What is the difference between a 1 pipe and 2 pipe geothermal heating system?

In a one-pipe geothermal system, the supply and return pipe to/from the heat pump all flow from and into the same primary pipe that is one big loop to and from the well-field. The disadvantage of this system was that is more than one heat pump is connected (typical for commercial buildings), then there would be a temperature decrease along the loop because the colder return water from the heat pump (in heating mode – the opposite would occur in cooling mode) would mix with the supply water in the loop going to the next heat pump in line. This is why one pipe systems are more typical for residential applications because there is usually only a single heat pump in the system.

In a two-pipe system, the supply comes into the heat pumps through one primary pipe from the well-field, but the return water goes to a different secondary pipe that goes back out to the well-field. In this configuration there is no mixing of the water in one pipe, so there is no temperature drop from each heat pump. For this reason, two pipe configurations are more common for commercial applications.

That said, there have been recent improvements to the one-pipe system to make them more viable for commercial applications. In an article the October 2009 edition of ASHRAE Journal, Kirk Mescher describes a one-pipe system design that uses a varied water flow rate to determine the temperature in the primary loop.

Advantages of a one-pipe system over a two-pipe system include reduced initial cost due to less pipe and materials ($0.50-$1.50/SF according to Mescher’s article), design simplicity and better maintainability, and decreased energy costs from less fluid use and less mechanical equipment.

I hope this helps!

Best of Luck,
Dave