What Is a Closed-Loop System?

A closed-loop geothermal system circulates a sealed mixture of water and antifreeze through buried high-density polyethylene pipe. The same fluid recirculates indefinitely, exchanging heat with the surrounding earth through the pipe wall without ever contacting groundwater. Loop joints are heat-fused rather than mechanically clamped, so the circuit stays sealed underground.

Also called: closed loop · ground loop

Why it matters to you

Closed loop is the common residential configuration, and the sealed design is why it avoids the water-quality and withdrawal questions an open-loop system raises. Loop joints are heat-fused rather than mechanically joined, which is what allows them to be buried and left alone.

What to do about it

Ask how the loop will be pressure-tested before backfill and whether that test is documented — it is the point at which a problem is cheap to fix. Record the loop field's location and depth; you will want it before any future digging.