How Do You Read Geothermal EWT, LWT, GPM, PSI and Delta T?
EWT is entering water temperature, LWT is leaving water temperature, GPM is flow, PSI is pressure, and Delta T is the temperature change across the heat pump. None of those numbers diagnoses the system by itself. The pattern across all readings, in heating or cooling, tells the story.
What each reading means
Entering water temperature is the loop-fluid temperature coming into the heat pump. Leaving water temperature is the fluid temperature leaving it. GPM tells whether enough fluid is moving. PSI shows pressure clues at the gauge location. Delta T is the temperature difference between entering and leaving water. A good diagnostic compares all of them with airflow, load, pump operation, controls, and manufacturer targets.
Heating readings versus cooling readings
In heating, the heat pump pulls heat from the loop, so leaving water is normally colder than entering water. In cooling, the heat pump rejects heat to the loop, so leaving water is normally warmer. That directional change matters. A reading that looks suspicious in heating may be normal in cooling, and the reverse can also happen.
Oklahoma application
Long Oklahoma cooling runs can warm the loop. A hard winter stretch can pull loop temperatures lower. Dave looks for trend, balance, and whether auxiliary heat, fault codes, long runtimes, or comfort complaints match the water-side data. This is why a photo of one number cannot safely diagnose a loop.
Measurement or decision table
| Reading | Plain meaning | How a technician uses it |
|---|---|---|
| EWT | Fluid temperature entering the unit. | Compares loop condition and season against unit operation. |
| LWT | Fluid temperature leaving the unit. | Shows how much heat is being added to or removed from the loop. |
| GPM | Fluid flow rate. | Separates poor heat transfer from low flow or pump trouble. |
| PSI | Loop pressure at a gauge point. | Finds pressure trends, air clues, pump effects, and possible leaks. |
| Delta T | Difference between EWT and LWT. | Shows heat transfer only when flow and operating mode are known. |
Professional diagnostic boundary
The heat moved by a water-source heat pump depends on both flow and temperature change. Delta T without GPM can mislead. GPM without EWT and LWT can miss a heat-transfer problem. PSI without temperature and pump status can mislead. A complete geothermal diagnostic also checks airflow, static pressure, electrical data, control calls, refrigerant-circuit behavior, and fault history.
Safe homeowner actions
- Record displayed readings only if they are visible without opening panels.
- Write down heating or cooling mode, setpoint, outdoor weather, and whether auxiliary heat is running.
- Take a photo of the thermostat or service display if it is safe to do so.
- Do not attach gauges, open cabinets, adjust pumps, or change valves.
Related geothermal pages
FAQ
What is a normal geothermal Delta T?
There is no single safe number without knowing mode, GPM, equipment, airflow, loop type, and manufacturer targets.
Is low EWT bad?
Low EWT can be normal during heating or can point to loop, flow, sizing, or load issues. The trend and the rest of the readings matter.
Why does Hartzell's ask for fault codes and readings?
They help Dave decide what test path to start with, but the diagnosis still requires professional measurements at the system.
Sources
These sources are used for general technical context. The final diagnosis still depends on the specific equipment, loop, control board, measurements, and site.