So wait a minute... free energy?
That's most people's reaction when they hear about ground source heating for the first time. But if you've got a decent sized back garden or land surrounding your property, you already have a passive store of free energy. Put simply,
- the sun's warmth is for free,
- the earth absorbs and stores it, and
- a ground source heat pump enables you to make use of it in your home.
Ground source heating offers overall the lowest carbon emissions method of heating a building. It creates no pollution on site, and it runs at 3 to 5 times efficiency for the electricity used to run it. In other words: unlike even the best boiler-based systems, which convert less than 100% of their energy input into useable heat, ground source heating actually multiplies the energy you put into it.
It sounds incredible, so let's go into a bit more detail about how it works.
Ground source heat pumps work on the same principle as refrigerators or air conditioners. They take heat from one place and exchange it with another, which can create either a heating or a cooling effect. (This is why you might have noticed your refrigerator is warm at the back -- it's the effect of all the warmth taken out of the chilled compartment).
As a heat source, the earth around us might not feel warm, especially during winter. But that's just at the surface. Below ground, it's a surprisingly constant store of warmth. Regardless of the weather up above, the subsoil (1-2 metres below the surface) stays at a reliable 11-12°C all year round. What's happening is that the ground acts like a colossal rechargeable battery -- absorbing solar warmth that we can put to good use by moving it indoors.
And that's exactly what the heat pump does. Those kind of temperatures don't sound warm enough for heating, but that's where the technology comes in.
Ground source heating works by laying a closed circuit in the ground, containing a water and antifreeze mixture that is circulated by your heat pump. This circuit can be drilled into the ground as a borehole, or more popularly nowadays - installed horizontally in slim trenches in the soil, in a loop form known as the "slinky".
The heat pump is a unit installed just inside your property. It contains a compressor that takes care of the heat conversion, warming the actual heating circuit to its operating temperature.
Applications
The most suitable application for ground source heat pumps is underfloor heating. The heating circuit runs at 30-35°C and keeps the living space warmed through from below. Rather than warming up a cold room like a radiator does, underfloor heating gives a consistent comfortable living temperature.
The specification of a ground source heating system is based on the heat loss from your building. In other words, it depends how well insulated your home is. The less heat that escapes from your house, the smaller system you need.
If you are on (or can get) an Economy Seven tariff from your electricity supplier, running your heat pump only at night will give the best cost efficiency. And for the ultimate green heating system -- you could run the pump on solar energy, or combine it with a solar hot water system.
OK, so how practical is this idea for you and me? There are several factors to bear in mind:
- What you're starting with. Tearing out an existing heating system and replacing it with underfloor heating would be too big a job for most. But if you're looking at a renovation, a new build, or even an extension, ground source heating would be well worth considering.
- Do you have enough land for the ground loop? For horizontal loops the rule of thumb is 10 metres of trench for every 1kW heating requirement. To supply a typical UK home with between 5-8kW requirement, you need upwards of 150 sq m land in order to dig and space the trenches. Alternatively, if you don't have enough land you can used vertical boreholes - although these add to the cost of the project.
- Insulation. If your home leaks warmth, there's no point beginning until the insulation is of a good standard. It's more cost effective to do that than to specify a bigger than necessary heat pump system.
- Cost. This is usually the "scare factor" for any energy efficiency measure. Like any home improvement though, there is a payoff period -- the lower running costs will see you make your money back within a number of years, depending on which fuel you are replacing and how quickly that will rise in price. The installed cost of a heat pump system could be in the region of £4,000 to £10,000 for a professional installation. This depends on size and the type of ground loop you opt for (trench or borehole). It's also possible to install the loop as a DIY measure, trimming hundreds of pounds from the bill.
This is not including the cost of the distribution system, e.g. underfloor heating, which you'd need to price separately.
A bonus is that grants are currently available under the Low Carbon buildings programme.
How soon could a system like this pay off? We suggest between 6 and 12 years, depending on the costs of the fuel being replaced. What nobody can say for sure is how much the fossil fuel and electricity prices might rise in the meantime, possibly bringing forward the moment of payoff.
Our conclusion is that if you're looking at installing a new heating system and can consider underfloor heating, or if you're specifying a new build property,provided you have room for the ground loop, this kind of heating system could be just as good as it sounds.
For further information try one of the following:
Mark Churchill