Up to three-quarters of homes in study ‘fitted with wrong size’


19 March 2024

Around 70% of homes with heat pumps tested in a study were found to have had the wrong size installed, new research suggests. It means that the householders faced higher energy costs - or a colder home - as a result.


The Department of Energy Security and Net Zero under the Heat Pump Ready Programme funded a joint study by three energy monitoring experts: Build Test Solutions, Veritherm and Elmhurst Energy.

Their findings will fuel the argument of those who point to the success that other European nations have had with heat pump installation.

“The UK Government has set a target of 600,000 heat pumps installed by 2028 but without a more accurate approach to measurement, people will never trust the new green technology and that figure won’t be achieved,” says Richard Jack, technical director at Build Test Solutions.

“Heat pumps are really important for decarbonising heat and achieving net zero but it is crucial that we maximise efficiencies. Before any heat pump is installed, specific heat loss measurement is needed to ensure the correct size for the property’s requirements.

“The performance gap between the assumed and actual heat loss calculations will have significant implications for residents if we don’t make changes, with the risk of higher costs and colder homes.”

The measured and BS EN12831 calculated heat loss was compared for 56 homes, (measuring heat loss through BTS’s SmartHTC system or a Veritherm overnight test) this included several types of dwelling, weighted towards detached and more recently built homes. The calculated heat loss only matched the measurement to within its uncertainty margin for 30% of homes, this means as mentioned that for 70% of homes, a heat pump would be incorrectly sized using a BS EN12831 heat loss calculation which takes into account various factors such as the building's thermal characteristics, outdoor climate conditions, and internal heat gains.


Oversized heat pumps cost more to run

Measurement provides accurate heat loss assessments, bespoke to individual houses.

Traditional heat loss calculations based on visual surveys are prone to inaccuracy resulting in incorrectly estimating the actual heat loss. This is because critical factors like airtightness and the presence and continuity of insulation are hard if not impossible to identify.

The research found that the difference in heat loss between that calculated and the measured heat loss was a substantial 35 percent. There was a significant bias towards overestimating the heat loss; 59 percent of houses. An oversized heat pump incurs higher capital costs, of around 10 percent on average, alongside higher maintenance costs and lower running efficiency. With an undersized heat pump, the risk is that the home will not be warm enough during the coldest periods, driving the occupant to turn to more expensive heating methods, such as a fan or oil heater.


Resident feedback

The residents generally found the experience of having their home measured fairly easy. All respondents replied that they would recommend a performance measurement to a friend.

The majority indicated that they would pay between £250 and £500 for the measurement and reporting service. In roughly equal measures interestingly the results showed that their motivations for making energy efficiency improvements were related to cost and carbon, with these scoring higher than to improve comfort or add value to the property.

Overwhelmingly, the results were viewed as very or extremely useful and more than 50% of people receiving the measurements went on to get a heat pump installed in the following 6 months, and more than 60% getting other energy efficiency improvements to the fabric of the house.


Making measurement mainstream

Measuring the thermal performance of a home should form the first step in any consumer heat pump journey.

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The field trial has demonstrated direct measurement of heat loss in buildings causes minimal disruption, helps installers to better manage risk in ensuring the right size heat pump is specified and is something that residents value given the potential capital cost and longer term operational performance savings on offer. The measurements also served to engage the resident, leading to a very high uptake rate of heat pump installation and fabric improvements.

MCS standards should be updated to recognise the role of measured heat loss as part of the heat pump design process for sizing and specifying the correct heat pump solution for a home.

The traditional desktop-based heat loss calculation method only delivered the correct sizing in 30% of cases with an average absolute difference between the predicted and measured heat loss of 34%. This presents significant cost, performance and industry reputation risks in relation to oversizing or undersizing.

Heat loss calculation software should allow for the input of heat loss measurements to calibrate their calculations and provide more accurate outputs.

Heat loss measurements provide a more accurate method to size the overall heat demand of a building, but there are clearly many more steps in the heat pump design process that this does not address. There are several existing software packages which do a great job of more detailed design tasks like specifying the required emitters, pipework or hot water provision. This software should include a user input for heat loss measurements to calibrate their outputs, which would in turn feed down to detailed heat pump design work.

Measurement based heat loss assessments are a valuable additional service that installers should consider integrating into their service delivery model.

Build Test Solutions, Veritherm and Elmhurst Energy’s study has clearly shown that residents see great value in heat

loss measurements; helping to build trust and confidence and ensure the system they commit to is ‘right first time’. A very simple analysis has shown that capital cost savings alone would cover the cost of the measurement in the majority of cases. Heat loss measurements could be conducted either by heat pump installers or dedicated businesses offering measurement services. It’s important to recognise these assessments as valuable services for which people are willing to pay.


To read the report in full and find further resources from Build Test Solutions, Veritherm and Elmhurst Energy visit: www.measuredheatloss.com