Do Air Source Heat Pumps Work in the UK?

The simple answer to this question is probably, yes, air source heat pumps do work in the UK. It might be more accurate, however, to say that air source heat pumps can work in the UK. It turns out that not all heat pump installations are equal.

Air source heat pumps absorb heat from air outside and deliver heat inside the building via underfloor heating, radiators, convectors etc.

You can think of an air source heat pump as a refrigerator working in reverse. Where a refrigerator extracts heat from air inside the fridge and vents it outside, an air source heat pump extracts heat from outside air and makes use of it inside the building. According to The Energy Saving Trust, air source heat pumps can work even at outside air temperatures of as low as -15℃.

Measuring Heat Pump Efficiency

There are two key measures of performance efficiency of heat pumps and if you are looking into the subject you will come across these a lot. They are seasonal performance factor (SPF) and coefficient of performance (COP).

COP is the efficiency of the heat pump or the amount of heat generated for the electricity consumed. A heat pump with COP of 3 will generate 3 units of heat for each unit of electricity consumed under test conditions. The higher the COP, the higher the efficiency of the unit. Similarly, SPF is the annualised COP (i.e. the annual heat generated for the electricity used). SPF is used to account for seasonal variations as outside temperatures change.

How Well Do Air Source Heat Pumps Work in UK Conditions?

A key question that probably everyone faces when thinking about heat pumps for your own applications is do they work in UK conditions? Heat pumps are designed to work with low temperature heating systems. So, the first concern is does that mean a lower temperature for your home? The simple answer to this question is no. A heating system designed with lower flow temperatures does not mean lower room temperatures. A properly designed underfloor heating system, for example, can still keep your house toasty warm. These systems are common in countries much colder than ours.

The next issue you will likely come across is defrosting. At low temperatures the refrigerant in the system can freeze in the outside air and if that is allowed to happen the pump will stop working. Our winters in the UK are pretty mild compared to many places but no-one wants a heating system that is going to break down on the very days you need it most. Just from looking at feedback across the web, it is pretty clear that this is an issue that has caused problems for some consumers.

At the heart of the matter is the question of running costs. Just how much electricity does an air source heat pump consume and can they provide sufficient heat for a reasonable cost under real world conditions in the UK?

Given that air source heat pumps are common in countries that have much harsher winters than the UK, it seems strange that these issues would be a problem here. If air source heat pumps can provide warm, cozy homes and function properly through frozen German winters, it stands to reason that they must be able to work here. Right?

Heat Pump Performance in the UK vs Germany

A comparative study done by CIBSE (Chartered Institute of Building Services Engineers) and Northumbria University gives some insight into the performance of air source heat pumps in the UK as compared to other EU countries like Sweden, Switzerland and Germany. The CIBSE quotes UK field trials conducted by the Energy Savings Trust between 2008 and 2013. The mean SPF for air source heat pumps was 2.45.

Germany (existing) Air 2.60
Germany (new build) Air 2.89
UK phase 2 trials Air 2.45

Phase 1 results for the UK trials found an SPF of just 1.83, which was much improved to 2.45 by phase 2 (more about that later, because it could be important).

What factors make heat pump performance better in Germany than here, despite harsher conditions?

Incorrect heat pump sizing – The optimal sizing of the heat pump for your system and conditions is vital and the margin for error is small. If the heat pump is too large, it will be inefficient. Too small and it will not provide enough heat, resulting in supplementary heating being used too often. It may surprise some, and it is worth noting, that the main culprit here is over-sizing.

Control systems – defrosting is an issue for air source heat pumps when outside temperatures get low. The CIBSE study highlights inappropriate or non-existent use of weather compensating control.

UK mostly has radiators in houses – Air source heat pumps work well with underfloor heating which is deployed more in other countries and is likely part of their superior performance.

Temperature controls with proper buffering are required with ASHP systems.

Phase 1 of the Energy Savings Trust trials took place between 2008 and 2010 and the mean SPF was 1.83. Disappointingly low to say the least. Phase 2 took place between 2010 and 2013 and showed an improved mean SPF of 2.45 for air source. The difference in the 2 phases was essentially improved standards of specifying and installing the systems. Several of the sites had corrective works done between the 2 phases and showed improved performance.

The field trials and the RHPP field trials were used to inform the development of standards and MCS installer standards were improved over time.

Phase 2 of the field trials included the following conclusions:

Heat pumps can provide an efficient alternative for householders.
The technical data obtained and the users’ feedback indicates that well installed and operated heat pumps can perform to a very high standard in UK homes.

Although standards have improved, heat pumps are sensitive to design and commissioning.
The field trial provides early indications that the reasons for underperformance are understood and have been addressed by the new Microgeneration Certification Scheme (MCS) installer standards. The revised standards have led to improved design and commissioning of heat pumps.

Customers provide positive feedback, but require more information.
The majority of the customers were satisfied with the heating and hot water provided by their systems but there were varying levels of understanding amongst customers of how to best use the various controls in order to achieve the best performance from the equipment.

Different aspects of a heat pump system impact on its performance.
Based upon a number of performance calculations, different aspects of the heat pump system can impact efficiency. Customers may benefit from feedback about which parts of the system impact operating efficiency, particularly auxiliary and immersion heaters.

The differences in measured performance suggest that behaviour impacts heat pump systems.
A broader system boundary, which incorporates auxiliary and immersion heaters and pumps, etc., enables the impacts of control and use to be calculated. SPF H4 appears to be the most relevant method to calculate performance as it is less sensitive to end-user hot water usage patterns than system efficiency.

Basically what we are finding here is that air source heat pumps can work in the UK and the field trials actually found very high levels of satisfaction among customers. The efficiency of an air source heat pump is very dependant on correct system design, installation and is sensitive to control systems and user behaviour.

“80% of users interviewed were either
satisfied or very satisfied with the space
heating supplied by their heat pump…”

In 2013, Mitsubishi released results from its own, independently audited, field trial. The year long trial looked at the performance of the Ecodan air source heat pump at 23 sites in the UK. The trial found an average SPF of 2.9 a performance that would likely provide savings even over gas boilers and more so over oil or electric heating.

According to Mitsubishi, the data they gathered was audited and analysed by the Energy Savings Trust using the same methodology as the EST Phase 2 trials above.

What the data shows us, would seem to be an improvement in the performance of air source heat pumps over time. This is not so much the development of the technology but rather a development of the understanding of installers, customers and those setting the standards. As the EST field trials concluded, heat pumps are sensitive to design and commissioning. The performance of the whole system will depend on the design of the whole system, not just the heat pump. Performance will also depend on the control system and how the heating is used by the homeowner.

MCS data gives us an insight into the Design SPF of air source heat pumps being installed over the last number of years.

Average Design SPF for Air Source Heat Pumps

Sep 2015 Sep 2016 Sep 2017 Sep 2018 Sep 2019
3.0 3.0 3.1 3.2 3.3

The above table shows the average design SPF for all air source heat pumps under the domestic RHI at the given time. It’s not clear how well “Design” SPF would correlate with the real world SPF numbers from the field trials and I suppose only more field trials will answer that question. The numbers would appear to indicate however an improvement in the design efficiency of systems over time and this can only be a good thing.

Air source heat pumps can and do work in the UK. How well they work, however, is sensitive to the design and use of the overall system and the home. A successful outcome depends strongly on the knowledge of those designing and installing the system and the understanding of the homeowner when it comes to use. Even among satisfied customers, not all homeowners feel they are adequately informed as to the factors that will affect the performance of their system. Given the variability in outcomes, it seems clear that not all installers are adequately educated and skilled.

It is clear that certification standards have a role to play in ensuring air source heat pumps can provide warm, comfortable homes here in the UK as they do elsewhere. It is clear also that finding the right installer is important. One with the knowledge and the ability to help the homeowner understand the system being specified and its use.

For the customer, it is important to understand that a whole house system affects the performance of a heat pump. Heat pumps might form the basis of a great solution for your home but you are not simply buying a machine. The solution will likely involve improvements to insulation standards. Heating use will be different. Probably more constant but lower temperature than the usage patterns we are accustomed to with the boiler systems we had in the past. Probably including changes to the internal heat distribution systems such as moving to underfloor heating or increasing the size of some radiators.

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