Geothermal Heat Pumps

A geothermal heat pump, also called a ground-source heat pump, transfers heat between a building and the ground or groundwater.

Instead of relying directly on outdoor air temperature, the system uses comparatively stable underground temperatures as a heat source in winter and a heat sink in summer.

DOE notes that ground temperatures around 30 feet below the surface remain relatively stable year-round, commonly around 50°F to 59°F across much of the United States.

Source: U.S. Department of Energy - Geothermal Heat Pumps

How Does a Geothermal Heat Pump Work?

A residential geothermal system has two major sides:

1. The ground loop or groundwater connection
2. The heat-pump equipment inside the building

In winter, fluid circulating through the ground loop absorbs heat from the earth. The heat pump raises that energy to a useful indoor temperature and distributes it through the home.

In summer, the cycle reverses. Heat is removed from the building and rejected into the ground.

The core refrigeration principle is similar to an air-source heat pump, but the external heat-exchange medium is different.

Is Geothermal the Same as an Air-Source Heat Pump?

No.

Both use heat-pump refrigeration, but they exchange heat with different environments.

DOE notes that geothermal systems are typically more expensive to install than air-source systems but can benefit from the relatively stable temperature of the ground.

Source: DOE Geothermal Heat Pumps

Air-source heat pumps

What Types of Geothermal Ground Loops Are There?

Closed-loop systems

A closed-loop system circulates water or a water/antifreeze mixture through sealed underground piping.

Common configurations include:

• Horizontal loops
• Vertical boreholes
• Pond/lake loops where appropriate

The site, soil, lot size, drilling cost, and local regulations influence which configuration is practical.

Open-loop systems

An open-loop system uses suitable groundwater directly as the heat-exchange medium.

This requires an appropriate water source and introduces water-quality, discharge, permitting, and local-code considerations.

Open-loop should not be treated as a generic substitute for a closed loop simply because the equipment efficiency looks attractive.

How Efficient Are Geothermal Heat Pumps?

ENERGY STAR uses EER for cooling and COP for heating when qualifying geothermal heat pumps.

Current criteria include:

Source: ENERGY STAR Geothermal Heat Pump Key Product Criteria

These metrics are not directly interchangeable with SEER2 and HSPF2 used for standard residential air-source systems.

Why Can Geothermal Be So Efficient?

The ground several feet below the surface changes temperature far less dramatically than outdoor air.

That gives the heat pump a more stable heat source in winter and heat sink in summer.

Air-source equipment may have to extract heat from very cold outdoor air on the worst winter day. A ground-source system works against the much steadier underground environment.

This does not make geothermal "free heat."

The compressor, pumps, blower, and other components still use electricity.

It does mean the refrigeration system can operate under favorable source conditions across much of the year.

How Long Does a Geothermal System Last?

DOE's 2021 consumer guide estimates an average life expectancy of more than 20 years for the heat pump itself and 25 to 50 years for the underground infrastructure.

Source: DOE - Consumer Guide to Geothermal Heat Pumps (2021)

Those are planning estimates rather than a warranty or guaranteed service life.

Actual life depends on equipment quality, water chemistry where applicable, installation, maintenance, operating conditions, and component selection.

The long ground-loop life is economically important because a future indoor-equipment replacement may not require rebuilding the entire underground system.

What Are the Main Advantages of Geothermal?

Stable source temperature

The system does not rely directly on extreme outdoor-air temperatures.

Strong efficiency potential

Ground-source systems can achieve high COP and EER values.

Quiet outdoor environment

There is no conventional air-source condenser fan operating outside the house.

Long-lived ground infrastructure

A properly installed loop can outlast the indoor mechanical equipment.

Heating and cooling from one system

Like other heat pumps, geothermal can provide both.

What Are the Disadvantages?

High upfront project cost

The loop field, trenching, drilling, excavation, or well work makes geothermal fundamentally more capital-intensive than swapping a conventional outdoor unit.

Site dependency

Lot size, geology, water conditions, drilling access, landscaping, and local rules affect what can be built.

Specialized installer requirements

The loop design and installation are not ordinary air-conditioning work.

Payback is not universal

A high-efficiency system can still be a poor investment when installation costs are extreme, the current heating system is inexpensive to operate, or the owner will not stay in the property long enough.

DOE notes that geothermal systems are typically more expensive to install than air-source systems, and that energy savings can offset the higher upfront cost over time.

Source: DOE Geothermal Heat Pumps

Is Geothermal Good for a New Home?

New construction can be one of the strongest geothermal use cases because excavation, mechanical design, and site work can be coordinated before the house is finished.

It is also easier to integrate distribution, mechanical-room layout, and loop installation when they are designed from the beginning.

That does not mean every new home should use geothermal.

High-performance air-source heat pumps can be much simpler and may have better project economics.

Get both systems priced against the same building load.

Is Geothermal Worth It for an Existing Home?

Potentially, especially when:

• You expect to own the property long term.
• Existing HVAC is due for complete replacement.
• The property can accommodate the loop economically.
• Heating and cooling loads are substantial.
• Local energy prices favor electric heat-pump operation.
• Rebates or financing materially improve project economics.

It is less compelling when a retrofit requires extremely expensive drilling or site restoration for a modest annual energy reduction.

Does Geothermal Need Ductwork?

Not always.

Water-to-air geothermal systems commonly use ducts to distribute heated and cooled air.

Water-to-water systems can serve hydronic distribution depending on the design.

The fact that a house lacks usable air ducts does not automatically disqualify geothermal, but it changes the system architecture and cost.

What Should You Ask a Geothermal Contractor?

Ask for:

1. A Manual J load calculation.
2. Proposed loop type and sizing basis.
3. Soil, drilling, groundwater, or site assumptions.
4. Pumping-energy assumptions.
5. Exact heat-pump model and performance data.
6. Distribution-system changes.
7. Expected auxiliary heat, if any.
8. Drilling/excavation scope.
9. Permits and restoration included.
10. Ground-loop warranty.
11. Equipment warranty.
12. Commissioning procedure.
13. Current incentives.
14. Total installed cost.

For a complex property, DOE recommends consulting qualified geothermal designers/installers and notes that local professionals or engineers may be appropriate for assessing site suitability.

Source: DOE Geothermal FAQs

Geothermal vs. Cold-Climate Air-Source

Cold-climate air-source systems have improved substantially, which means geothermal should be compared against the best realistic air-source alternative rather than against old resistance heat.

A useful comparison includes:

• Complete installed price
• Annual modeled energy use
• Electrical upgrades
• Ground-loop cost
• Outdoor-space constraints
• Equipment replacement horizon
• Maintenance
• Rebates
• Financing
• Expected ownership period

Cold-climate heat pumps

Compare system types and the best heat pumps before choosing equipment.

Compare Geothermal and Air-Source Quotes

Get complete project quotes before deciding whether the additional ground-loop investment makes sense for your property.