Why Does Geothermal Loop Pressure Change With Temperature?

By Dave Hartzell, Master HVAC, IGSHPA Accredited, ClimateMaster GeoElite dealer, 47 years in the trade. Hartzell’s Heat & Air, Kingfisher, Oklahoma. Updated October 3, 2026.

Why does my geothermal loop pressure go up and down?

Because the fluid in a closed loop changes temperature all year. In summer the heat pump dumps your house heat into the loop, and in winter it pulls heat out. The polyethylene pipe and the fluid in it both grow and shrink with temperature, and the loop is sealed, so the gauge moves. ClimateMaster’s loop leak bulletin says low loop pressure “may be the result of pipe expansion from seasonal fluid temperature changes not a leak.” Its Tranquility 30 installation manual says loop static pressure “will fluctuate with the seasons,” with higher pressure in the winter months than during the cooling season, and calls that fluctuation normal. ClimateMaster units built with the internal flow controller carry an expansion tank that soaks up some of that swing. The same bulletin calls it normal for a pressurized loop to need at least one more top-off after a complete heating and cooling season. So a different number in January than in July is not, by itself, a reason to call.

What should geothermal loop pressure be?

There is no one number for every house. When a ClimateMaster loop is filled at startup, the Tranquility 30 installation manual says to pressurize it to at least 50 psi and not more than 75 psi, which is enough to carry it through every season. That is an installer’s fill range, not a target for you to chase with a hose. ClimateMaster’s older loop leak bulletin also says the unit’s performance depends on flow, not pressure. Pressure is there so the flow center pumps get the flow and lubrication they need. Before the loop is ever filled, the buried pipe gets a hydrostatic pressure test, and ClimateMaster calls for at least 100 psi on that test. What I watch on a service call is the trend: the reading against last season, against the water temperatures, and against flow.

Does falling loop pressure mean my Oklahoma geothermal loop is leaking?

Usually not the buried loop. In the 700+ geothermal jobs in my records since 2017, I have not replaced one loop field, and I have loops in the ground on their third generation of equipment. What I do a lot of is service work at the house end. Pressurizing and flushing a loop that picked up air came up 150+ times in those records. Loop pumps and flow centers came up 45+ times. ClimateMaster says a loop is only properly purged when a dead-head test shows a fluid drop of 3/8 inch or less in the flush tank. In my experience, air that was not fully purged at install can keep working its way out long after startup. When pressure keeps falling, I check the flow center, the fittings and the hoses before anybody talks about digging. ClimateMaster’s bulletin says the same thing: isolate and test the inside piping first, then the outside, before digging.

“We don’t replace the loops. They last.”Dave Hartzell, Hartzell’s Heat & Air, Kingfisher

That matches the pipe itself. The Plastics Pipe Institute puts the design life of heat-fused HDPE ground loop pipe at more than 50 years. If you want the full leak checklist, read where geothermal loop leaks actually are.

Why can’t I just add water to my geothermal loop?

Because topping off a loop is a technician job: a gauge in one P/T port and a repressurization tool in the other, not a hose hooked straight to the loop. ClimateMaster’s bulletin has the tech purge every bit of air out of the supply hose and the tool first, then add only a few ounces, since that changes the pressure a lot. If the loop has a lot of air in it, the fix is a full reflush, and ClimateMaster’s procedure for that calls for a flush cart with at least a 1.5 horsepower pump, 2 feet per second through all the piping, and 100-micron filtration. Done wrong, you put air in, and repeated top-offs with plain water thin the antifreeze. ClimateMaster says freeze protection should hold to 15°F below the lowest loop temperature you expect, checked with a hydrometer, and plain water knocks that down. On a farm or a rent house where nobody watches the gauge, a thinned loop is how a heat exchanger freezes, and ClimateMaster says a frozen coax can permanently damage the heat pump.

Is loop pressure different on an Oklahoma pond loop or horizontal loop?

The physics are the same, but the loop fluid does not swing the same amount. The DOE notes that ground temperature about 30 feet down stays fairly steady year-round, between about 50°F and 59°F. A horizontal loop a few feet down sits well above that steady zone, and a pond loop follows the pond, so I expect their fluid temperature, and their pressure, to follow the seasons more than a deep vertical bore. I work all three loop types across Oklahoma, and I read every one against its own history, not against the house down the road.

What does Hartzell’s charge to check geothermal loop pressure?

At Hartzell’s Heat & Air, the geothermal diagnostic is $348 flat. Weekday, night or weekend, it is the same $348, and it is the same for my maintenance plan members. I check loop pressure, refrigerant charge, the desuperheater, controls and the heat exchanger, and you get a written report and a quote before I leave. I am IGSHPA Accredited and a ClimateMaster GeoElite dealer, with 4.8 stars and 300+ Google reviews. I work geothermal across Oklahoma out of Kingfisher.