I’ve done a scary thing this week – I’ve signed up to replace our gas boilers with a ground source heat pump (GSHP for short). I’m in good company though because Cambridge Terrace, the largest residence in London after Buckingham Palace is doing the same, although on a rather larger scale. Switching from gas to renewable heat can be a hard decision because the financial incentives are less attractive compared to switching from oil or electric storage heaters but I’m making the leap for a number of reasons:

  • Our ancient gas boilers are about to die;
  • If I install the GSHP before April 2017 I should get it paid for, albeit over 7 years, through the renewable heat incentive (RHI);
  • The efficiency levels of heat pumps have improved significantly – they are now able to produce high enough flow temperatures to heat older houses; and also
  • In this world of ever rising global temperatures, it feels good to be moving away from fossil fuels.

New research shows that solid walls retain heat far better than previously thought, resulting in GSHPs working well because they run constantly and heat the whole thermal envelope of the house rather than just the air within it. This is similar to the way old houses were heated in the past, with open fires kept constantly lit. If you have an old house though, before going down the GSHP route, you must make sure that you’ve done everything you can to keep the heat in which means stopping draughts and maximising insulation.

How it works. A ground source heat pump circulates a mixture of water and anti-freeze around a loop of pipes buried in a garden or field. Heat from the ground is absorbed by the fluid and then, through the aid of a heat exchanger, is transferred to the water in your radiator system and hot water tank. If you’ve got enough space, the ground loops can be laid horizontally in trenches about 1m deep. If space is a problem then they can sit vertically in deep boreholes.

One thing’s for certain that when you switch to GSHP your electricity bills will increase a lot. The key, however, is to install a system that has a high seasonal coefficient of performance (SCOP). It took me a while to get my head around SCOPs but it is basically a ratio measuring how much electricity you need to put into a heat pump to generate heat output, the seasonal bit just makes sure the ratio covers the whole heating season.  For instance, a SCOP of 4 means that for every 4 kwhs of heat generated you’ve had to use 1 kwh of electricity.  The higher the SCOP of your heating system the less money it will cost you to run it. Remember, that SCOPs don’t simply relate to the heat pump but to the whole heating system and this is particularly important in old houses where heat losses can be considerable. If you live in a well-insulated, modern house then GSHP is ideal.

A few things I’ve learnt along the way. Our system isn’t installed yet so I can’t tell you that we’ve never been so toasty but I can give you a few tips on what to look out for and some of the steps you might need to take.

  1. The RHI payments are paid on the total annual heat demand of your house (both space and hot water). If you have an energy performance certificate (EPC) dated after July 2012 it will have all the information on it to calculate your RHI payments. If you don’t have one then you need to get one.  EPCs are very cheap and as a result many are calculated without the assessor even coming to your house. My advice is to pay a little bit more to get a decent assessor who produces an accurate heat demand figure reflective of your house – if you live in an old house make sure your assessor understands old houses.
  2. You can get an idea of the likely scale of your RHI payments by using the government’s domestic RHI calculator.  The payment tariff for GSHP is currently 19.33p per kwh so on a house where the total annual space heating and hot water demand is 30,000 kwh, the government will pay you £3,500 per year for 7 years amounting to a total payment of £24,500. Depending on the complexity and size of your system, this should easily cover the installation costs.
  3. Like all new technologies, especially when there’s a government incentive scheme in place, there are companies who jump on the bandwagon and install systems that have a high chance of failure.  It is absolutely essential to find the right installer and this means going with recommendations, getting competitive quotes, asking for references, checking warranty periods etc. What I have found, is that the RHI payments over 7 years are basically designed to cover the installation costs. If a company comes in with a quote that enables you to make a lot of money from the RHI then either your system won’t work or they won’t stick to their budget! The Green Homes Network, part of the Energy Saving Trust, has case studies and helps you to find people near you who have installed GSHP.  Three companies that I have been very impressed by their knowledge on GSHP are Isoenergy (installing our system), Eco-Wright and Ecovision.
  4. If you want an idea of how much electricity your GSHP will use then divide the total annual heat demand of your house by the SCOP that your installer tells you they will achieve and multiply that by what you pay for your electricity, e.g. 30,000 kwh/4 x 11.5p = £862 which is what it will cost you per year to run your GSHP.
  5. Finally, two other useful pieces advice that I’ve been given by people who have installed GSHP are 1) don’t undersize your hot water tank – it takes longer to reheat a tank of water using a heat pump than it does using gas or oil; and 2) think carefully where you put your heat pump because although the noise level is low, it is constant and if it’s located below your bedroom it might drive you crazy.

This post feels as if it’s geared to people with a field or at least a garden with access for heavy machinery.  However, there are lots of urban community GSHP schemes being rolled out where multiple houses tap into a single heat loop network. I’ll cover this in another post.