How Do You Read Geothermal EWT, LWT, GPM, PSI and Delta T?
Short answer: In HVAC, LWT means leaving water temperature and EWT means entering water temperature. On a geothermal heat pump, the difference between them is delta T, and with flow in GPM it tells you how much heat the unit is moving. At Hartzell’s Heat & Air I read all of them together, never one number alone.
What do EWT and LWT mean on a geothermal heat pump?
EWT is the temperature of the loop fluid as it enters the heat pump, and LWT is the temperature as it leaves. In heating, the unit pulls heat out of that fluid, so LWT comes out colder than EWT. In cooling, the unit dumps your house heat into it, so LWT comes out warmer. On an older ClimateMaster iGate ATC32U03 thermostat you can see both, plus flow rate on some units, on the System Status screen. Hold the left arrow from the home screen. My ClimateMaster thermostat walkthrough shows where.
| Reading | What it means | ClimateMaster reference point |
|---|---|---|
| EWT | Entering water temperature: loop fluid coming into the heat pump. | Loop temperatures can range from 25°F to 110°F. Below 40°F entering, the loop needs antifreeze. |
| LWT | Leaving water temperature: fluid going back out to the loop. | Colder than EWT in heating, warmer in cooling. |
| GPM | Gallons per minute of loop flow. | 2.25 to 3 gpm per ton of cooling capacity on a closed loop. |
| Delta T | The difference between EWT and LWT, the change across the unit. | At 3 gpm per ton on a closed loop: about a 4°F to 8°F drop in heating, a 9°F to 12°F rise in cooling. |
| PSI | Pressure at the gauge point. Used for pressure drop across the water coil and for loop pressure. | Loop filled to 50 to 75 psi at startup. Performance depends on flow, not pressure. |
What is a normal delta T on a geothermal system?
It depends on the flow, so a delta T without a GPM number can fool you. ClimateMaster’s Tranquility 30 installation manual gives startup ranges for a closed loop at 3 gpm per ton: the water should drop about 4°F to 8°F through the unit in heating and rise about 9°F to 12°F in cooling. Cut the flow in half and the delta T goes up, even if nothing else changed. That is why I never call a system good or bad off delta T alone. On a 100-degree Oklahoma afternoon, a long cooling run warms the loop and EWT climbs. On a January cold snap, EWT drops. The readings have to be compared against the season and against that system’s own history.
How do you calculate how much heat a geothermal unit is moving?
ClimateMaster’s DXM2 troubleshooting guide uses one formula: heat moved in Btuh equals GPM times delta T times a fluid factor, 500 for plain water and 485 for an antifreeze mix. Here is a worked example. A 3-ton unit moving 9 gpm of antifreeze mix, with a 6°F drop in heating, is pulling 9 × 6 × 485, or about 26,190 Btuh, out of the loop. If that same unit only moves 5 gpm with the same drop, it is pulling about 14,550 Btuh. Same delta T, very different result. When I need efficiency, I take it a step further: how to calculate COP on a geothermal heat pump walks through it.
What does PSI tell you on a geothermal loop?
Two different things. Pressure drop across the water coil, measured at the unit’s P/T ports, is how a technician checks flow against ClimateMaster’s pressure-drop tables for that model. Loop pressure is a separate number. ClimateMaster’s Tranquility 30 manual says to fill a closed loop to 50 to 75 psi, and its loop leak bulletin says unit performance depends on flow, not system pressure. Pressure is there so the flow center pumps get their flow and lubrication. For what a changing loop pressure means, see why geothermal loop pressure changes with temperature.
Why does Hartzell’s read the readings together?
Because the loop itself is rarely the problem. In the 700+ geothermal jobs in my records since 2017, I have not replaced a loop field, and I have loops in the ground on their third generation of equipment. Low flow from air or a worn loop pump, a drain problem, or a control issue is far more common: loop pressurizing and flushing came up 150+ times, loop pumps and flow centers 45+ times. A bad delta T usually points me at flow first. ClimateMaster’s troubleshooting guide lists delta T, delta P, GPM and antifreeze percentage as the water-side checks, and says not to hook up refrigeration gauges for that analysis. On systems I installed, I also have the startup numbers to compare against, which is what a geothermal commissioning record is for.
“We don’t replace the loops. They last.”Dave Hartzell, Hartzell’s Heat & Air, Kingfisher
What does a geothermal diagnostic cost in Oklahoma?
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.
Safe to do yourself, and what to leave alone
- Write down any EWT, LWT or flow number your thermostat shows, with the date and the mode.
- Write down any fault code exactly as shown.
- Check the air filter. Low airflow changes these numbers too.
Leave alone: P/T ports, gauges, the flow center, loop valves and refrigerant connections. Those readings are technician work.
EWT, LWT and delta T questions
What does LWT mean in HVAC?
LWT means leaving water temperature: the temperature of the water or loop fluid as it leaves a heat pump, chiller or boiler. On a geothermal heat pump it pairs with EWT, entering water temperature. In heating, LWT is colder than EWT. In cooling, it is warmer.
What is a normal delta T on a geothermal heat pump?
ClimateMaster’s Tranquility 30 manual gives startup ranges for a closed loop at 3 gpm per ton: about a 4 to 8 degree drop in heating and a 9 to 12 degree rise in cooling. Delta T changes with flow, so it only means something next to a GPM number.
How many GPM per ton should a geothermal loop have?
ClimateMaster recommends 2.25 to 3 gpm per ton of cooling capacity on closed loops. The exact number for a unit is checked against the manufacturer’s pressure-drop tables for that model.
Can I diagnose my geothermal system from the thermostat readings?
You can spot a pattern worth calling about, like EWT falling every week in winter or a fault that keeps coming back. A diagnosis takes flow, both water temperatures, airflow, controls and fault history together, and that is technician work.
Does a bad delta T mean my geothermal loop is failing?
Usually not. In 700+ geothermal jobs in my records since 2017 I have not replaced a loop field. A bad delta T more often points to low flow from air or a worn loop pump, which is service work, not a new loop.
How much does it cost to have geothermal readings checked in Oklahoma?
My geothermal diagnostic is $348 flat, any hour, and the same for plan members. It covers loop pressure, refrigerant charge, the desuperheater, controls and the heat exchanger, with a written report and quote before I leave. Call 405-375-4822.
Sources
Ranges and the heat formula come from ClimateMaster’s manuals below. Job counts come from my service records since 2017, rounded down. Every unit is checked against its own model data.
- ClimateMaster Tranquility 30 Digital (TE) installation, operation and maintenance manual, 97B0045N04 (rev. March 20, 2023)
- ClimateMaster Basic Troubleshooting Information and Guidelines
- ClimateMaster Tranquility Digital (DXM2) troubleshooting guide, 97B0601N01 (heat of extraction and rejection formula)
- ClimateMaster Technical Bulletin TB-C0001, Loop Leak Verification (rev. 4/1/02)
- ClimateMaster ATC32U03 iGate thermostat user manual, 97B0055N02 (System Status screen)