HEAT PUMPS

70% saving - too good to be true?

At the National Homebuilding & Renovating show, the big topic again this year was heat pumps. The level of confusion around this equipment just seems to keep growing; “do they work, can I run radiators, will I need a back-up boiler?” These questions, and more, seem to recur every year. And with more suppliers making ever-more attractive claims for their products the confusion just deepens.

Heat pumps are a relatively simple technology and one of the best known and oldest of the renewable energy technologies. The first significant heat pump installed in the UK went into the Royal Festival Hall in 1952. Having said that, it is not a renewable energy technology – they run on electricity, so unless you get your electricity from a renewable source it is still fossil fuel based. It is, however, a very efficient way to use electricity. Or at least it can be and a little explanation may be called for.

Types of Heat Pump

There are 3 main types :-

1. Air source
2. Ground source
3. Water source

What this refers to is where the heat comes from. All heat pumps work in basically the same way and it is exactly the same technology as used in a fridge (which is, in essence, an air-source heat pump).  Essentially heat pumps move heat from one place to another (that is why they are called pumps!). They absorb heat from the air, ground or water, extract the heat absorbed, pass it over a heat exchanger which transfers the heat to air or water, which in turn heats our house. Used in that way they are good, efficient technology. It is when they try to do more than just move heat that the problems occur. Increasingly manufacturers are adding things like immersion and electric heaters to increase the apparent output, but which actually reduce the efficiency.

Apart from the heat source, the difference between the 3 types of heat pump relates to potential efficiency. The ground source heat pump has an array of pipes some 1.2m below ground level where the ground is a fairly constant 7⁰C to 12⁰C through out the year. Air temperatures can vary greatly, with summer temperatures of 25⁰C and winter of maybe -2⁰C. Water falls somewhere in between being subject to more variation than the ground but less than air.

Coefficient of Performance

The efficiency of a heat pump is stated as the Coefficient of Performance (COP) and there is more confusion around this figure than any other issue. The Energy Saving Trust suggest that we should look for a COP of 4 – which means that for every 1kW of electricity the machine consumes, it produces 4kW of heat. Manufacturer’s COP figures range from 2.2 to as high as 6. All of which are probably true, but not in the way we may think.

The COP is calculated on the relationship to the heat output and electricity delivered to the compressor pump. An immersion heater, or any other equipment used to increase the output temperature is not included in the calculation. It also relates to the difference between the source temperature (that is the temperature of the air, ground or water we take heat from) and the delivered temperature (that is the temperature of the air or water used to heat the house). The closer those two temperatures are together then the less heat that needs to be extracted and the higher the COP.

So, in the case of an air source heat pump for example, if the outside air temperature is 20⁰C and we want an inside temperature of 25⁰C, the heat pump has very little to do and we can therefore expect a high COP.

Take the Dimplex range for example. This is a reputable manufacturer that gives good, if not clear, information. Their LA-TR air source range offers the following figures :-

Looking at the first line, what these figures mean is that with a heat output of 8.7kW, and outside air temperature of -7⁰C and a delivered water temperature of 35⁰C the machine delivers a COP of just 2.5. Increase the heat output or the water temperature and the COP will fall.

By comparison the Dimplex WI-TE range of water source heat pumps give these figures:-

Notice that the COP in this case varies from 3.6 to 5.2 and that the difference lies in the delivered water temperature. When the delivered water temperature (that is the water temperature we ask the heat pump to produce) rises to 50⁰C the COP falls to 3.6.

Which brings us to the nub of the problem. Heat pumps work best when they are only moving heat from one place to another. Ask them to do more than that and the efficiency drops. To put it simply, underfloor heating needs water at around 35⁰C and heat pumps work well at that temperature. Ask the heat pump to deliver domestic hot water at 55⁰C and the COP falls dramatically.

As an example, on a Barratt’s development in Chorley, Lancashire showed that the average COP for ground source heat pumps was just 2.6, probably because they we being asked to provide domestic hot water as well as space heating.

Why do the manufacturers do it?

In short, because we want them to. To get good efficiency from any type of heat pump means running them with a low output temperature – below 40⁰C. And that means that they cannot be used for radiators or for domestic hot water, both of which have to be 50⁰C or above. Which is inconvenient and means that the home owner will need another heat source. The manufacturers try to be all things to all men and deliver everything we want in one package. The result is an apparently useful package that does everything, except deliver good efficiency. And heat pumps are all about efficiency.

So we have adverts that suggest a heat pump will deliver a COP of 5 and a hot water temperature of 50⁰C – both of which are true, but they are not true at the same time! Suppliers advertise savings of 70% on heating bills, adjacent to an image of a beautiful young lady relaxing in a hot bath. The implication being that you can have both. Which, of course, you can, but you won’t get that 70% saving when you are heating water for the bath.

When is it a good idea?

Only 2 things bear on whether heat pumps are a good idea; the heating distribution and the heat source.

If the heating distribution is anything other than underfloor heating then maybe think of a different boiler. Specifically if the idea is to replace a gas boiler with an air source heat pump running radiators, then forget it. Heat pumps can cope with radiators, and skirting heaters, but they have to be very efficient radiators, over-sized and the COP will still fall.

As to heat sources, the best are :-

1.      Water – a stream or spring, by preference, will deliver all the heat needed and the best efficiency at lowest cost. Boreholes are an option but drilling the borehole can be expensive, or very expensive.

2.      Ground – next in efficiency and about the same capital cost. The critical issue is that there must be enough room to accommodate the pipes buried in the ground. As a rough guide, an area of 50m² is needed for every 1kW of output temperature. So a 10kW heat pump (suitable for the average 4-bedroom house) will need a 500m² garden. Do not be tempted to try and squeeze it into a smaller area. No matter what the salesman says. There is a limited amount of heat in the ground, which is recharged every summer. Putting the ground pipes too close together means that they will extract too much heat too quickly, with a potentially devastating affect on the efficiency of the heat pump. Similarly pipes must not be put under buildings or in areas permanently in shade. 

3.      Air – if all else fails… Air source are always less efficient than water or ground source, but capital cost tends to be lower. They will be least efficient (and have highest running costs) in winter when you need them most and most efficient in summer when you don’t need them at all. Air-to-air is more efficient than air-to-water but needs warm-air heating system. Combined with a mechanical heat recovery system this can prove a good option. Some are reversible, providing cooling as well as heating which can have advantages but there are generally better, more efficient ways of cooling a house.

Typical Cost

The capital cost of equipment will be about the same for water or ground source at around £5000 for a typical 10kW heat pump. An air source will be a bit less at perhaps £3000. But the installation costs will vary enormously. The range could be as great at £500 for an air source to £15,000 to drill a borehole and install a water source heat pump. The Energy Saving Trust suggest a budget of £800 to £1,200 per kW as a guide to the installed cost – and that is probably about right.

Running Costs and Carbon Dioxide Emissions

A house of 200m² floor area, built to Part L standards will require about 11,000 kWh of heating each year (excluding domestic hot water). To provide that heat from conventional sources will cost :-

Below is a rough guide as to what to expect for a 10kW heat pump providing heating to the same house.

The figures above suggest a worst case with electricity bought at standard tariff. Using one of the economy tariffs, Economy 10  for example, will significantly reduce the running cost of the heat pump.

Conclusions

The wrong heat pump, used in the wrong way, makes no economic or environmental sense. An air source heat pump will, over winter and operating at COP 2.5, cost as much as oil to run and more than gas. And will have a capital cost 4 to 5 times that of a high efficiency gas boiler.

And in those circumstances the air source heat pump will still need some other form of heating to provide domestic hot water in winter.

Used in the right way, in the right circumstances, heat pumps are an efficient and cost effective way of heating the home. As part of an integrated system, with solar panels to produce domestic hot water and some, at least, of the electricity also produced on site, heat pumps really come into their own. The typical 10kW ground source heat pump operating at COP 4, running underfloor heating only, will achieve a saving of £150 per year over gas and £200 over oil – increasing to more than £350 if Economy 10 is used. And it will save around 1.2 tonnes of CO₂ each year.

Heat Pumps – Quick Q&A

Q. Will a heat pump supply all the hot water I need?

A. Some can but they don’t do it well and it is an expensive way of heating water

Q. Is a heat pump a boiler or do I need a separate boiler as well?

A. It is not a boiler but will supply your space heating. You will need a separate heat source (probably a boiler) to supply hot water.

Q. Does the size of my garden affect whether I can have a heat pump or not?

A. It does if you are putting in a ground source heat pump. The average house will need a garden of around 500m² for the pipes.

Q. I want to use underfloor heating downstairs and radiators upstairs. Can a heat pump do this?

A. Not easily. The heat pump is great with underfloor heating, but the radiators will need to be at least 50% bigger than normal.

Q. What is the best way to provide hot water if I am using a heat pump for space heating?

A. Solar panels and an immersion heater. The heat pump should be run on Economy 10 electricity tariff which means that when the immersion heater is needed in the winter, it will be cheap to run as well.

Q. Which is the best type of heat pump?

A. Depends on what you have got. Water source if you have a stream, Ground source if you have a big garden (and no stream) and air source if you have neither of the above.