Heat pumps are great. Let’s make them even better
Lessons from a deep dive into heat pump efficiency
Heat pumps are highly efficient heating devices, and their efficiency really matters. The difference between an efficient and an inefficient heat pump system for customers can be hundreds or even thousands of pounds a year in energy bills. As heat pumps are installed in increasing numbers over the coming decade, being able to monitor their performance and ensure they are achieving a good level of energy efficiency will become ever more important. At Nesta we recently undertook a rapid literature review to understand how real world heat pump efficiency data could help us to surface issues and identify adjustments and strategies that heat pump users can deploy.
Key findings
- Studies have found that while heat pump technology is becoming more efficient over time, heat pump systems don’t always perform as well in homes as they do under factory conditions.
- The efficiency of a heat pump system is affected by many different factors — from the equipment itself, through to its design and installation, to the way that it’s used.
- Greater access to efficiency data would lead to a greater understanding of these factors and may unlock opportunities to create tools and services that benefit installers and consumers.
How do we define efficiency?
The efficiency of a heating system is simply the ratio of the heat energy it provides in relation to the energy it uses. Due to conservation of energy, traditional fossil fuelled boilers and electric heaters can’t reach efficiencies greater than 100% as they work by converting other forms of energy into heat — via combustion in the case of a boiler, or resistance in the case of an electric heater.
Heat pumps, meanwhile, can comfortably exceed 100% efficiency because they use electricity to move heat from a heat source such as the air or ground rather than producing the heat from scratch. A new gas boiler will generally have an efficiency of 92–94%; heat pumps can achieve efficiencies of 300–400%, meaning that the heat energy they provide is 3–4 times more than the electrical energy used to power them. Technological improvements have meant that heat pump efficiencies have steadily been increasing over time.
Manufacturers measure the efficiency of their heat pumps in standardised tests which allow for comparison between different models. These theoretical values can differ from what most systems achieve when they’re installed. To some extent this difference is to be expected due to the variation between homes and factory conditions. To account for this, heat pump installers use methods to refine these estimates, taking the property’s heat loss and heating system settings into account.
Yet research by rb&m suggests that heat pump systems in some homes still don’t achieve these predicted efficiencies — either because the prediction methods overestimate efficiency or because of decisions made during the system’s design and installation or a combination of the two.
A more efficient heat pump will use less electricity to heat a property, so it’s important that heat pump systems are made as efficient as possible — both to keep households’ electricity bills low (which also makes heat pumps more attractive as an alternative to fossil fuelled heating) and to reduce the pressure on the electricity grid resulting from increased heating decarbonisation. And it’s important that estimation methods are accurate to protect consumers and avoid overpromising fuel bill savings. But the factors that affect efficiency are varied and interconnected and can be difficult to untangle.
What affects efficiency?
Every home has a specific space and water heat demand (the amount of heat energy required throughout the year to meet its comfort and lifestyle requirements) which depends on the property’s size and thermal efficiency and on the size of the household. Setting aside ways in which the heat demand can be reduced in the first place (such as extra insulation), a heat pump needs to be powerful enough to meet this demand, bearing in mind that an air source heat pump will need to work harder in cold weather and average outdoor temperatures will vary depending on where the property is located. Note that the efficiency of the heat pump is different to the thermal efficiency of the property — a property with minimal insulation may need a more powerful heat pump to meet its heat demand, but this heat pump won’t necessarily be less efficient than a smaller heat pump in a property with lower heat demand.
In practice, it’s difficult to measure the heat demand exactly, so this is estimated by the installer in order to determine the heat pump’s capacity. However, if the real heat demand turns out to be much higher than the estimate, the heat pump will need to work at maximum capacity for longer and may have to resort to backup resistance heating more frequently, lowering the overall efficiency. On the other hand if the real heat demand is much lower, the heat pump will cycle on and off rather than operating continuously, which can use more electricity. It’s therefore important that heat demand estimation methods are accurate to make sure that heat pumps are sized in this “sweet spot” where they’re most efficient.
Another key factor is the flow temperature, which is the temperature at which water is sent from the heat pump to the home’s radiators to provide space heating. In general, the lower the flow temperature, the higher the efficiency. To work at a lower flow temperature, a home will need radiators that are big enough to transmit enough heat to the rooms. This means radiators sometimes need to be replaced as part of a heat pump installation to get the best possible efficiency. Changing radiators is a relatively routine job, but it does slightly increase upfront costs (the UK government estimates this at about £300 per radiator). However, if it allows for the heat pump’s flow temperature to be reduced then the increase in efficiency can pay off in the long run.
The efficiency is also impacted by decisions made during the design and installation process. For instance, if an air source heat pump’s external unit is placed far away from the property then more heat is lost along the way. If it is placed somewhere with limited airflow then the unit will struggle to pull enough heat energy from the air. Errors during the installation could also result in faulty equipment or energy-saving settings such as weather compensation being turned off.
Finally, even if a heat pump system has been designed and installed correctly, the way it’s used can affect its efficiency. Occupants who are unfamiliar with heat pumps may try to operate them like a boiler, running them at a high flow temperature and turning them on and off, even though heat pumps generally perform at their best when running continuously at a lower temperature. User behaviours like this can be due to insufficient guidance provided by the installer or manufacturer, or unintuitive control panels not providing enough feedback about the system’s operation and running costs.
Bearing this in mind leads to several questions.
- Which of these factors have the greatest impact on efficiency, which issues are most common and are current design and installation standards sufficient to ensure that installations achieve maximum efficiency?
- How do we make sure that the design of a heat pump system accurately accounts for the household’s needs and usage?
- How do we ensure that occupants use heat pump systems in such a way that the efficiency is maximised?
- How can prospective customers be given a more accurate indication of the efficiency they are likely to achieve?
How we could answer these questions
One way we could start to address some of these questions is by collecting and sharing more data. Most data relating to efficiency in the UK comes from a small number of field trials, so we have very little visibility over how efficient the majority of installations are.
It’s difficult for an individual household to monitor its heat pump’s efficiency without having a heat meter installed, which generally has only been done by those who signed up for Metering and Monitoring Service Packages (MMSPs) while the RHI scheme was available. If Ofgem were to make MMSP data available for researchers to analyse then it would provide an opportunity to fill in some of the knowledge gaps. So far, only data relating to those for whom metering was required for RHI payments has been made available; these are homes that are unoccupied for over half the year or use backup heating systems, so they aren’t representative of most households.
If consumers had easier access to their own efficiency data then it would empower them to seek redress when their efficiency doesn’t meet expectations. It’s important, however, that consistent measures of efficiency are used to enable comparisons. Measures of efficiency can vary significantly depending which parts of the equipment are included (eg, whether water heating is included as part of the measure, or just space heating) and the time period over which they’re calculated (as the efficiency varies throughout the day and with the external temperature). Heat pumps that have built-in efficiency monitoring aren’t always clear about which measure they’re using, which can make them difficult to interpret.
The Energy Saving Trust’s field trials in 2008 and 2013 found that noticeable efficiency improvements could be made when installers revisited installations and made changes. If this was the expectation in the industry then it could lead to cost savings and a better customer experience. That said, installers are already finding it challenging to meet the current demand for heat pump installations. Data collection and monitoring could lead to tools that automatically diagnose problems, or allow installers to do so remotely, so that aftercare can be targeted where it’s needed most.
As we’ve seen, estimates of heat demand are used to determine heat pump capacities and if these are inaccurate then this can lead to inefficiencies. Current methods of estimating heat demand make assumptions about a household’s lifestyle and heating needs which may not reflect reality. Alternative estimation methods such as BRE’s DAHPSE have been developed which may be more appropriate and could be adopted by the MCS standards. These still don’t take specific household needs into account — but then again, rigidly personalising a heating system to its current residents may lead to problems down the line when a new household moves in.
There are also several technical innovations which show potential. Variable compression heat pumps might make it matter less if the heat pump is oversized because they remain efficient even when the heat demand is low. In their Electrification of Heat trial report, Energy Systems Catapult state that high flow temperature heat pumps can in some cases be more efficient than low temperature alternatives, which is a claim that’s worth exploring further. One other part of a heat pump’s operation that reduces its efficiency is the need to regularly sterilise water by heating it to 60°C, and research is ongoing to find more efficient ways of achieving this.
Summary
While there are improvements to be made, it’s worth bearing in mind that even the least efficient heat pumps are more efficient than gas boilers or resistance heating. But increasing efficiency benefits everyone and is important for making sure that heat pump running costs can be reduced below those of gas boilers in all homes.
To achieve this at scale consumers and researchers need greater access to data. This would allow for a greater understanding of the factors that influence efficiency, better estimation methods, and the creation of tools and services that reduce the burden on installers and empower consumers.
With thanks to Naoise Boyle, Kyle Usher and Andrew Sissons for their contributions.
