HVAC Guide
How to Check Subcooling on a Heat Pump
A heat pump charges exactly like an air conditioner in cooling mode, but the reversing valve changes the rules in heating. Here is where to measure, what to target, and when to weigh the charge in instead.
Verified. The calculator's PT interpolation is re-derived from its cited manufacturer pressure-temperature data, and the target ranges come from the same locked refrigerant module, both checked by a deterministic test that blocks the deploy if any value drifts. That is the Intry Verified standard, our own gate, not a third-party audit.
Cooling vs Heating Mode: Which Coil Condenses
A heat pump is an air conditioner with a reversing valve. That valve decides which coil rejects heat (the condenser) and which absorbs it (the evaporator). Subcooling only means something at the condenser outlet, so the moment the valve flips, the place you measure subcooling flips with it. This is the one fact that makes heat-pump charging different from straight-cool AC.
| Mode | Condenser (where subcooling matters) | Charging indicator | Target or method |
|---|---|---|---|
| Cooling | Outdoor coil | Subcooling at the outdoor liquid line (TXV) | 8 to 14°F subcooling |
| Heating | Indoor coil | Weigh-in (charge is not set by gauges) | Nameplate charge + line-length adjustment |
The cleanest way to verify charge on a heat pump is to run it in cooling mode (force it with the thermostat or the service test mode) and read subcooling at the outdoor unit like an AC. If the weather is too cold to run cooling, weigh the charge in.
How to Measure Subcooling on a Heat Pump (Cooling Mode)
- Run the system in cooling mode until it stabilizes (about 15 minutes on R-410A).
- Read the liquid-line pressure at the outdoor unit service port.
- Convert that pressure to a saturation (condensing) temperature with the PT chart for your refrigerant, or enter it in the calculator above.
- Clamp a pipe thermometer to the liquid line at the outdoor coil outlet and read the actual temperature.
- Subtract: subcooling = saturation liquid temperature minus measured liquid temperature.
- Compare against the target (8 to 14°F with a TXV) or the nameplate value, and adjust charge only after ruling out low or restricted condenser airflow.
Worked Example: R-410A Heat Pump in Cooling Mode
An R-410A heat pump running in cooling with a TXV. Liquid-line pressure 340 psig, liquid-line temperature 93°F.
Saturated liquid temp at 340 psig = 105°F
Subcooling = 105°F − 93°F = 12°F
Diagnosis: 12°F sits inside the 8–14°F TXV target, so the charge is good. Pair it with superheat in the 8–15°F window to confirm. If subcooling read low with normal superheat, suspect low charge; if it read high, suspect overcharge or a condenser airflow problem.
Heat Pump Subcooling FAQs
How do I calculate subcooling on a heat pump?
Subcooling equals the saturation temperature at the liquid-line pressure minus the measured liquid-line temperature. Read the liquid-line pressure at the service port, convert it to a saturation (condensing) temperature with the PT chart for your refrigerant, clamp a thermometer to the liquid line at the condenser outlet, and subtract. In cooling mode the outdoor coil is the condenser and the target with a TXV is 8 to 14F, the same as an air conditioner. In heating mode the reversing valve makes the indoor coil the condenser, so the "liquid line" you measure is different and manufacturers charge by weigh-in rather than a subcooling number.
What is a good subcooling for a heat pump?
In cooling mode, a good subcooling for a heat pump with a TXV is 8 to 14F, measured at the liquid line near the outdoor coil, exactly like an AC. Many nameplates print a specific subcooling target (often 10 to 12F); the nameplate value always wins over a generic range. In heating mode there is no single good subcooling number to chase: charge the unit by weigh-in per the manufacturer, not by a subcooling target.
Can you check subcooling in heating mode?
You can measure it, but you should not set the charge by it. In heating mode the reversing valve sends hot discharge gas to the indoor coil, which becomes the condenser, so subcooling is measured at the indoor coil outlet and shifts with outdoor temperature as the outdoor coil (now the evaporator) sees widely varying conditions. Manufacturers direct you to charge a heat pump in heating by the weigh-in method (nameplate charge plus a line-length adjustment), or only by a mode-specific chart within a stated outdoor-temperature window. Below roughly 55 to 60F outdoor, verify by weigh-in, not gauges.
Where do you measure subcooling on a heat pump?
Measure subcooling on the liquid line at the outlet of whichever coil is acting as the condenser. In cooling mode that is the outdoor coil, so you read the liquid-line pressure and temperature at the outdoor unit, the same place as an AC. In heating mode the indoor coil is the condenser, so the relevant liquid line is at the indoor coil. Clamp the pipe thermometer to clean, bare copper near the coil outlet and insulate it from ambient air for an accurate reading.
Why is subcooling different on a heat pump than an air conditioner?
In cooling mode there is no difference: a heat pump charges exactly like an AC, so target subcooling with a TXV is the same 8 to 14F. The difference appears in heating mode. A heat pump has a reversing valve that swaps the roles of the two coils, so the coil that condenses refrigerant (and therefore where subcooling matters) flips from outdoor in cooling to indoor in heating. That is why the reliable charging method in heating is weigh-in, not a gauge-based subcooling reading.
What subcooling should a heat pump have in cooling mode?
With a TXV in cooling mode, target 8 to 14F of subcooling and 8 to 15F of superheat, the same windows as an air conditioner on the same refrigerant. Low subcooling with normal superheat usually means low charge; high subcooling with normal superheat usually means overcharge. Confirm against the nameplate target before adding or recovering refrigerant.
Do R-454B and R-32 heat pumps use the same subcooling target?
Yes. R-454B and R-32 use the same 8 to 14F TXV subcooling target as R-410A in cooling mode, because their pressure-temperature behavior is close. What changes is the PT scale you convert with: use the correct refrigerant's chart to turn liquid pressure into saturation temperature, and charge A2L blends like R-454B as a liquid. The heating-mode weigh-in rule is the same regardless of refrigerant.
Related Calculators & Charts
Charge verified. Was the heat pump sized for the house?
Chronic charge and comfort complaints on a heat pump often trace back to sizing. Run the load by climate zone, insulation, and windows before you trust the nameplate tonnage.