Air-source heat pumps vs electrical heating

Posted by: David Miles

Topic: heating

We often specify and control electric heaters and ASHPs in projects, and thought it would be interesting to compare and contrast what each has to offer. There are 2 main points to consider, the energy efficiency of each, and the capital cost of installation and maintenance.

We have been unable to find a case study that has made a direct comparison between direct electrical heaters and air-source heat pumps in homes. However, the energy models published in the Low Carbon Heat: Heat Pumps in London [PDF] study commissioned by the Greater London Authority (GLA) included both approaches in their comparison of heating efficiency in hypothetical homes.

The GLA's headline result was that mains-powered electrical heaters produced around three times as much carbon per unit of heating energy than air-source heat pumps. However, that headline figure is based on assumptions about the energy efficiency of the different systems that are not supported by data reported from real-world studies.

They assumed their air-source heat pumps would deliver an energy efficiency of 300%: every watt of electrical energy used by the heat pump would deliver three watts of heating energy to the home it is fitted to. When compared to field trials of air source heat pumps in the UK, 300% appears to be overly optimistic. The Energy Saving Trust (EST) reported an average of 245% [PDF] while a  University College London (UCL) trial reported an average of 293% [PDF], although both studies reported considerable variation with many installations delivering much higher or lower efficiency. 

The same analysis was then repeated with the assumption that heat pumps are 20% less efficient than the 300% prediction, placing them in the range reported by the field trials. Using more realistically modelled air-source heat pumps, electrical heaters proved to be the more efficient option but the result is buried in the report's Appendix K.

While the GLA assumed an over-optimistic efficiency for their heat pumps, they also assumed rather high energy demand in their buildings, placing a high energy demand on the electric heaters. Their baseline space and water heating demand for a 70m2 flat with two occupants is 4,200kwh/yr. In Appendix K, they model a building of equivalent size built to what they call 'very high construction quality' and show that with the lower demand, electric heaters are once again more efficient than heat pumps, even with the heat pumps running at the unlikely 300% efficiency.

Further, the model's 'very high' efficiency does not look that high. It assumes the same 70m2 flat has a total space and water heating demand of 2,500kwh/yr, or 36kwh/m2/yr. In our published case study, the demand for space and water heating was below 30kwh/m2/yr. Further, the buildings in the case study were divided into flats accommodating seven people, who would likely take a lot more showers than the two individuals modelled in the GLA study.

Annual space and water heating demand in modelled homes in GLA report and actual homes in Atamate case study.

Source

GLA report:

standard model

GLA report:

high efficiency model

Atamate installation:

14 Cogan Terrace

Atamate installation:

16 Cogan Terrace

Area (m2)

70

70

159

159

Residents

2

2

7

7

Space heating (kwh/yr)

   

1,874

1,960

Water heating (kwh/yr)

   

2,579

2,716

Total (kwh/yr)

4,200

2,500

4,453

4,676

Total (kwh/m2/yr)

60

36

28

29

 

The potential role of heat pumps

We do not argue that heat pumps have no role to play in the future of housing in the UK, and indeed the case study flats used air-to-water heat pumps to extract heat energy from heated air being ventilated out of the building for domestic hot water. However, even by the standards used in the GLA's model, the conclusion that heat pumps are more efficient than direct electrical heating did not hold when either realistic standards of heat pump efficiency or modern construction standards were applied. There does not seem to be any consideration given to the capital cost of heat pumps as well as maintenance costs, something that is an obvious disadvantage when compared to electric heaters.

MHCLG does not recognise the potential for direct electrical heating, stating that they 'anticipate that direct electric heating will play a minor role in our plan for the future' (Section 2.13) and that direct electrical heaters are so expensive to run that they are only likely to be appropriate in buildings with the 'very highest' fabric standards. There is no consideration of the impact that intelligent smart control can have on the running costs of electric heaters. Using an intelligent, advanced control using occupancy as well as temperature as a trigger, electric heaters can be an energy efficient and costs effective way to heat new homes.

They offer no evidence in support of their preference for heat pumps over direct electrical, hence our view is that their position flies in the face of the evidence published in the case studies published by Innovate UK and ourselves as well as the GLA's model.

It is worth again reiterating the point about capital cost. Electric heaters are much cheaper to purchase and to install than air-source heat pumps and do not require the same level of maintenance. When this is factored in, an air-source heat pump would need to demonstrate consistently better energy efficiency to compete with them.

 

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