ASHP do not deserve their reputation as a low-carbon technology, argues David Strong. Oh yes they do, counters Max Halliwell overleaf.

Problems persist with the technology

Professor David Strong
Professor David Strong, CEng, FEI, FCIBSE

Chief executive, Inbuilt Consulting

I was involved in the development of the first generation of air source heat pumps in the late 1970s. It was an exciting new technology – a simple way to absorb heat from outside a building and use it to warm the inside, and smaller and easier to install than its soil-covered cousin, the ground source heat pump.

The technology took some serious knocks in the 1980s because of reliability issues and failure to deliver on its promises. Today, there are many applications where it can contribute to energy efficiency. When used for swimming pool heating in spring, summer and autumn, for example, a well-designed system can be very cost-effective and deliver significant carbon savings. Air source systems can also be effective in recovering energy from building extract systems and industrial processes.

As awareness of their potential benefits has grown so, inevitably, have the marketing promises. There are claims that air source heat pumps (ASHP) provide a “low-carbon technology” for all space heating needs, with manufacturers stating coefficients of performance (COPs) up to 4.

There is, however, a basic flaw that no one is talking about, and it’s to do with the UK climate – specifically when outside temperatures fall below about 5C, as in a big proportion of a typical British winter.

Most of these systems are based on technology originally designed for air-conditioning. Little objective, reliable, independent performance data is available on the seasonal performance of the latest technology being installed in the UK. Despite this, ASHP have been classified by the Department for Business, Enterprise & Regulatory Reform as a renewable low-carbon technology and three manufacturers are listed in BRE’s Green Book as providing “approved products”, which are eligible for grants from the government’s Low Carbon Buildings Programme.

BRE has tested ASHP performance in different operating conditions in the laboratory, and the Energy Saving Trust is planning performance trials. Meanwhile there is a conspicuous lack of independent data on actual seasonal performance.

Trials in the 1970s and early 1980s found a significant amount of energy was wasted via evaporator defrosting. Ice build-up on the evaporator is a serious problem for ASHP, typically occurring when outdoor air temperatures fall below about 5C (as high as 7C for some systems). In extreme conditions COPs fall to less than 1 (ie, worse than direct-acting electric heating).

The most common way of removing the ice is for the heat pump control system to switch the unit into a reverse cycle mode. Then the outdoor heat exchanger (evaporator) becomes the condenser, with hot refrigerant used to remove the ice. In this mode, electricity continues to be used by the compressor and heat is removed from inside the building (ie, the condenser temporarily becomes the evaporator). Other systems use hot-gas bypass or direct-acting electric elements for defrosting.

Clearly, whatever type of de-icing system is used, the energy needed will markedly reduce the seasonal performance of the system. The energy used has a big impact on economic performance too. There are three other serious concerns:

  • noise from the fan and compressor can be intrusive in domestic/urban locations;
  • snow could block the airflow around an ASHP, preventing the system operating unless it is manually removed, which would be unacceptable to many householders (particularly the elderly or infirm); and
  • the heat output from an ASHP reduces markedly as outdoor air temperatures fall, so a system may have difficulty meeting demand when it is most needed. Most manufacturers include an additional heating system to avoid this problem, usually a direct-acting electric flow boiler or a bivalent system which includes a gas- or oil-fired boiler, with all the attendant additional costs and complexity.

These issues caused trouble with the first generation heat pumps in the 1970s and 80s and there are concerns that problems persist with the latest technology despite improvements in compressors, heat exchangers and controls, including the inverter/variable-speed drive systems.

It is also worrying that ASHP, which are specified ostensibly as a “renewable” low-carbon form of space heating, can also be used to provide cooling in summer. This may be great news for electricity companies (by providing additional summer load), but from a sustainability perspective it is perverse. This Trojan horse aspect of ASHP is never mentioned in the context of their green credentials. It is, though, cited by some manufacturers as an extra “benefit”.

I fear that when the true operating performance and/or operational limitations of outdoor air source become known, there will be a negative market reaction to heat pump technologies of all types, as happened in the 1980s. This could have dire consequences for the manufacturers and installers of well-designed ground source heat pumps and air source systems used for heat recovery and swimming pool heating.

We desperately need independent monitoring of external ASHP in different geographical locations to ensure systems originally designed and tested primarily to provide air-conditioning in hot climates can operate efficiently in cold, wet and humid climates.

I also have grave misgivings about the rapid deployment of variable refrigerant volume/variable refrigerant flow heat pump systems in the UK for applications such as offices, retail, hospitality and healthcare. Often, particularly when internal heat gains within a building are low or when installed in poorly insulated buildings, the defrost energy requirements will be extremely high.

Our climate hasn’t changed much since the first generation of ASHP was marketed in the 1970s. Nor have the basic laws of physics and thermodynamics.

De-icing, noise, operation in heavy snow and reduced heat output at low outdoor air temperatures are still big problems today, despite improvements in technology. Before embracing these systems as a renewable low-carbon technology, research to measure performance in the field is needed urgently.

Today’s ASHP have huge green potential

Max Halliwell
Max Halliwell

Product manager, Mitsubishi Electric Heating

Modern ASHP offer a tangible way for each household to reduce energy use. Our Ecodan system, which has been independently tested by BRE, can deliver an average 50% cut in CO2 emissions against a gas-fired boiler. It also makes sense financially, as it can cut running costs by up to 30% over gas. Savings in CO2 and monthly bills are even greater against oil, LPG and direct electric.

As one of the ASHP manufacturers listed in BRE’s Green Book, and one of the few whose whole range is fully accredited to the Microgeneration Certification Scheme (MCS), we welcome the opening of a debate on the seasonal performance of ASHP in the UK. We too are concerned that the low- carbon potential of modern systems will be overshadowed by claim and counterclaim for different manufacturers and competing technologies.

Much of what Professor Strong has stated relates to ASHP technology of 10-20 years ago and bears little resemblance to modern systems. The point about no system working efficiently or reliably below 5C, for example, is easily countered by the evidence of widespread adoption of ASHP across north Europe – 200,000 units last year alone – including Scandinavian countries where average winter temperatures are far lower than the UK.

We undertook trials in four UK locations this winter, when temperatures dropped to -9C in many parts of the country, and believe that Ecodan works effectively down to -20C.

Almost 75% of the energy used in UK homes goes on space and water heating. Ecodan will provide heating and hot water for the majority of UK homes, regardless of location, provided each building’s heat loads are calculated properly and the system sized accordingly. It will work with radiators and underfloor heating systems in all properties, old and new, that have modern insulation standards.

Using technology from our air-conditioning products has helped to reduce noise and maximise energy efficiency as well as to ensure reliability. Ecodan is not an air-conditioning system, however, nor does it provide a reverse cooling function. Detailed modelling has shown that cooling is neither necessary nor sustainable in most homes. What is needed is better ventilation for the few days a year when our bedrooms get a bit sticky. We have decided to forgo the benefits of short-term air-conditioning sales in the residential market and focus on where we believe our technology can make a difference – residential heating.

Coming back to seasonal performance, our winter trials showed average COPs from 3.0 to 3.33 despite some of the lowest recorded outdoor temperatures for decades. These were measured in four properties: a three-bed terrace in Hertfordshire (average outdoor temperature 7C), a four-bed semi near Luton (4C), a large five-bed detached home in Newcastle (6C), and the BRE visitor centre in Watford (4.4C).

We appreciate that these are our own tests but we are also involved in field trials with the Energy Saving Trust. which will provide the “independence” Professor Strong demands. Our literature, like all other manufacturers, lists the COP measured in lab tests. For the Ecodan range, this is 3.85-4.19. We hope our winter tests will show beyond doubt that modern ASHP can offer a reliable alternative to fossil-fuel based heating systems, and provide accurate in-use figures for the market.

The professor’s point regarding defrost cycles is, again, applicable to older-generation, fixed-speed ASHP, which do indeed consume large amounts of energy when defrosting under certain ambient and humidity conditions. Inverter-driven compressor technology means Ecodan has a typical defrost cycle of three minutes, keeping energy use to a minimum, and this is likely to occur about once an hour in exceptional conditions.

Ecodan would not have achieved MCS accreditation without stringent testing, including defrost performance, and the published figures reflect this. The same can be said for noise levels as this is also independently tested as part of the MCS scheme. A modern ASHP is no louder than a condensing boiler.

Following up the professor’s other concerns, heavy snowfall is highly unlikely in the UK and where it does happen regularly, such as Scandinavia, a snow hood can be fitted easily with no impact on performance. In terms of a back-up heating system, if the ASHP is correctly sized and installed then nothing is required, although an electric immersion heater is probably a worthwhile safeguard for any residential system, whatever the primary source.

We see a big future for ASHP as the best mass-market alternative to gas and oil-fired heating and have invested heavily in development. Later this year UK production of Ecodan will start at our factory in Livingston.

We are delighted with the results of our seasonal tests, especially as we know that we will be able to improve on them in real terms, not just in the lab. What the market needs now is enforceable targets for efficiency, independently monitored, so that consumers can have real confidence in the products and the figures.