Lower temperatures, higher heat pump efficiency

A large-scale study of UK heat pump installations has revealed a major gap between hypothetical performance and real-world data. Changes to commissioning could make all the difference. pv magazine caught up with Oxford University’s Jan Rosenow and Heat Pump Monitor’s Trystan Lea to hear how installers can achieve higher seasonal performance factor (SPF) ratings for their customers.

UK heat pumps have been falling well short of their potential, but a change in approach to commissioning could lead to a step-change in efficiency.

While the UK government expands grant funding for heat pump installations, poor performance from some installations puts the sector’s reputation at risk. Real-world data shows average annual seasonal performance factor (SPF) values of 3.86 can be achieved by a well-designed, well-optimized system. However, the UK government’s large-scale Electrification of Heat trial found heat pump SPFs averaging closer to 2.81.

Poor commissioning could be the culprit, as revealed by analysis of heat pump performance across hundreds of UK homes. Research based on real-world data drawn from the open-source HeatpumpMonitor.org platform found UK heat pumps could be 38% more efficient with good optimization – a significant prize for households in cost saving terms.

Sizing errors

Before a heat pump is sized and installed, the installer will typically estimate how much heat a property would lose on the coldest day of the year. This figure has major downstream effects. Decisions on how powerful the heat pump needs to be and what flow temperature must be reached are made based on heat loss calculations. Overestimating heat loss will leave a lot of efficiency on the table.

The current methodology for heat loss calculations in the United Kingdom is straightforward in principle. The installer will assess the U-values of walls, windows, and roof, account for the building’s volume and air infiltration, apply an estimated outdoor temperature, and arrive at a peak heat demand in kilowatts.

At least in the UK market, these calculations are significantly overestimating actual heat loss. Jan Rosenow from Oxford University’s Environmental Change Institute, who co-authored a peer-reviewed analysis comparing data from the open-source platform HeatpumpMonitor.org with the UK government’s Electrification of Heat trial, pointed to a real-world example close to home. A colleague in his own research group had their home surveyed and received a heat loss estimate at least double what the household was actually losing.

“Something is not right here,” Rosenow said, noting that the problem is not restricted to one methodology – different software, such as Energy Performance Certification-based (EPC) standard assessment procedure (SAP) calculations, or tools used by specialist installers, can inflate figures by varying degrees.

There are a few reasons why. EPC assessments, which underpin most residential heat pump designs in the United Kingdom, are built on standardized assumptions about occupancy, behavior, and building fabric that often diverge from reality.

The latest UK government assessment from May 2026 revealed electric-heated homes used 31% less energy than expected, and 47% less in the month of December, although the sample size assessed was smaller than for gas-heated homes, as less than 10% of UK households are electrically heated. This is also supported in academic literature: studies have identified a consistent “prebound effect” whereby buildings that look poorly insulated on paper actually consume less energy than predicted, partly because residents have adapted their behavior, and partly because default assumptions about air leakage and thermal bridging missed the mark. Reforming this methodology has been discussed in the United Kingdom for years, but implementation on the ground has lagged.

Oversized heat loss

An inflated heat loss figure has a major impact on system efficiency because it drives up flow temperature – the single strongest predictor of heat pump performance. Research from HeatpumpMonitor.org found that the correlation between weighted average flow temperature and annual SPF explained more than half of the variation across 169 monitored systems. Systems achieving an SPF of 4.0 were running at flow temperatures around 36 C to 37 C on the coldest days. Those ­averaging SPF 3.5 were running at average flow temperatures around 39 C to 40 C on the coldest days. Conventional wisdom among installers is to set flow temperatures of 45 C to 50 C, which is a major contributor to low SPF values.

Trystan Lea, co-founder of HeatpumpMonitor.org and co-author of the research, explained the mechanism in simple terms. When an installer sets a heat pump’s weather compensation curve based on an overestimated heat loss, the heat pump “thinks” it needs to reach a much higher flow temperature than the house actually needs. Left to run under pure weather compensation, the indoor temperature would overshoot dramatically. The system therefore relies on a room thermostat to cut it off. This means the compressor runs flat to a high temperature, the thermostat registers the room is warm enough and kills the heat pump, then when the temperature drops the whole cycle repeats.

“The thing that’s really hitting efficiency is not cycling per se, but that it’s running a full load when it is running and that it’s running to a high temperature,” Lea said.

Efficient cycling would involve a heat pump modulating at low output to maintain a steady low flow temperature. That’s not the kind of cycling that was observed in the Electrification of Heat trial systems, contributing to the low SPF achieved.

The problem appears widespread. HeatpumpMonitor.org examined 165 heat pumps from one of the biggest brands in the UK market and found 55% had weather compensation set too high and were cycling on room temperature, with an average SPF of just 2.69. Only 25% had well-configured settings, achieving an average SPF of 3.3 – still well below what can be achieved through proper system design.

Weather compensation settings and heat pump performance
Description	Number of heat pumps	Percentage of total	SPF H4
Weather compensation too high, cycling on room temperature	90	55	2.69
Mixed, harder to classify	34	21	2.85
Weather compensation is overall set quite well; other issues may be present	41	25	3.3
Total	165	100	2.88
Source: ‘Bridging the efficiency divide: open-source insights into UK heat pump performance gaps,’ Energy & Buildings 352 (2026)

Researchers inspected operational data from 165 heat pumps of the same model in the UK government’s Electrification of Heat trial to see how weather compensation affects performance.

Note: SPF H4 includes a comprehensive accounting of electricity use, including by the heat pump itself and other contributors such as any electricity needed to run source-side circulation, internal backup or boost heaters heating distribution pumps, and hot water cylinder charging pumps.

Cost spiral

Overestimating heat loss is a compounding problem. If an installer believes a house needs 10 kW of heating capacity when it actually needs 6 kW, they will fit the larger heat pump. These are more expensive for the consumer, undermining the economic case for switching from a gas-fired boiler. Heat pumps with greater capacities are also harder to commission effectively because they must modulate their minimum output cycle at mild temperatures, and that minimum is a larger share of a modest heating load than would be the case for a correctly sized system.

Installer performance should therefore be a key consideration for consumers, as Rosenow explained.

“Just looking at capex is probably not the right approach,” he said. “Increasingly, it should be really important to see if there is a track record of installing high-quality heat pumps.”

This is because system efficiency has a marked influence on running cost over the life of a heat pump. An SPF of 2.8 – the average recorded in the UK government Electrification of Heat trial – results in running costs roughly equivalent to gas heating when electricity prices are at current UK levels. The HeatpumpMonitor.org average SPF of 3.86 yields savings of around 26%, or approximately GBP 224 ($302) per year for a typical household.

“That is a massive difference in efficiency,” said Rosenow. “You’re talking about maybe 20%, 30%, maybe even 40% difference in running costs.”

Solving the problem

The good news is that solutions exist. For heat pump system designers and installers, low-hanging fruit involves committing to a more accurate heat loss assessment. Methodologies such as the EN 12831-1:2017 standard – the European standard for calculating the design of heat load – can support installers, ideally applied in combination with air permeability testing. Crucially, when more accurate methods yield lower heat loss figures, the response should be to lower the target design flow temperature rather than reduce emitter capacity. Given the choice between running at 37 C or running at 50 C with fewer radiators, the lower flow temperature wins on efficiency.

Lea also highlighted other improvements at the commissioning stage that can provide efficiency gains. Getting weather compensation settings correct following an installation can take time, but can also have a major impact.

More tools are emerging to support this. Third-party optimizers can learn building thermal characteristics and adjust settings remotely, and newer heat pump models are starting to incorporate auto-tuning. Lea argued manufacturers could go further by embedding the algorithms needed to determine correct weather compensation into the devices themselves, removing the commissioning burden from installers entirely.

Rosenow also suggested adding a performance element to the current grant framework that supports heat pump deployment in the United Kingdom. The UK government offers households a cash grant for heat pump installations through its Boiler Upgrade Scheme, but the scheme itself is performance-agnostic. Rosenow argued there is scope to change this without a major administrative burden.

Enhanced grant payments could be offered for verified high-performing installs. Accreditation standards for installers could be introduced that mandate accurate heat loss calculations, with regular reviews of those standards based on real-world evidence. Performance guarantees, as already offered by some specialist installers, could be encouraged or required.

There are lessons here for all markets. The pattern of overestimated heat loss, oversized equipment and poor commissioning is not a UK-specific failure. Field studies in Ireland, Central Europe, and elsewhere have documented the same performance gap between rated and real-world output.

This efficiency gap is the result of working practices that can be modified. As the UK data reveals, just a few changes to commissioning can significantly strengthen the economic case for electrifying heat.

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