Heat pumps - should you believe the hype?

It is not at all clear to the Low Carbon Kid why heat pumps are described as renewable energy technologies. Since they use electricity, they are not. It is especially clear that air source heat pumps, unless powered by renewable electricity, and unless replacing electric heating, should not be used at all.

Read on to find out why... this is an extract from my forthcoming book The Expert Guide To Sustainable Home Refurbishment, to be published by Earthscan next summer.

How heat pumps work

Heat pumps can take heat from the ground, air or a nearby body of water if it’s available. All of them basically work like a fridge backwards.

For example, typically, in an air-source heat pump, air flows over two refrigerant-filled heat exchangers, similar to those in a fridge, one outdoor and one indoor. In the heating mode, liquid refrigerant within the outside coil extracts heat from the outside air, making the refrigerant evaporate into a gas. It then is pumped to the indoor coil, which reverses the process. The refrigerant condenses back into a liquid and returns to begin the cycle again. As the volume of air outside is much greater, the amount of heat in it, when transferred to a smaller volume, results in a higher temperature.

Ground source heat pumps work the same way, but the element containing the coolant is buried in the ground, and so takes the heat from there.

Judging efficiency

Heat pumps are judged by their coefficient of performance (CoP). This is the ratio of the amount of heat produced divided by the electricity consumption of the pump. So for example a heat pump with a CoP of 3 (or 3:1) will produce three times as much heat energy as the electrical energy it consumes. The higher the CoP the better the performance.

You can maximise the CoP by choosing a heating system requiring a lower final water temperature - radiant heating like underfloor or skirting board heating rather than domestic hot water and radiators - and by choosing a heat source with a high average temperature (e.g. the ground rather than air).

The final efficiency will be significantly better for underfloor heating covered by solid screed (tiled etc.) finishes than timber, and/or carpets.

Benefits of heat pumps

Other benefits of heat pumps over conventional boilers include:

• no combustion or explosive gases in the building

• no need for flues or ventilation

• no local pollution (although noise from the outside fan may be a problem if air-source)

• long life expectancy

• low maintenance costs

• the payback period can be as short as 4-5 years and save up to 75% of conventional heating costs.

Ground source heat pumps

These require a network of underground coils or loops to extract heat from the ground. A hole must be dug and the collecting coil buried - usually a closed circuit loop of 20-40mm high-density polyethylene piping filled with a mixture of water and glycol anti-freeze. Holes take two forms: the commonest is a series of horizontal trenches (wet ground is better than dry); or one or more boreholes. The system also includes a heat exchanger, pump and delivery pipes passing under an exterior wall (typically a French window or other door) to the destination.

Care must be taken that the coil makes good contact with the ground. As the depth increases the maximum and minimum soil temperatures begin to lag the surface temperature. At a depth of about 1.5m the lag is about one month. Below 10m the ground temperature remains effectively constant at around the annual average air temperature. Sizing is complex and specialised software is required, available, amongst other places, via the website of The International Ground Source Heat Pump Association (IGSHPA).

Ground-source heat pumps are more expensive but the payback is reduced, financially and carbon-wise, if a hole is being dug anyway, for example for foundations. However they have a long life expectancy (typically 20-25 years and up to 50 years for the ground coil) and are a great idea if the opportunity’s there.

One where the ground is well above freezing (ten degrees) outputting to radiant heating (underfloor or skirting) would be ideal, especially if it is replacing electric heating. It will yield significant carbon savings.

But if it is replacing a modern gas-condensing boiler, which can have over 95% efficiency, the carbon savings are much less.

Air-source heat pumps

Like air-conditioners, they suck in outside air, the units being placed a distance from the dwelling to reduce noise. Despite their reduced efficiency, an advantage of air-source heat pumps over the ground-source variety is their lower installation cost. They are thus more appropriate for renovation projects than ground-source models.

Air-source heat pumps extract heat from the outside air, even in the coldest months. However, the colder it is, the less efficient they become and the more warmth you need.

In mild weather, the COP may be around 4, but at temperatures below around 8°C (17°F) an air-source heat pump can achieve a COP of 2.5 - below the magic 3 level at which carbon savings are realised.

The average COP over seasonal variation is typically 2.5-2.8, but obviously this depends how cold it gets in the winter. As soon as it drops below freezing, the CoP plummets. Of course, it will never reach 1, but it will be much less carbon-efficient than gas or biomass.

Further questions have been raised about the power used by the pump to de-ice itself. Professor David Strong, chief executive of Inbuilt, has observed that “ice build-up on the evaporator of an air-source heat pump is a serious problem, with icing typically occurring whenever outdoor air temperatures fall below about 5°C (this can be as high as 7°C with some systems). In these situations COPs fall to less than one (i.e. worse than direct acting electric heating).”

The de-icing process means that the outdoor heat exchanger becomes the condenser, hot refrigerant being used to melt the ice. But electricity continues to be used by the compressor and pulls heat from inside the building - not you want in cold weather. Some systems use hot gas bypass or direct acting electric elements. Professor Strong has called for an objective assessment of the technology's effectiveness. One is being conducted by the Fraunhofer Institute, and preliminary results show average annual CoP was 2.99. For ground source it was 3.72, significantly better.

Are they noisy? The exterior pump - around 1.2m x 0.7m x 1m tall - generates around 50dB at full fan speed at one metre distance. This is similar to that of an air conditioning unit. The heat exchanger unit, inside - fridge-sized, around 1.8m tall - is about 42dB at one metre distance, similar to a large refrigerator.

Whole house passive stack ventilation with incoming air pre-warmed using a heat pump

Heat pumps can transfer their heat to air or water. If to air, it is directed through vents in the ground floor. The air is drawn through and up the building by pressure differences (heat rises). An advantage of air destination heat pumps is that air into which the heat is passed usually needs a lower temperature than water heating for the same level of comfort, resulting in a higher CoP and increased heat output.

Borderline efficiency

Once the COP descends to 3 or less, if the electricity supplying it is not from a renewable source, and if it is replacing electrical heating, then there is no carbon saving from using the heat pump, since generation and distribution inefficiencies account for two thirds of the energy in the original carbon fuel.

Most of the time air source heat pumps will not achieve a CoP which saves carbon emissions - it will happen only if the air is at or above zero degrees and the target temperature is 35 degrees. A ground source heat pump will perform better.

This is because during the heating season (winter) the outside air temperature is often much lower than the ground temperature (at a depth at which heat is extracted by a ground-source heat pump).

Hot water and heating can be provided 365 days a year; the hot water can be at 55°C; but the CoP will be low, though not as low as 1 - that of a gas or immersion water heater. In other words, it will be more cost-efficient but not as carbon efficient.