Ground source or air source

An air source heat pump collects heat from outside air. A ground source heat pump collects it from pipes buried in the ground. Everything else about them is much the same, and almost every practical difference between them follows from that one choice.

The temperature they collect from

British air ranges from about −5 °C on a bad night to 30 °C in a heatwave, and it is at its coldest exactly when the house wants most heat.

Two metres down, the ground barely moves. It sits close to the local annual average air temperature all year, which in Britain is between about 8.5 °C in north-east Scotland and 11.3 °C in the Thames valley, lagging the seasons by a few months so that it is at its warmest in autumn.

On a February morning an air source unit might be collecting from air at −2 °C while a ground source unit collects from a loop at 2 or 3. That difference in the lift is worth a great deal: a well designed ground source system will return a seasonal efficiency around a full point higher than an air source one on the same emitters, which is roughly a quarter off the running cost.

Ground source also has no defrost cycle, because the collector is not exposed to moist air, and no fan, which means no outdoor noise and no MCS 020 assessment.

What it costs to get at

The advantage is bought with groundworks, and they are substantial.

Horizontal loops are pipe laid in trenches about 1.2 m deep, or coiled as slinkies. They need a great deal of land: as a rough guide, two to three times the heated floor area of the house. The digging is straightforward but the garden does not survive it in recognisable form.

Vertical boreholes are drilled, typically 80 to 150 m deep, and need only a few square metres of surface. They perform better and more consistently than trenches, because they reach ground that is deeper and steadier, and they are considerably more expensive. They also need a rig on site, which needs access.

The result is that a ground source installation commonly costs two to three times an air source one. The running cost saving is real, but recovering that difference from it alone takes a long time at domestic scale.

The design problem air source does not have

A ground loop is a finite resource. Heat is extracted from the ground faster than the surrounding earth conducts it back, so the ground around the pipe cools through the winter and recovers over the summer. If the loop is too small for the load, it cools further each year than it recovers, and the system's efficiency declines season by season.

MIS 3005-D deals with this by setting limits on how cold the loop may be allowed to run, and by requiring the design to establish the actual site characteristics, not assume them: the thermal conductivity of the ground, the presence of groundwater, the load profile of the house. MGD 007 carries the sizing tables and the hydraulic design method.

The standard is explicit that simplified methods, look-up tables and nomograms should be used only in limited circumstances. A ground array sized off a chart, on ground nobody characterised, is the single most expensive mistake available in this technology, because putting it right means digging it up.

Air source has no equivalent failure mode. The air outside a badly sized air source heat pump is exactly as warm as the air outside a well sized one.

Where each makes sense

Air source suits most British houses, and that is why it is what most British houses get. It is cheaper, it fits in a garden or on a wall, it needs no land, and a modern unit on a well designed low temperature system will do everything asked of it.

Ground source earns its cost where several things line up: a large heat demand, so the running cost saving is large in absolute terms; land or drilling access; groundworks that are happening anyway, such as a new build or a major renovation; a site where an outdoor unit is unacceptable, whether for noise, planning or appearance; or a shared array serving several dwellings, where the borehole cost is divided.

Two further points. A ground array laid in a new build costs a fraction of what it costs retrofitted, because the ground is already open. And ground source can provide passive cooling in summer, circulating cool loop water through the emitters without running the compressor, which is a genuine advantage as summers warm.

Lifetimes

A heat pump of either kind has a working life of fifteen to twenty years, much like a boiler.

A ground loop does not. Buried polyethylene pipe is expected to last fifty years or more, and it has no moving parts. So the expensive half of a ground source installation is bought once and reused by two or three generations of heat pump, which changes the arithmetic on a long view in a way a simple payback calculation does not capture.