Ground as a Heat Source for Heat Pumps

The ground can supply reliable environmental heat for a heat pump. Below the surface, soil and rock act as a long-term thermal reservoir. This stored heat can be transferred to a heat pump and upgraded to the temperature level needed for space heating (and, depending on the system design, also cooling).

This page explains the ground as the heat source. It does not describe system sizing, installation planning, costs, or product selection.

What “ground heat source” means

A ground heat source is environmental thermal energy stored in subsurface soil and rock. The heat pump does not generate this heat. It collects it from the ground via a heat exchange interface and then upgrades it for use inside the building.

What this is

• A thermal reservoir outside the building
• A stable source compared with outdoor air in many climates
• A site-dependent heat source that depends on ground conditions

What this is not

• A different heat pump technology
• A fuel or combustion process
• A statement about efficiency on its own (efficiency depends on system design and operating temperatures)

A ground source heat pump uses the ground as its environmental heat source.

Why the ground is considered “stable”

Compared with outdoor air, subsurface temperatures change more slowly over time. The ground has thermal mass and seasonal “lag,” which can reduce short-term temperature swings at the heat collection interface. This is the main reason ground is often described as a consistent heat source through the heating season.

The interface between ground and heat pump

The ground itself is the heat source, but the heat pump needs a heat exchange interface to access it.

Ground heat exchanger

A ground heat exchanger is the component (or infrastructure) that transfers thermal energy from the ground into a circulating heat transfer loop. The exchanger belongs to the heat source side of the system.

Heat transfer loop

A circulating loop transports collected heat to the heat pump. In many systems, the loop uses a water-based fluid with freeze protection.

This separation is useful:

• Heat source side: collects heat from the environment
• Heat pump: upgrades temperature
• Building side: distributes heat through the heating system

The internal process is explained on the Heat pump technology page.

What influences how much heat the ground can provide

Ground heat availability is not the same everywhere. It depends on a set of physical and operational factors:

• Soil and rock composition (how well heat moves through the ground)
• Moisture content (often affects heat transfer behavior)
• Local climate and seasonal patterns (how the ground replenishes heat)
• Heat extraction rate over time (how intensively the ground is used)
• Operating temperatures in the building (higher required temperatures increase the “temperature lift” demand on the heat pump)

A well-matched design aims to extract heat at a level the site can sustain over time.

When ground is a suitable heat source

Ground is typically considered when one or more of the following apply:

• You want a heat source that changes slowly across seasons
• The site conditions allow reliable heat exchange with the ground
• The building can operate with suitable heating distribution temperatures
• You want a heat source that does not depend on short-term outdoor air temperature swings

If site constraints limit ground works, air as a heat source can be more practical.

Where a suitable source is available, water as a heat source may offer stable conditions.

In technical environments, waste heat as a heat source can be an alternative.

When another heat source may be more practical

Another heat source may be preferred when:

• The site has constraints that limit ground works or heat exchange infrastructure
• Planning or approvals for subsurface work are restrictive
• Project timelines favor a heat source with less site dependency
• The system concept is designed around another available source (for example, a suitable water source or recoverable waste heat)

(These are decision criteria, not performance claims.)

Ground compared with other heat sources

Use this comparison as orientation only. A correct choice depends on site conditions and the overall system concept.