There is no dependable geothermal tons-per-square-foot number. A rough estimate may put many homes near one ton per 400 to 600 square feet, but insulation, windows, air leakage, climate and occupancy can move the correct size well outside that range.
Size the heat pump from a heating and cooling load calculation, then size the ground loop separately for the equipment, ground conditions and long-term heat balance. Using square footage alone risks high bills, poor humidity control or excessive backup heat.
What a Ton Means in Geothermal HVAC
One ton of HVAC capacity equals 12,000 Btu per hour. It describes a rate of heat transfer, not the weight of the equipment and not the amount of buried pipe.
If a load calculation shows a 24,000 Btu/h design cooling load, that equals two tons of cooling capacity. The selected unit must also be checked against the design heating load and its published performance at the expected entering-water temperature.
Variable-capacity equipment can operate across a range, but it still needs to be selected from calculated loads and manufacturer data.
Why Square-Foot Rules Are Only a Screening Tool
Two houses of the same size can require very different equipment. The load changes with:
- local winter and summer design temperatures;
- insulation levels and air leakage;
- window area, orientation and shading;
- ceiling height and exposed surface area;
- duct location and leakage;
- internal heat and humidity loads; and
- ventilation requirements.
A contractor should perform a recognized load calculation such as ACCA Manual J for a home. The result should show both heating and cooling loads, not just a proposed tonnage.
Equipment Size and Ground-Loop Size Are Different
The heat pump is selected to meet the building load. The ground heat exchanger is then designed to keep entering-water temperatures within the equipment’s operating range over many years.
Loop length depends on soil or rock thermal conductivity, moisture, borehole or trench geometry, grout, pipe, groundwater movement, spacing and the balance between annual heat extraction and rejection.
A rule such as feet of borehole per ton may be useful for an early estimate, but it is not a final design. Large or uncertain vertical projects may justify an in-situ thermal response test.
Questions to Ask the Contractor
- Can I see the room-by-room heating and cooling load report?
- What percentage of the design heating load will the heat pump cover?
- How much auxiliary heat is expected at the winter design temperature?
- What entering-water temperature range was used for equipment selection?
- How was trench length or borehole depth calculated?
- What soil, rock and groundwater assumptions were used?
- How will the loop be pressure-tested, flushed and balanced?
- What energy use does the model predict at current utility rates?
Bottom line: Use tons per square foot only for a preliminary sense check. The final geothermal size should come from documented building loads and a site-specific ground-loop design.
References: DOE geothermal heat-pump purchasing guidance and NREL ground-source heat-pump design considerations.