Ground Source Heat Pump Monthly Loop Fluid, Pressure & System Performance Log

ContextBefore starting the heat pump, or at least 10 minutes after it has shut down, note the pressure gauge reading at the flow center. A properly charged residential closed-loop system typically reads 35–60 PSI at rest when cold; consult your installer's commissioning sheet for your installation's specific target range. Always record the exact number — not a vague notation of 'normal.' A drop of 5 PSI or more from one monthly reading to the next is diagnostically significant even without visible wet spots: HDPE loop joints and fittings can weep slowly underground without moisture ever reaching the surface. A consistent decline across three to four consecutive monthly logs justifies a nitrogen pressure test to locate the source.

ContextWith the heat pump running at full load — during a cold morning in heating mode or peak afternoon heat in cooling — note the high-side and low-side loop pressures if your flow center has two gauges. The differential between supply and return should remain stable from month to month at similar conditions. A sudden increase in differential pressure at the same operating conditions suggests a partially clogged strainer, a failing pump impeller, or a developing restriction in the loop. A sudden decrease suggests pump wear or air entrainment causing cavitation. Neither finding should be dismissed as a gauge issue until the gauge accuracy has been independently verified with a separate calibrated instrument.

ContextThe diaphragm expansion tank on a closed geothermal loop has an air pre-charge that should match the system's cold fill pressure — typically 30–50 PSI depending on installation design. Check the Schrader valve on the tank's air side with a standard tire pressure gauge when the loop circuit is fully depressurized. If the air pre-charge reads more than 3 PSI below specification, the tank has lost its charge and will fail to absorb pressure swings, causing relief valve chatter or hydraulic hammer on startup. If you depress the Schrader valve and water exits instead of air, the diaphragm has ruptured and the tank requires replacement ($80–$250 installed). Perform this check at least quarterly, not just monthly.

ContextWalk the accessible loop path — flow center inlet and outlet unions, purge and fill ports, pressure gauges, and any above-ground transitions in a mechanical room or crawl space — looking for mineral deposits (white crystalline crust), glycol residue (a slightly oily or tacky film with a faint sweet smell), or active drips. Propylene glycol evaporates more slowly than water, so a dried drip leaves a sticky residue that is easy to miss on a casual walkthrough. Pay close attention to dissimilar-metal connections — brass fittings threaded into plastic manifolds are common leak initiation points. Photograph any new staining and date each image to track whether the deposit is growing over months.

ContextEWT is the loop fluid temperature arriving at the heat pump's refrigerant-to-water heat exchanger and is the single most important performance input parameter to monitor consistently. In heating mode, EWT should typically be 25–50°F for a closed-loop residential system; below 20°F the unit approaches low-temperature lockout. In cooling mode, EWT should stay below 90°F; above 100°F efficiency degrades sharply and a high-pressure lockout becomes likely. Record this at the supply line temperature sensor — most units provide a thermistor well or a clip-on measurement port — during peak operating conditions. Comparing the same calendar month across multiple years reveals slow loop field thermal saturation before it causes lockouts or climbing utility bills.

ContextLWT is the temperature of loop fluid exiting the heat pump's coaxial heat exchanger on its way back toward the ground. Record it from the return line sensor at the same moment you read EWT so your delta-T calculation is valid. In heating mode, LWT will be colder than EWT because the heat pump is extracting heat from the fluid; in cooling mode it will be warmer because the unit is rejecting heat into the ground. If your temperature sensors are thermistors embedded in the unit's manifold, verify their accuracy against a calibrated contact thermometer once per year — thermistors can drift 2–4°F over time without triggering any diagnostic fault condition.

ContextLoop delta-T is the difference between EWT and LWT: EWT minus LWT in heating mode, or LWT minus EWT in cooling mode. For a properly sized, properly flowing system with antifreeze solution, heating-mode delta-T runs 3–6°F and cooling-mode delta-T runs 8–14°F. A delta-T significantly lower than baseline suggests flow is too high, the load is less than the system was designed for, or the coaxial heat exchanger is scaled and transferring less heat per unit of fluid. A delta-T significantly higher — 10°F or more in heating — indicates flow is too low due to a blocked strainer or worn pump, or that the loop cannot keep pace with the load. A steady upward trend in heating-mode delta-T across 12 months is a leading indicator of loop field thermal depletion requiring professional evaluation.

ContextFlow rate context is essential for interpreting delta-T readings correctly. Most residential geothermal units specify a minimum flow rate — commonly 1.5–3 GPM per ton of rated capacity — in the installation manual; retrieve this from your commissioning sheet. If your flow center has an integrated flow meter, log it directly. If not, calculate approximate flow from the pump's performance curve using the measured pressure differential across the pump. Operating below minimum flow reduces turbulence inside the coaxial heat exchanger, cuts heat transfer efficiency, and can cause liquid slugging in the refrigerant circuit. Operating more than 25% above design flow wastes significant pump energy — pump power scales with the cube of flow velocity, meaning a 10% flow excess adds roughly 33% more pumping energy cost.

ContextUse a clamp-type ammeter on each compressor conductor during the first 5 minutes of a run cycle at conditions close to design. Compare readings to the nameplate RLA (Rated Load Amperage). A reading within 10% of RLA is healthy. Amperage 15–20% above RLA often points to elevated head pressure from a loop that is too warm in cooling mode, refrigerant overcharge, a mechanically binding compressor, or low supply voltage forcing compensatory current draw. Amperage significantly below RLA with inadequate output temperature typically indicates low refrigerant charge. Always measure both legs of a 240V single-phase system: a leg-to-leg imbalance greater than 2% creates unequal heating in motor windings and can substantially shorten compressor life over years of cumulative operation.

ContextMeasure line voltage at the unit's dedicated disconnect or main terminal block — not at the breaker panel — while the compressor is running at full load, because voltage sags under load and a no-load measurement hides the real problem. For 240V single-phase systems, both legs should read within 10% of nameplate voltage. For three-phase systems, voltage imbalance between any two legs must not exceed 2%. Low voltage forces higher amperage draw; by the standard motor insulation degradation rule, every additional 10°C of winding temperature from excess current halves the remaining insulation service life. Consistent readings below 220V on a 240V system warrant an electrician evaluating the service entrance conductors, panel connections, and utility transformer tap.

ContextAfter starting the heat pump, stand near the unit and listen actively for 3–5 minutes. A healthy scroll compressor produces a steady, low-frequency hum with no rhythmic clicking or intermittent clattering. Metallic sounds on startup can indicate refrigerant liquid slugging — often caused by very low loop temperatures in heating mode or a malfunctioning expansion device. A high-pitched whine or intermittent grinding from the flow center pump suggests bearing wear or air trapped in the loop causing cavitation; the pitch varies unpredictably when air is the cause. Record any new sound with your phone's voice memo app and date-stamp the file — an audio recording of an intermittent noise can save a technician hours of diagnostic time during a follow-up visit when the problem may not be actively occurring.

ContextWith the unit off and power locked out, open the electrical cabinet and locate the main compressor contactor. When the contactor is de-energized, carefully observe the contact face surfaces: they should be flat, silver-gray, and free of deposits. Pitting, carbonization (black buildup on contact faces), or heat discoloration in the surrounding wiring indicates arcing and elevated resistance at the contact surface. A pitted contactor set costs $30–$80 in parts and takes roughly 30 minutes to replace — far less costly than a compressor hard-start failure or a burned motor lead that results from leaving it in service. Replace any contactor showing pitting or heat discoloration regardless of the unit's overall age or hours of operation.

ContextInsert a thermometer probe into the supply register nearest the air handler and into the return grille after the unit has been running for at least 10 minutes. In heating mode, supply air should reach 90–105°F; in cooling mode, 50–58°F. The difference between return and supply air — the air-side delta-T — should be 15–25°F in either mode. A supply temperature below 85°F in heating while loop and refrigerant parameters are within spec often points to a dirty or partially iced evaporator coil, a failing reversing valve, or low refrigerant charge. A supply temperature above 110°F in heating mode usually traces back to restricted airflow — check the air filter before calling a technician.

ContextRemove the air filter and hold it up to a bright light source. If you cannot see light passing through it cleanly, it is overdue for replacement regardless of the calendar schedule. A clogged filter reduces airflow across the evaporator coil, raises refrigerant-side pressure drop, reduces system capacity, and in cooling mode can cause coil icing within hours of sustained operation. Geothermal systems running two-stage or variable-speed compressors operate more total hours per day than conventional heat pumps — a filter that lasts 3 months in a standard system may need replacement every 4–6 weeks during heavy-use seasons. Write the replacement date on the filter frame with a permanent marker each time so the record is always with the hardware.

ContextMost communicating thermostats — Ecobee, Honeywell T6 Pro, Nest — record daily runtime data accessible through their app or web portal. Log the average daily hours for the preceding 7 days. In a well-insulated home at design outdoor conditions, a properly sized geothermal system runs approximately 8–14 hours per day. Runtimes consistently exceeding 18 hours per day when outdoor temperatures are within normal seasonal range — not during record cold or heat events — point to loop performance decline, equipment efficiency loss, or an undetected building envelope change such as a failed window seal. Runtimes under 4 hours per day with frequent short cycling may indicate oversizing, thermostat calibration error, or both, each of which causes excessive compressor wear over time.

ContextIf your geothermal unit has an integrated desuperheater — a secondary heat exchanger that recovers compressor discharge superheat to preheat domestic hot water — measure the water temperature at the storage tank connection after the heat pump has been running for at least 30 minutes. In heating season, desuperheater output typically reaches 90–120°F; during cooling season, recovered heat can push output to 130–140°F as the unit rejects additional heat overall. If outlet temperature is significantly below these ranges while the unit is confirmed running, the desuperheater circulator pump may have failed — a common failure mode for these small wet-rotor pumps — or the coil is scaled from hard water. A fully functional desuperheater offsets 25–50% of annual water heating energy cost.

ContextLocate the small wet-rotor circulator — often a Taco 006, Grundfos UP26-99, or equivalent — plumbed into the desuperheater water line. While the heat pump is running, place your hand on the pump body: it should be noticeably warm (50–80°F) from fluid moving through it. A cold pump body during a confirmed heat pump run cycle indicates the pump is not operating; check its wiring connection to the heat pump's control board, as these pumps are typically powered directly from the unit's control outputs and fail silently. A pump running extremely hot suggests cavitation from scale buildup or a partially closed isolation valve. Replacement for a failed desuperheater pump runs $80–$200 in parts and labor — modest compared to weeks of full-cost resistance-element water heating while it goes undetected.

ContextRecord every measurement with the date, time of day, outdoor temperature at the moment of the reading, and the system's operating mode (heating, cooling, or standby). A spreadsheet with conditional formatting — red fill for out-of-range values, yellow for readings approaching limits — transforms a table of numbers into a visual early-warning dashboard that anyone can read at a glance. Track at minimum: EWT, LWT, loop delta-T, static loop pressure, glycol concentration, compressor amperage on both legs, supply and return air temperatures, and desuperheater outlet temperature if equipped. If using a paper logbook, keep it in a clear sleeve mounted near the equipment — technicians with access to 12 months of handwritten data can often diagnose intermittent faults in a single visit that would otherwise require a dedicated monitoring device and a return call.

ContextYear-over-year comparison eliminates the seasonal variability that month-over-month comparisons cannot remove, making it far more sensitive to genuine performance decline. If January this year shows an EWT 4°F higher than January last year under similar outdoor temperatures and usage patterns, loop field thermal depletion is a possibility worth investigating — a condition that, if confirmed, may require loop expansion or supplemental load shedding and can cost $8,000–$20,000 to remediate. If compressor amperage has increased 8% over 12 months with no change in supply voltage or occupancy patterns, refrigerant charge may be drifting. Print or export the previous year's same-month column and place it beside current readings every time you complete this log.

ContextMost modern geothermal units have diagnostic LED indicators or communicating controls that store recent fault codes. After a lockout — the unit shuts down and requires a manual reset to restart — write down the fault code before resetting, then power-cycle to clear it and restore operation. Common codes to know: a high-pressure limit trip (HPS) frequently means insufficient loop flow or a cooling-mode loop temperature that has climbed too high; a low-pressure limit trip (LPS) suggests low refrigerant charge or an EWT approaching the freeze-stat threshold in heating mode; a freeze stat trip means loop fluid temperature at the coaxial heat exchanger dropped below approximately 25°F. Never dismiss a fault code as a one-time glitch without logging it — three occurrences of the same code within any 30-day period constitutes a pattern requiring a service visit.

ContextUse your accumulated trend data to determine when a technician is needed rather than relying exclusively on a fixed annual service schedule. If three consecutive monthly logs show compressor amperage rising, loop pressure declining, or any key delta-T drifting outside your established baseline, schedule service regardless of when the last tune-up occurred. A geothermal technician arriving with access to your 12-month log can typically diagnose issues in 30 minutes that would otherwise require two or more hours of active testing — directly reducing both the service invoice and the total time your system runs in a degraded state. Before any service call, export your log to PDF or photograph each page, and share it with the technician in advance of the appointment so they arrive prepared.