A Guide to Thermal Response Testing

Thermal response testing (TRT) is an in-situ method for measuring the thermal properties of the ground and borehole heat exchanger. This data is essential for accurate design of ground source heat pump systems, particularly for larger installations.

The test involves circulating heated water through a test borehole while measuring the temperature response over time. Typically, a constant heat injection rate is maintained for 48-72 hours while inlet and outlet temperatures are logged at short intervals.

Analysis of the temperature data yields two key parameters: ground thermal conductivity and borehole thermal resistance. Ground thermal conductivity indicates how readily heat moves through the subsurface. Borehole thermal resistance describes the thermal impedance between the circulating fluid and the surrounding ground.

These parameters enable accurate sizing of the ground loop. Higher thermal conductivity means fewer or shorter boreholes are needed for a given load. Lower borehole resistance improves system efficiency. Without site-specific testing, designers must rely on conservative assumptions that may result in oversized, more expensive installations.

Test interpretation requires careful attention to data quality. Ambient temperature fluctuations, groundwater movement, and equipment issues can all affect results. Professional interpretation considers these factors and provides reliable design parameters.

For large commercial or institutional projects, the cost of thermal response testing is modest compared to the potential savings from optimised design. We recommend TRT for any project with more than 10-15 boreholes or where ground conditions are uncertain.