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Ground source heat pumps (GSHP) are used to heat water for washing etc. and to run underfloor heating systems. Some people also use them to run conventional radiator central heating, but this is very inefficient as GSHP is best at heating water to around 40oC; the system breaks down a bit when trying to produce the much higher temperatures needed for radiators.
Air Source Heat Pumps are similar to GSHP, but use air as the heat source rather than the ground. Air source is slightly less efficient than ground source, but they are much cheaper and easier to install and can be more versatile. However, very cold weather will affect air source heat pumps a lot more than ground source and can stop them working altogether when damp as well. Ice on the evaporators is a terrible thing for heat pumps!
Note that GSHP and ASHP are not really renewable forms of heating, as they need electricity to work. They are efficient, but to make them anything like carbon neutral they need to be run on green grid electricity, or preferably electricity generated by your own wind turbine or solar panel (etc).
How they work. GSHP work along similar principles to a fridge, but in reverse. It takes electricity to run, producing around four times as much heat energy as it consumes electrical energy. This makes them slightly more efficient than standard condensing boilers, but only when heating water up to around 40oC.
They work by collecting the heat from the soil and releasing it into your hot water tank. To do this, a lot of pipework needs to be buried in your garden somewhere. If you have the room, it is best and cheapest to bury it in a trench. A heating-only installation for a three bedroom house would need at least two narrow trenches, each 30cm wide, 2m deep and 40m long. The trenches can be straight or curved and laid in any direction to suit your garden, providing they are always a minimum of 5 metres apart. When building a house or extension, the cost of this work is reduced dramatically when combined with foundation excavations. If there is not room in your garden, the pipework will need to be buried vertically into the ground via a borehole, which is rather more expensive.
Once the pipes are in the ground, they will need to be filled with a mixture of antifreeze and water, then connected to the heat pump. This is about the size of a small fridge, will be located in your house somewhere and contains the same sort of gubbins as a fridge. The antifreeze is pumped around the underground pipes where it heats up a bit (the ground is usually around 10oC), enters the heat pump where it is compressed to squeeze the heat out of it, then expands again to get good and cold before being returned to the ground pipes to heat up. That is how the mildly tepid ground can be used to make hot water.
Once the water is hot, it can be pumped into the underfloor heating system or hot water tank. It is likely that the hot water (for washing etc) will need further heating by a standard boiler or other renewable heating source. The GSHP will get the water to around 40oC really well, but hot water should be kept at around 65oC for domestic use.
GSHP can also be used in a well, stream or in groundwater rather than just a trench in the ground. In fact, the water-based options are more efficient, so if you have a handy well or stream you may not need to dig a trench at all.
ASHP do the same thing as GSHP but, fairly predictably, they get their heat from the air instead of the ground. The slight difference is that the heat pump is usually sited in the garden and is about a big as a large fridge/freezer. The heat is extracted from the air so there is no need for any underground pipework, other that to transfer the heat from the heat pump into the house (unless very close together). Amazingly, ASHP do actually work when it is freezing outside, although they get much less efficient in cold and damp weather.
Specs and cost. GSHP units for domestic application usually come in sizes between 5kW and 8kW, with the larger end able to supply most of the hot water and underfloor heating for a three bedroom house. Systems cost between around £7,000 and £12,000 fully installed; the borehole ground pipe installations will be around the top end of this figure and the trenches the lower. The actual installation process accounts for quite a lot of this cost due to the groundworks, so DIY pipe laying can save a packet. For instance, Navitron sell a 9kW heat pump for £2,100. There will be extra costs for connecting it up to the heating system etc., but I think you can see the potential savings here.
ASHP units are usually slightly cheaper then GSHP, but they take a lot less to install. This usually makes the total installed cost of ASHP much less than GSHP - half the price is a reasonable rule of thumb. ASHP is less efficient but the reduced costs will make payback times appreciably shorter than for GSHP, apart from the colder and wetter parts of the kingdom.
GSHP are cheaper to run than heating systems powered by standard electricity, LPG or oil. However, they are about the same or slightly more expensive than heating by grid-supplied natural gas (due to the electricity they consume to power the pumps, compressors, etc.).
If your heating system currently runs on LPG or other expensive fuels, a GSHP could have a payback time of around 10 years if you pay for installation.
If you currently have a mains supplied, gas-fired boiler the payback will only drop from the cosmic scale if gas prices rise more than electricity prices.
If you have an Economy 7 or Economy 10 deal with your electricity supplier, it will reduce payback times as this electricity is cheaper. You will need a large and well lagged tank to take advantage of this.
If you have or will have a wind turbine or solar PV system, GSHP makes a lot of sense.
GSHP design and installation needs to be done by someone who knows what they are doing, so either get good advice or only think about DIYing the groundwork phase. To this end there are two makers and suppliers of heat pumps that I like.
ASHP case study of a friend suggests that a small four bedroom house that is well insulated and has retro-fitted underfloor heating costs about £600 a year for all heating and hot water. Apparently this includes taking lots of baths. The underfloor heating went straight over the old floor boards and took about 2cm off the hight of the rooms. The ASHP, rather clever control unit and heating system cost about £3,000 fully installed, although my friend is a bit of a property magnate so may have got a good deal.
Navitron are my personal favourites and sell a 5kW or 9kW ground source heat pump at low cost. The site is very useful, with full installation and operation manuals there to download. It is quite techy and gives the impression of great enthusiasm rather than sales quotas. Navitron give some assistance in choosing the right model for your home, but assume you know what you are doing. If you need serious assistance in getting the thing connected and running, they will recommend a local installer you can ask for help. Their ASHP unit is basically an air conditioning unit with a reverse gear, so is only really able to heat one room per unit.
NIBE are a Swedish bunch that manufacture top end, high spec heat pumps for all situations. A little slicker than Navitron, their products are a bit more pricey but come with all the whistles and bells. They have a list of accredited distributors and installers on their website, so it is easy to get in touch with your local bunch. How they quote and design a system will vary as they are all independent of NIBE.
Alternatively, you can do a regional search for certified microgeneration installers at greenbook. They also have lists of certified microgeneration products.
The Feed In Tariff grant system does not cover heat pumps, so you are on your own until the Renewable Heat Incentive comes along.
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