What is a geothermal heat pump and how does it work

A geothermal heat pump is an electrically powered system that moves heat between a home and the ground. In Australia, the name generally describes a ground-source or water-source machine rather than equipment tapping into hot rocks deep inside Earth. Its job is to provide winter heating, summer cooling, or both, using stable ground conditions to reduce the energy needed for comfortable indoor temperatures.

That distinction matters because a heat pump does not create heat in the way an electric resistance heater does. It uses electricity to run a compressor and move heat that is already present. During winter, it brings warmth out of the ground; during summer, it takes heat from the house and transfers it to the ground. The same reversible cycle can handle both jobs.

Australian homes often rely on ducted reverse-cycle air conditioners for year-round comfort, so a geothermal installation may be considered alongside a familiar split system or ducted gas heater. Geothermal equipment can supply air through suitable existing ducts, or it can circulate water through underfloor pipes. In either arrangement, a compressor, refrigerant circuit, heat exchanger and controls do the central work.

The attraction is steady year-round output and potentially lower operating energy use, while the trade-off is a more involved installation. A buried loop or borehole needs suitable space, drilling or trenching, and careful coordination with the home’s heating and cooling layout. Prices, incentives and installation rules differ across Australian states and cities, so local quotations matter more than a generic national figure.

Why the ground is a useful heat source

Ground temperature changes much less than outdoor air temperature over a year. Shallow soil tends to track the annual average, staying cooler than the surrounding air during summer and warmer during winter. A deeper borehole can draw water from an even more stable temperature. This moderates the conditions the heat pump must handle and helps maintain consistent output.

The buried circuit acts as a heat source in winter and a heat sink in summer. Fluid flowing through the loop exchanges heat with the surrounding soil before entering a separate refrigerant circuit inside the building. This does not mean the ground is being used as an unlimited reservoir: the loop must be correctly sized, and the design needs to account for local soil, rainfall and building loads.

Efficiency is another important benefit. A well-designed ground-source system can often deliver around three to five units of heat for each unit of electricity used by its compressor. That figure varies with temperatures, system design and operating conditions, but it explains why electricity use for space heating can be much lower than with resistance heating. The same principle supports reverse-cycle cooling, although performance figures use a different efficiency measure.

How the heat-moving cycle works

A typical system has two connected circuits. The ground circuit usually consists of buried plastic pipes carrying water or a water-glycol solution. A pump moves that fluid between the property and a heat-exchange unit. Horizontal trenches suit properties with open space, while a vertical borehole uses less room. Neither circuit needs exceptionally warm ground to function.

In heating mode, the ground loop transfers warmth to refrigerant in an evaporator. The compressor raises the refrigerant’s pressure and temperature, allowing a condenser to release that heat into an indoor coil. A fan then distributes the warmed air, or the energy heats water for underfloor piping. An expansion valve reduces the refrigerant’s pressure before it returns to the evaporator, completing the cycle.

In cooling mode, the process reverses. The indoor coil absorbs heat from the returning air, and the compressor sends the warmed refrigerant to the ground-loop heat exchanger. Heat is released into the soil, while the cooled refrigerant circulates back indoors. A reversing valve controls the change between heating and cooling, and a thermostat determines when each mode operates.

Compare geothermal with common alternatives

Before comparing equipment, it is worth checking whether the building can retain heat effectively. Guidance on attic insulation explains how the roof space affects comfort and energy use, which shapes the results available from any heating or cooling system.

At the equipment level, the main choices for many Australian homes are a ground-source heat pump, an air-source heat pump and ducted gas heating. The practical differences are:

Option	Heat source	Installation profile	Running characteristics	Main limitation
Geothermal heat pump	Ground or water at a relatively stable temperature	Indoor unit plus a buried horizontal loop or vertical borehole	Usually low electricity use, with relatively stable performance as outdoor weather changes	Higher installation effort and site-specific drilling or trenching costs
Air-source heat pump	Outdoor air	Outdoor unit connected to a split-system or ducted indoor unit	Efficient in mild conditions, though output and efficiency can change in extreme weather	Simpler installation, but an exposed outdoor unit and site-dependent performance
Ducted gas heating	Combustion of natural gas	Flued indoor heater connected to a gas supply	Heat production does not use electricity, but fuel is consumed whenever heating runs	Requires gas availability and leaves fuel costs and on-site combustion in the equation

A ground-source system offers stable conditions and avoids a large fan mounted outdoors, but drilling can be expensive where rock is close to the surface. An air-source heat pump is generally easier to install and often cheaper initially, making it worth considering in many existing homes. Gas heating remains familiar in Australian homes, particularly where a gas supply and suitable flued heater are already installed.

Installation options for Australian homes

The indoor equipment usually sits in a cupboard, underfloor void or garage, while the ground loop occupies trenches or a borehole outside. Horizontal trenches need considerable lateral space and can disturb established gardens. Vertical boreholes suit tighter sites, but drilling access, soil conditions and underground services must be investigated before work begins.

Site assessment is particularly important in established suburbs. Concrete slabs, limited outdoor space and nearby structures can affect the available layout. Drilling contractors may encounter rock, groundwater or buried services, and local council or water-authority approvals may apply. The installer should explain the proposed route, expected disruption and any requirements for ongoing inspection or maintenance.

The building envelope still matters even when the heat source is underground. During hot Australian weather, a poorly ventilated roof space can add substantial heat to the home. Understanding attic ventilation helps identify a separate cooling issue that a ground loop cannot solve by itself. Climate also shapes the priorities: Melbourne’s variable seasons favour reliable year-round heating and cooling, Brisbane’s humid summers make moisture control valuable, and Adelaide’s hot, dry conditions increase the benefit of efficient cooling.

Make a well-informed investment

Geothermal heating and cooling normally carries a higher upfront cost than a comparable air-source installation because of the ground works. The final price depends on the property, loop configuration, indoor equipment and the existing ductwork or underfloor system. A quote should separate these items so that savings, incentives or future changes can be assessed accurately.

Electricity tariffs, household usage and available rebates affect the operating-cost comparison. Government assistance varies by state and may also depend on local council programs, installer accreditation and system specifications. Programs can change, so Melbourne households should check current Victorian requirements, while Brisbane and Adelaide residents should review the schemes operating in Queensland and South Australia. Confirm eligibility before ordering equipment.

Routine maintenance is smaller than the initial installation, but it should not be overlooked. Filters, airflow, refrigerant charge and control settings influence performance. Ice forming on an indoor coil can indicate restricted airflow or another cooling fault; this AC freeze troubleshooting guidance explains the checks that may be needed. Ask the geothermal installer what maintenance the manufacturer requires and which parts remain covered by the warranty.

Contact a qualified geothermal installer and request a site assessment, room-by-room heat-loss calculation, loop layout, installed price and projected running costs. Compare that proposal with a correctly sized reverse-cycle split system and any viable gas-heating replacement. Schedule the assessment with a local contractor before committing to installation.