How warm the earth stays beneath the surface

Why does ground temperature change with depth?

The surface soaks up summer heat and loses it in winter, so the top metre follows the weather with a delay. Soil and rock conduct heat slowly, so each deeper layer sees a smaller, later version of the surface swing. By a depth of several metres the daily changes are gone. Further down the yearly swing fades too. Deep enough, the ground simply sits at a temperature close to the local annual average, with a slow upward drift in deep boreholes from heat flowing up from the earth's interior.

What temperatures can you expect?

Exact numbers depend on location, soil, snow cover and groundwater. As a rough guide, the steady deep-ground temperature is near the long-term mean air temperature of the place, usually a little higher. Across southern Canada that is typically somewhere from low single digits to around 10 degrees Celsius, and colder in the north. For a specific lot, ask a local driller or designer for measured values, since design should rest on site data rather than a national figure.

Snow acts like a blanket, which can keep shallow soil warmer than bare ground in the same winter. Dry, sandy soil behaves differently from wet clay or rock.

Where is the frost line?

The frost line is the depth to which ground freezes in a normal winter. It is set locally in building rules and is deeper in colder regions, commonly more than a metre on the Prairies. Footings and buried water lines go below it. Horizontal ground loops are usually installed near or below that depth, but they still feel seasonal swings, which is one reason the choice between trenches and boreholes matters (see horizontal vs vertical ground loops).

Why does steady ground help heat pumps?

A heat pump works best when the source is warm and stable. In late winter, air can be far below the temperature of the ground at depth, so a ground-source unit stays efficient when an air-source unit has to work harder. The cycle explained in how ground source heat pumps work depends on the loop fluid arriving at a decent temperature. Take too much heat out of the same ground over many winters and temperatures drop, which is why sizing and spacing are so important.

Why does steady ground help storage?

The same property that makes ground stable also makes it slow to lose heat. That is the principle behind borehole thermal storage. At Drake Landing, solar heat from 800 collectors was pushed into 144 boreholes during summer, warming a block of earth that approached 80 degrees Celsius in the core by late summer. The district loop then drew that heat out in winter. Because the ground was warmed gradually over several years, output climbed toward full coverage; the borehole storage page covers this, and the BTES guide explains the mechanics.

How can you find the ground temperature at your site?

The practical answer is to ask the people who drill. Local drillers and geothermal designers keep records of undisturbed ground temperatures from earlier jobs, and for larger systems a thermal response test measures the real value and the ground's conductivity in an actual borehole. Geological surveys and provincial groundwater databases can give hints about rock type and water, which affect how heat moves.

For a quick feel, note that a basement stays remarkably steady through the year because it sits within that stable zone. The same logic is behind the design advice in geothermal borehole depth and spacing, where depth is chosen partly to reach steadier ground.

Quick answers