Heat pumps are at their most efficient when delivering relatively low temperatures, typically around 45°C. This immediately lends itself to underfloor heating as the preferred delivery method, as this works well at these relatively low temperatures, compared with radiators.
Garden Space Requirements
If you have a large garden, it is cheap to lay pipework in shallow trenches, excavated by a digger. But if garden space is tight, there are alternatives available. You can drill a borehole, at an additional cost of around £3,000, which can accommodate all the ground source pipework, in an area as little as 2 metres x 2 metres. Ice Energy has also developed a ground radiator collection system that enables large installations to be fed from a single trench; and it has even placed some ground source pipes under the footprint of a new house.
Trench collectors work best with relatively wet soils, such as clay, where there is a mechanism for the heat taken from the ground to be readily replaced. In dry ground, such as sand or gravel, the ground itself can eventually freeze because the heat being taken out isn’t replenished. This, of course, doesn’t stop the heat pump working, but low ground temperatures do reduce the efficiency.
Some systems are capable of being run in reverse so as to provide cooling in summer. Generally this comes at a cost, not only in additional installation fees, but in operating efficiency as well. A better option is to pump some of the waste cold air (the fridge in reverse) into cooling units, known as passive cooling. Whilst it’s not quite as effective as reversing the whole system, it is much less energy hungry.
Additional Heat Sources
You may want to harness other power sources to boost your hot water supply. For instance, you may have a stove that can produce hot water, or you may have solar panels fitted. Most heat pumps can accommodate additional heat sources but it is important that the whole hot water supply side is designed correctly at the outset.
One of the favourite applications for heat pumps is to provide heating for swimming pools. It’s something ideally suited to heat pumps as they work at their best when delivering a constant trickle of heat, rather than a few short sharp bursts which typifies how oil- and gas-fired boilers work.
Domestic Hot Water
It’s important to assess your domestic hot water needs carefully because this is an area where heat pumps are arguably not at their best. Most heat pumps are set to deliver around 10kW maximum: this is easily enough to keep even a very large house warm throughout the coldest winter snaps. It is also quite adequate to provide stored hot water for a family. However, the recovery time taken to replace hot water is fairly lengthy in comparison with typical boilers.
The only situation where this is likely to be a problem is if you have heavy use of power showers, which drain the heat out of tanks at between 12 and 20 litres per minute, depending on how hot the water is stored. A heat pump could take an hour or so to replace the hot water, whereas a boiler, rated at 20kWh, would take half the time. If your hot water needs were more modest, then you probably would not even notice any difference.
Standards are becoming progressively tighter but it can’t be emphasised enough that the success of heat pump heating systems is very dependent on good insulation levels in walls, roof and floor and good air-tightness levels throughout the house. A really well-insulated house requires remarkably little heat to stay warm throughout the winter and by reducing your heat demand, you reduce both the installation costs and the running costs of any heating system you might choose to fit.
Noise and Siting Issues
Heat pumps behave to all intents and purposes like refrigerators. Although clean – there are no emissions – they are probably best located in a utility room or garage rather than in living space. Most designs work with large water tanks, often around 200 or 300lts, about double the size of a larder fridge. There are very few house designs that couldn’t incorporate a heat pump but equally it is not something to fit in as an afterthought.
Requirement for Three-phase Electricity
Some larger units are best operated of a three-phase supply. The additional cost of three phase is extremely variable. If it is present nearby, it can often be supplied at no extra cost, and the running costs are identical to normal single-phase supplies. But on some sites, the cost of three-phase supply may be many thousands of pounds — effectively prohibitive. In such cases, it is usually possible to work around the supply restrictions.
There are around a dozen commercially available heat pumps in the UK at the moment. They are by no means identical and are consequently not really interchangeable. The best advice is to choose a manufacturer to work with early on and, if possible, get them to design and even possibly install your whole heating system. It needs an integrated approach, especially if you plan to use other heat sources as well.
Heat Pumps and Global Warming
Heat pumps currently enjoy Government support, by way of an installation grant, as an important technique for combating CO2 emissions. By reducing the heating demand by threefold or even fourfold, the power used to heat a home is reduced by the same amount. However, heat pumps are still a form of electric heating and most electricity in this country is generated by burning fossil fuels. In fact, the production and transmission of electricity around the National Grid is itself a very inefficient process and the CO2 released by producing a unit of electricity is often three or four times as great as it would be just by burning oil or gas. This would appear to all but cancel out any carbon emission savings. However, the supply of electricity is not straightforward and other sources of power are increasingly being utilised, principally nuclear power and renew¬able sources. If you really want to make a difference environmentally, you could consider switching to green tariff (which uses mainly renewable sources) or to a renewables-only supplier such as Good Energy.
How much do heat pumps cost?
A typical 8kW system costs £6,000-£10,000, plus the price of the distribution system around the house, maybe another £2,000-£4,000. That’s expensive when compared to a standard gas boiler system, but the price differential is markedly less when compared to oil boilers. Not only are oil boilers expensive compared to gas, but they require expensive and unsightly tanks.
Funding and Advice
The good news is that funding and advice is available to help with the cost of installing ground source heat pumps, which significantly reduces the cost. In Scotland householders can access capital funding of 30% of the installed cost, up to a limit of £4,000 from the Scottish Community and Householder Renewables Initiative (SCHRI). Call 0800 138 8858 or visit est.org.uk/schri In England and Wales, there are grants available under the Low Carbon Buildings Programme which is introduced in April 2006. Additionally, Ice Energy offers a £1,500 installation grant in conjunction with nPower.
The issue of payback time is one that is frequently raised by would-be green technology installers. In essence, it’s a simple equation, dividing the extra installation costs by the annual running cost savings: if, for instance, a heat pump cost an additional £3,000 to install and it saved £300 a year on fuel bills, there would be a ten-year payback (being £3,000 ÷ £300). In fact, this example isn’t far removed from what people who install heat pumps are currently working on: a ten-year payback is reckoned to be pretty good and beats most other renewable technologies.
There are, however, a number of variables that need to be considered, all of which impact on the payback equation.
1. Fuel costs
At the moment oil is very expensive, at around 35p/litre, equivalent to 3.5p per kWh. Mains gas is costing around 2p per kWh and electricity is available at around 7p per kWh. However, this ratio of oil to electricity prices is unusual; historically, electricity has tended to be between three and four times the cost of oil, whereas currently it’s just twice the cost. At current prices, the payback sums are very favourable to electric heating in general and heat pumps in particular. But many commentators think that electricity prices are set to increase in the medium term and that the old ratio of 1:3 (oil to electricity) will be re-established. This would suggest an electricity price rise to around 10p to 12p per kWh.
Having said that, moves are afoot in the industry to establish special heat pump tariffs which should hold down running costs. There is currently a successful heat pump tariff operating in Jersey.
2. Installation costs
Whilst heat pumps are invariably more expensive to install than gas- or oil-fired boiler systems, just how much more varies enormously from site to site. It may be £3,000, it could be double or treble. Realistically, all you can do is obtain quotations for your specific installation and do the calculations after you have some firm prices.
The work you require from your heating system is a function of the size of your house, its insulation levels, its air-tightness and your lifestyle. By reducing demand, you also reduce the scope for savings in running costs and hence you actually lengthen payback periods for all green technologies.
4. Actual efficiencies
A good heat pump will easily achieve its stated objective of raising water temperatures 35°C at a CoP of 4, sometimes even higher. But for every 1°C extra heat you require, the efficiency of the heat pump drops by around 3%. The temperature lift required is itself dependent on a number of factors.
Firstly, the external ground temperature, which may vary by 10°C or more, depending on ground conditions and the depth the ground loop is installed. Secondly, the temperature of the water circulating in the underfloor heating pipes and the storage temperature of your domestic hot water. The effect of increasing the temperature is to reduce the CoP (efficiency) so that the average CoP of a heat pump throughout a year is considerably less than the advertised rate. Obviously, a reduction in the efficiency adds to running costs and hence lengthens the payback time.
Which begs an important additional question. What is the achieved performance of heat pumps? There is only limited data available on this. The most authoritative source of information is a paper published by the BRE in 2000 called Heat Pumps in the UK – a monitoring report. Its scope was limited to one house over one year so it could hardly be called wide ranging, but it found that the average year round CoP to be 3.16.
Another data-collecting survey is currently underway on eight separate installations in Yorkshire and early indications are that there is a wide range of outcomes with some ostensibly similar households using twice as much power as others. The best of the Yorkshire sample looks to be achieving a CoP of 4, the worst is close to 2 — i.e. only half as efficient. Two key factors which effect this seem to be the preferred space heating temperature and the demand for hot water: where both these are high, the CoP is significantly reduced.