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Heat pumps (both ‘ground source’ and ‘air source’) are being heavily sold at present. GSHPs work a bit like fridges and freezers by shifting heat from one place to another. With houses they extract heat from the ground which tends to have a constant temperature of about 10°C, increase its temperature a bit (to 30 or 40 or 50°C) and move it into the house. The ground extraction can either be via long loops of plastic pipe buried over a metre deep in your garden or, if space is limited, by sinking a number of bore holes in your garden. There is also a method for using sets of pipe ‘grids’, almost like buried radiators, but this relies on many pipe connections which may leak if there is subsequent ground movement. See below – Copt Hewick
The problem with GSHPs is that they use quite a lot of electricity while operating and the generation of electricity is usually expensive and polluting in the UK context (unlike the hydro electricity in countries such as Canada and Sweden where GSHPs have been pioneered).
If using electricity is the only option for heating then there is a clear choice. -
burn a kilowatt of electricity (say with a fan heater) and get that 1 kilowatt straight into your house
burn a kilowatt of electricity in a ground source heat pump and get 3 or more kilowatts into your house
and the second option wins. The arguement in favour of GSHPs is much greater in areas which are not connected to the natural gas network and these are often rural areas where the space requirement for storing wood fuel are easier to meet.
Electricity comes dear
However, if there is a choice of fuels then the decision is not so easy. Burning mains gas directly in a boiler in your house may be just as efficient and cause less pollution. This is because the electricity for a heat pump is mainly made from oil or coal or gas. Making and distributing electricity is inherently very wasteful – only about a third of the fuel energy ends up as electricity in your house; two thirds gets wasted in the power station (as heat into the air due to inefficiencies) and also a bit in the national grid (where it leaks out of the wires). This is why a kWhkilowatt hour. This is a unit to measure an amount of energy. If you run your 30 kW gas boiler for 2 hours a day you use 60 kWh per day of electricity almost always cost about three times as much as a kWh of gas on your bill!
Burning wood pellets would probably be cheaper and more efficient than either GSHPground source heat pump. A heat pump which extracts heat from the ground or gas and possibly cleaner than a GSHP. Burning seasoned logs causes by far the least CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming pollution
The coefficient of performance (COP)
The COPCoefficient Of Performance. Applied to heat pumps (in heating mode) this indicates the ratio of how much energy they can shift compared with how much they use to do it. So for instance, a ground source heat pump with a COP of 3 will be able to get 2kWh of heat out of the ground for every 1kWh it uses. So it gives out 3kWh when you include the heat off the pump itself. The COP in cooling mode is calculated differently since the heat off the pump is not useful. of a heat pump is the measurement of how much heat it puts out compared with how much electrical energy you need to power it and it tends to range between 2 and 3.5 in practice. In other words, if it is using one kWkilowatt - a measure of how fast energy is flowing. e.g. electricity might flow through an electric kettle at the rate of 2 kW of electricity to run it, it will give out 2.5 – 3.5kW of heat. Each manufacturer states the COP for their heat pumps. This figure is important because if it is over 3 then the heat pump will probably be a better option (ecologically and financially) than burning mains gas. If it is under 3 then it will be creating more pollution at the power station than a gas boiler in your house.
Manufacturers compete to have the highest COPs and they try to measure them under ideal load conditions and with brand new machines. This is how they get figures like 4 or 5.
Load conditions vary in practice and the COP is very dependent on how hot you want the output water. A GSHP finds it quite easy to raise the temperature from 10ºC (underground) to 30ºC and may achieve a COP of 4 at this degree of lift. It works very efficiently and this may be fine for underfloor heating which runs at a much lower temperature than radiators.
If, however you want water to be at 60ºc for radiators and Legionella proof domestic hot water then the heat pump starts to struggle and becomes much less efficient; maybe the COP drops down to 2. Some systems run at 30ºC for the radiators, 50ºC for the DHWDomestic Hot Water. see the section on domestic hot water and then have an ancillary electric heater which briefly raises the DHW to 60ºC each week to protect against Legionella.
The other factor is the age of the equipment. After a few years the COP is likely to drop by a significant amount.
The largest UK study by the ESTEnergy Savings Trust, Getting warmer: a field trial of heat pumps, was recently completed on 83 installations including private and social housing. It doesn’t read too well. Although some of the better examples come out satisfactorily there are far too few of them, with 76% of them not even reaching a COP of 2.8. Problems came from poor installation practices and inappropriate heating systems. Heating controls are also mentioned as a problem but this could apply to any heating installation.
There is also an interesting study which was done a few years ago at Copt Hewick in North Yorkshire. Although this was a rural local authority project to research the viability of retrofitting GSHP technology into retirement bungalows, it is very useful for exactly that reason and the results come out quite well (with a couple of exceptions). It is reasonably well documented and a couple of points to note are the problems of leakage with the ‘panel system’ of pipes in the ground and also the fact that each house was given only one thermostat, which would theoretically contravene the building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) requirement for all radiators to have thermostatic valves. Bedrooms may require to be kept cooler than living areas.
These studies contrasts with the many forums where people are coming up with all sorts of ‘facts’ and figures about GSHPs. See for example this one where all sorts of figures are being bandied about.
The advice is to go very thoroughly into the subject and don’t necessarily believe the ‘ideal’ figures quoted by companies. Try to get a legally binding forecast for your own particular situation especially since the installation of GSHP involves high capital cost. Bear in mind that the EST report above emphasizes the importance of high quality design and installation.
There is an interesting exception to the subject when it comes to PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. design. In this case a small heat pump is used and the heat is introduced into the incoming ventilation air and also into the hot water system. In summer the heat pump can be used to cool the house.
LOL – very funny
see the marketing men having orgasms about heat pumps :)