A ground-source heat pump works in the same way as an air-source heat pump. It moves heat from one place to another instead of burning fuel to make it. The difference is where it finds that heat. Instead of taking it from the outdoor air, it takes it from the ground.
Why the ground?
The ground is very good at holding on to heat. Engineers call this thermal mass: the ability of a material to store heat energy. The sun warms the ground over the year, and the soil and rock hold on to that warmth.
Just below the surface, the temperature changes with the weather. A little deeper down it stays fairly steady all year round. It is cooler than a heated room, but in winter it is much warmer than the air on a frosty night.
That steady temperature is the big advantage. On the coldest days, when an air-source heat pump has to work hardest, the ground is still giving up heat at much the same rate.
Collecting the heat
To reach that heat, pipes are buried in the ground. This is called a ground loop. There are two common ways to lay it:
- Horizontal loops: long lengths of pipe laid in trenches, usually a couple of metres deep. These need a fair amount of open land.
- Vertical loops: pipes lowered into deep, narrow holes called boreholes. These take up very little space at the surface but need specialist drilling.
Some systems use a lake, pond or river instead of the ground. Water holds heat well too.
The pipes are filled with water mixed with antifreeze. A pump pushes this fluid slowly round the loop. As it travels through the ground, it picks up a little warmth.
Inside the building
The slightly warmed fluid comes back to the heat pump unit inside the building. From here it is the same process as any heat pump. A refrigerant takes the heat from the fluid and turns into a gas. A compressor squeezes the gas, which makes it hot, in the same way a bike pump gets hot. That heat is passed into the heating system or hot water tank. The refrigerant then expands and cools, like spray from an aerosol can, ready to go round again.
Because the heat source is steady, a ground-source system can stay very efficient through the winter. Efficiency is measured with the coefficient of performance, or COP: the heat you get out divided by the electricity you put in.
Can it cool a building too?
Yes. In summer the process can run the other way, taking heat out of the building and putting it back into the ground. This needs to be part of the design from the start.
Getting it right
Ground-source heat pumps can work extremely well, but they are not right for every site. The ground loop is the part you cannot easily change later, so it has to be designed properly from the start.
If the loop is too small for the building, it can take heat out of the ground faster than the ground can replace it. The system then becomes less efficient over time. The type of soil or rock, the space available, and how much heating and cooling the building needs all affect the design.
Like most renewable technologies, ground-source heat pumps need to be used in the right situations, designed properly, tested and installed professionally. Done well, they are one of the most efficient ways to heat a building.