difference matters. A volumiser sits inline on the circuit and is fine. A buffer tank separates the heat pump's loop from the heating loop, and on a cold day it can quietly cost you efficiency.

A good answer. "A volumiser," a small inline thermal store on the same loop as the radiators. It adds the water volume the heat pump wants without forcing it to run any hotter than the radiators need.

A weak answer. A four-port buffer tank as the standard fit. To get, say, 45°C to your radiators through a buffer, the heat pump has to run hotter than 45°C to overcome the mixing, which lifts the working temperature and drops the SCOP at exactly the moment you most want it high.

Why it matters. This choice is invisible on the headline price and rarely comes up unless you ask. It is also one of the clearest tells of whether the designer is optimising for your running cost or for an easy install. A buffer is occasionally justified on a complex system, so the answer to listen for is a reason, not just a default.

Most UK heat-pump installs have no backup heater, and for a correctly sized system that is fine. The question is whether the installer has thought about the cusp case, a unit sized close to your peak demand in a colder spot, and whether there is a way to add support later if it turns out you need it.

A good answer. Either a designed-in backup option (rare in the UK, standard in colder countries like Canada), an add-on path such as a hydraulic module that can be fitted later on the units that support it, or an honest "on the two or three coldest mornings a year, a plug-in fan heater covers the gap for a few pence."

A weak answer. No backup, no add-on option, and no discussion of the boundary case at all. That usually means the risk of the unit sitting right on the edge of its capacity in a cold snap hasn't been worked through.

Why it matters. Whether any of this applies to you is largely geographic. A well-insulated home on the mild south coast almost never needs it. An exposed home in the colder north-east, on a unit sized tight to its heat loss, is a different conversation, and it is better to have it before you sign than during the cold week that exposes it.

The honest answer to this one is the clearest signal of whether an installer has thought the design through. A heat pump is sized for the design temperature, not for the one freak morning a degree or two below it, and a good installer will tell you that plainly rather than promise the impossible.

A good answer. "The unit is sized to hold temperature down to the design outside temperature, with a margin for defrosting. On a handful of colder days the house might sit a degree or two below the setpoint, and a fan heater covers it cheaply." That is what a well-engineered system does.

A weak answer. "Our unit will always keep up," which isn't plausible at the design margin, or no real answer at all. The first is over-promising, the second is a designer who hasn't worked through the boundary.

Why it matters. A unit that can modulate low enough, meaning it throttles right down rather than switching on and off, often beats an oversized one that short-cycles through the mild weather that makes up most of the year. What matters is the modulation floor, not just the headline size. Sizing tight and accepting a fan heater on the worst few mornings is usually the cheaper, more efficient system, not a compromise. The cold-weather guide covers what a UK winter really asks of a heat pump.

The nine questions interrogate the design. Two more things decide whether a good design gets installed without grief, and both live on the quote rather than on the fitting day.

Where the outdoor unit goes. This is the part the nine questions skip, and it sinks more installs than people expect. The position of the box on the wall is a design decision, and it has to clear three hurdles at once: noise, planning, and being reachable for servicing.

A good answer names the spot and shows it passes the MCS noise assessment from there. That calculation only gives a boundary wall its noise credit if the wall completely hides the unit from the neighbour, so a careful installer chooses the position and the unit, height included, to pass the test, with a clear drainage path for defrost water and safe access to service it.

A weak answer is "we'll work out the location on the day." Worth a second look too: a tall unit hard against a close boundary near an upstairs window, which can fail the noise test and need full planning permission rather than permitted development, or a high-wall or flat-roof mount with no permanent safe-access plan, which some manufacturers now decline to warranty.

What the quote actually contains. A heat-pump quote is only comparable once it is based on a home survey. An estimate worked up from your floor area is a starting point, not a number to sign, and the headline price is only the real price once you know what it leaves out.

A good answer is a fixed, itemised price produced after a survey, with the extras either included or named: scaffolding, any electrical or consumer-unit upgrade, the network connection if one is needed, and making good afterwards. You should be able to see the line items, not just one round figure.

A weak answer is a single number with no breakdown, an unsurveyed estimate dressed up as a quote, or "things like the consumer-unit upgrade get priced later." When two quotes differ by thousands it is almost always because they include different work, or rest on different heat-loss assumptions, not because the same job is priced differently.

The bottom line. Run the proposal past the nine questions, then these two checks. If the installer answers them well, you are looking at a competent design from someone who has thought the whole job through, and you can move ahead with some confidence. If they get defensive or vague on two or more, treat that as your signal to get another quote. A good installer will be glad you asked. The ones who aren't have told you something useful.

You may have noticed this guide hasn't pushed an installer on you. That is deliberate. HeatPass makes a single introduction, to one MCS-certified installer who covers your postcode, and only once your home is ready and you have asked for it. The introduction is exclusive to that installer, and your details are never shown to anyone else, unlike the sites that pass your postcode to three or four firms the moment you hit submit.

We have already put these nine questions to the installer we would introduce you to. So get your two or three quotes, ask every one of them the same nine, and line the answers up. The one we introduce has already passed them, which gives you a benchmark for the rest. If someone else answers better, that is worth knowing too. Either way you end up with a design you have checked yourself, rather than one you took on trust.

It starts with the same two-minute HeatPass check the rest of this guide points to: a straight verdict on your home, and the exclusive introduction only on a Full Pass, and only if you want it.