Air source heat pumps vs ground source. Which is best?
Posted by Tasha Kosviner on 22 April 2015 at 3:05 pm
Should I get an air source heat pump or a ground source heat pump?
Heat pumps take heat from the ground, air or water and use it to heat space and water. They are like a fridge in reverse. The fridge takes heat from the food you put in it, and pumps that into the kitchen, keeping the food cold. Heat pumps take heat from the ground, air or water and pump it into your house, keeping it warm.
We are going to leave aside water source heat pumps for now – how many of us really, have access to a private body of water? – and look at the relative benefits of air source heat pumps (ASHP) and ground source heat pumps (GSHP).
What’s the difference between an air source heat pump and a ground source heat pump?
ASHPs draw their heat from the air and consist of an outdoor unit that looks like a big fan on an air conditioner unit and typically measures about 1.5m x 1.5m for a three bed house. This is usually connected to an indoor blower unit which is around the size of a combi boiler.
GSHPs draw their heat from the ground and require the laying of a long loop of pipe about 1m underground. The pipe can be laid either horizontally, requiring an area at least twice the size of the property to be heated, or vertically into boreholes measuring up to 30cm in diameter and up to 120m deep. The GSHP unit can be mounted internally or in an adjoining shed about the size of a bike shed.
There are a number of factors on which we can compare the two.
Because of the amount of external works required for GSHP they are nearly always going to cost more. However the rewards are likely to be greater (see Financial Incentives, below). Both systems run more efficiently at a low flow temperature meaning your radiators need to be larger than normal – and may need replacing in a retrofit. Both systems run most efficiently with underfloor heating, which can be costly to install. All other things being equal, a GSHP in a typical three bed semi will cost in the region of £10,000-£15,000 compared to £5,000-£7,000 for an ASHP.
Ease of installation
In terms of physical disruption, a GSHP is always going to be more complex. They’ll be digging up your entire garden after all! Either that or you’ll need to get a drilling rig in to create the boreholes. This is the main reason GSHPs are costlier. However don’t disregard the practical and regulatory challenges connected to ASHP. It may be necessary to obtain planning approval for the siting of the external unit which can be a headache with an uninformed planning department or suspicious neighbours. Even if your installation is classed as permitted development (ie not requiring permission) you still need to fulfil certain criteria such as undertaking a standard simplified noise calculation. If your unit turn out to exceed 0.6m3 volume, full planning permission will be required.
In general GSHP systems are 10-25% more efficient than ASHP. Thus the long term energy savings with a GSHP are greater than ASHP, and the longer the system lasts the greater the savings. And even when the actual heat pump might need replacing in say 20-25 years, it is a relatively low cost item (less than an ASHP) and the ground array should still be fit for purpose for at least another 25, if not 75, years!
Ease of operation
There is little to choose between the two systems on this. Both GSHPs and ASHPs should have user friendly temperature and timer controls plus more complex controls that deal with the finer operation of the heat pump (e.g. weather compensation function) that should only be set by your engineer. If you do not have an annual service agreement you may be advised to periodically check the pressure gauge within the ground array and record the reading, particularly noting any drop in pressure and advising your installer as appropriate.
Assuming the ground array has been properly installed and tested with no water leaks, GSHPs are like your fridge in that the unit itself really needs no planned maintenance. The array should contain antifreeze and a biocide to stop any bacterial growth in the water. These chemicals have an anticipated lifespan in excess of 10 years so particular attention would be paid to water testing after this period, particularly if a leak were detected.
ASHP are similarly reliable requiring only periodic checking to ensure the external air way and coil surfaces are free of debris.
Many manufacturers offer extended warranties on their equipment provided an annual maintenance plan is entered into so you may be better advised to pay a small annual fee and leave things in the hands of the experts.
Both ASHP and GSHP are eligible for payments through the Renewable Heat Incentive (RHI) and here is where all the foregoing factors combine to make a very compelling case for GSHP. The RHI tends to fund 80-110% of the capital investment cost of the GSHP system (very much dependent on the ground array installation costs) over the seven years of the domestic RHI. After that you can sit back and enjoy the running cost savings which are significant if you’re replacing oil or electric and even possible, although slight, compared to mains gas.
With an ASHP you can expect the seven years of RHI payments to cover 70-80% of the capital investment cost. Thereafter you can expect your system to be far cheaper to run than a comparable oil or electric system, although this time, unless the system is particularly well designed and installed in a very well insulated home (more likely with a new build) it may end up costing you slightly more than central heating using gas.
If you have the space and access – and the money – a GSHP system will bring you greater rewards in the medium to long term. If you do not have the space or the money to invest in a GSHP system then a well designed and installed ASHP system will provide you with an attractive low carbon, low cost, heating solution off the gas grid.
This blog is independently written with expert input from both Graham Hazell from the Heat Pump Association and David Matthews from the Ground Source Heat Pump Association.
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