Walk into almost any conversation about residential heat pumps and the question that comes up first is the brand. Vaillant or Viessmann? Daikin or Mitsubishi? Grant or Samsung? It’s the wrong question, and it’s the reason a large share of UK heat pump installations underperform their brochure ratings within their first year.
The data is now too consistent to ignore. Public real-world performance data on platforms like Heat Pump Monitor shows seasonal coefficient of performance (SCOP) figures ranging from below 2.0 to above 4.5 on identical hardware in similar properties. The spread is the install, not the unit. And the single biggest variable inside that install is the heat loss calculation that should sit at the top of every design.
This article is about what that calculation does, why skipping it dooms the project regardless of the unit fitted, and what the specifying process looks like done properly.
The Brand Debate Misses the Point
Every major heat pump manufacturer in the UK market produces units that are technically capable of achieving SCOP 4.0+ in a well-designed system. The hardware specifications have converged. R290 refrigerant adoption, variable-speed compressors, modulating output, and weather compensation are now near-universal at the upper end of the market. Differences in factory performance are real but small in the context of typical residential installations.
What blows real-world performance apart is the way the system is sized, designed, and commissioned for the specific property. A heat pump running at 50°C flow temperature consumes roughly 30% more electricity than the same heat pump running at 35°C on the same property. The hardware is identical. The design is different.
The reason flow temperatures end up wrong is almost always the same: the heat loss calculation was rushed, templated, or skipped entirely. Without an accurate room-by-room figure for the property’s heat demand at design temperature, the installer cannot correctly size emitters, set flow temperatures, or pick a unit. So they pick a unit on the high end, run it hot to cover their bases, and walk away. The system technically works. It just costs the homeowner significantly more to run than the brochure promised.
This is precisely why a heat pump installation designed around your home starts with the heat loss calculation, not the unit selection. Get the calculation right and the unit specification follows logically from it.
What a Heat Loss Calculation Actually Tells You
A proper heat loss calculation, run to BS EN 12831 or equivalent, produces a room-by-room kilowatt figure for the property at its design outdoor temperature. For most of the UK that’s around -2°C to -3°C, depending on region. The total figure tells you the size of heat pump the property needs. The room-by-room breakdown tells you which radiators are big enough, which need upgrading, and what flow temperature the system can sustainably run at.
That last point is the part most installations get wrong. The flow temperature isn’t a setting; it’s a consequence of how much heat each emitter has to deliver and how much surface area it has to deliver it. A radiator sized for a gas boiler running at 70°C might only deliver a third of its rated output at 45°C. If the heat loss calculation reveals the room needs more output than the emitter can deliver at the target flow temperature, the radiator has to be upgraded or the flow temperature has to rise. Most installers raise the flow temperature, because it’s quicker. The homeowner pays for that shortcut every winter.
The calculation also flags the hot water demand pattern, the cylinder size needed, the pipework requirements, and the outdoor unit placement constraints. None of these are optional. All of them affect the install’s real-world running cost, which can vary by hundreds of pounds a year on the same property depending on how the system was designed. The depth of that variation is covered in detail in our running cost analysis for Berkshire and Surrey homes.
Why Identical Properties Deliver Wildly Different Running Costs
Take two semi-detached 1960s homes on the same street with comparable insulation. In one case, the installer runs the heat loss calculation, correctly specifies an 8kW air-to-water heat pump, identifies four radiators that need upsizing, calibrates the system to run at 38°C, configures weather compensation, and schedules domestic hot water on a smart tariff. Annual running cost: £450 to £550.
In the other case, the installer skips the heat loss calculation, fits a 13kW unit “to be safe,” leaves the existing radiators in place, commissions the system at 55°C, runs domestic hot water at peak rate, and disables weather compensation because it generated a single cold-room complaint in week two. Annual running cost: £950 to £1,200.
Same property type. Same weather. The difference is design and commissioning, and it starts with whether the heat loss calculation was done before the unit was chosen. An oversized heat pump short-cycles, runs hot, and wears out faster than a correctly sized one. Multiply this pattern across the UK’s heat pump fleet and you get the spread visible in monitored performance data.
The Specifying Process That Separates Good Installs From Bad Ones
For architects, specifiers, and main contractors involved in retrofit projects, the indicators of a properly designed heat pump installation are reasonably straightforward to check at quote stage.
A credible quote includes the heat loss figure for the property and shows it room by room, not as a single total. It specifies the target flow temperature and explains the emitter schedule that supports it: which radiators stay, which are being upgraded, and why. It documents the cylinder size and the domestic hot water strategy. It identifies the outdoor unit location with the MCS 020 a) noise calculation already run. It explains what weather compensation configuration the system will use at handover.
A weak quote skips most of this and quotes a unit size with a price. The unit might be from the same manufacturer as the strong quote. The result will be very different.
What This Means for Architects, Specifiers, and Homeowners
The implication for anyone advising clients on a heat pump installation is to push the conversation off brand and onto design discipline. The questions worth asking installers are not which manufacturer they prefer. The questions are: do you produce a room-by-room heat loss figure? What target flow temperature is your design built around? Which radiators will need upgrading? How is the system commissioned and what happens if performance falls below specification in the first year?
The heat pump industry doesn’t have a hardware problem. It has a design discipline problem. The brand on the outdoor unit matters far less than the calculation that should have determined which unit to specify, where to put it, and how to run it. Anyone procuring a heat pump installation, whether for their own home or on behalf of a client, who internalises that single point is already ahead of most of the market.
