Ground source, air source or water source?

We install three types of heat pumps: Brine to water, air source and exhaust air. Brine to water collects its energy from the ground (boreholes or horizontal loops) or from water (lake, pond or river). Air source collects energy from outdoor air and exhaust air collects the energy from indoor air.

In the table below we have ranked the different solutions in terms of efficiency (A is best) and cost (A is most expensive) assuming that the respective system has been designed correctly.  The efficiency of heat pump systems depend critically on design. Incorrect sizing can result in poor efficiency for all types of systems.

 So what are the pros and cons of the different alternatives?

Ground source – boreholes

Advantages

• The undisturbed temperature in the borehole is higher than in a horizontal loop and a borehole installation should therefore give a higher efficiency (COP) than both horizontal loops and air source.
• Requires less space than horizontal loops.

Disadvantages

• Drilling boreholes is expensive. It is questionable if the extra cost compared to horizontal ground source or air source can be justified by the higher efficiency.

 Ground source – horizontal

Advantages

• Considerably less expensive than a borehole installation.
• More efficient than air source for space heating.

Disadvantages

• Makes a big mess if done in a garden.
• More expensive than air source.
• Slightly less efficient than a borehole installation.

 Water source – pond, lake or river

Advantages

• Typically less expensive than ground source.
• Better efficiency than horizontal. Possibly better than a borehole installation, especially if it is moving water.

Disadvantages

• Most people don’t have access to water as an energy source.
• Might require permission from the Environmental Agency.

Air source

Advantages

• Less expensive than ground source.
• Perfect if you have an outdoor pool and need heating in the summer.

Disadvantages

• Makes a noise which can be disturbing in a garden.
• Not especially nice to look at (looks like an air conditioning unit).
• Might require planning permission in some cases.

Exhaust air source

Advantages

• High COP.
• Low cost.
• Perfect to meet requirements under the Code for Sustainable Homes.

Disadvantages

• Only feasible in relatively small, very well insulated dwellings with mechanical ventilation
• Very sensitive to correct sizing. If slightly undersized, the backup electric heating kick in with a detrimental effect on efficiency.

 Why is ground source more efficient than air source?

Typically the main advantage of ground source over air source is that the ground is warmer than the air during the winter when most heating is needed. This makes it possible to obtain better efficiencies (COP’s) with ground source. The average undisturbed ground temperature over the year reflects the average air temperature, i.e. around 10-12°C in the UK. The deeper you go the smaller the seasonal variation around this average.  Once a collector pipe is installed for a ground source heat pump, it starts to extract heat and the temperature around the pipe decreases. This means that the brine (the liquid in the collector pipes bringing energy from the ground to the heat pump) will be colder than the natural ground temperature. A short collector loop brings down the temperature more than a longer one. “Good practice” for loop design dictates that the loop should be long enough for the brine temperature not to fall below 0°C during winter. For this reason collector loops must not be undersized. Ground source heat pumps with undersized collectors can be less efficient that air source heat pumps which defeats the whole object.

Having said that, ground source heat pumps with correctly sized collectors have a better efficiency over the year than air source.