Parts of a geothermal HVAC system and what each does

What is in the underground loop?

The loop is plastic pipe buried in boreholes, trenches or a pond, joined by fused connections so that no joints sit in the ground. It circulates water with antifreeze, which carries heat between the earth and the geothermal heat pump. Pipes from each borehole or trench run to a header, usually in a vault or in the building, where they are collected and connected to the geothermal heat pump. Design choices are compared in horizontal and vertical ground loops.

Questions to ask: what is the total pipe length, what antifreeze is used and at what concentration, and was the loop pressure-tested and flushed before backfilling? Failures here are the hardest to fix, which is why the loop repair and replacement guide exists.

What does the circulation pump do?

One or two small pumps keep the loop fluid moving through the geothermal heat pump. They are often packaged in a flow centre with valves, ports for flushing and a pressure gauge. A pump that runs more than the system needs wastes electricity, and one that is undersized starves the heat exchanger. Variable-speed pumps match flow to demand. The circulating pump is a separate item from the geothermal heat pump, a point that confuses people searching for a geothermal pump. More on that is in the guide to geothermal pump and loop.

What is inside the geothermal heat pump cabinet?

• Compressor. Raises the pressure and temperature of the refrigerant. Single-stage, two-stage and variable-speed versions exist, and staging gives smoother heat and better efficiency in mild weather.
• Two heat exchangers. One exchanges heat with the loop fluid, the other with the house air or water.
• Reversing valve. Switches the cycle between heating and cooling, so geothermal systems cool the home in summer and heat it in winter.
• Expansion device. Lowers the refrigerant pressure so it can absorb heat again.
• Desuperheater, if fitted. A small heat exchanger that preheats domestic hot water with spare heat during the heating and cooling season.

The role of the indoor cabinet is described in the geothermal indoor unit.

How does the heat reach the house?

Distribution depends on the home. Ducted geothermal systems use an air handler with a blower and a coil, plus ductwork sized for the airflow. Hydronic systems send warm water to floor loops or radiators, often through a buffer tank. The same unit can sometimes serve both. Radiant floors suit the lower water temperature of a geothermal system, as described in the radiant floor heating guide, and the distribution side matters as much as the heat pump for comfort.

Many installations add a ventilation system and a backup heater. Whether backup is needed depends on how the geothermal system is sized and on energy use in the coldest weather; the sizing logic is in heat pump load calculation.

What do the controls and extras do?

A thermostat sets the temperature, while the control board in the unit manages compressor stages, pump operation, safety limits and air temperature control, and geothermal systems do not ice up like air-source units. Newer geothermal models report loop temperatures and energy use through an app or a monitoring panel, which helps spot a problem early. Ask which readings the system exposes and who can see them. The monitoring guide shows the same logic for solar equipment.

Keep all the documents the installer provides: loop layout drawings, pressure test records and the commissioning sheet. They matter later for service and for the house sale.

Quick answers