Superheat and Subcooling in Heating Mode
Flip a heat pump into heating and the refrigerant circuit turns inside out. The outdoor coil becomes the evaporator, the indoor coil becomes the condenser, and every probe you clamp on has to move with them. Here is how to read the system correctly.
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In This Guide
What the Reversing Valve Actually Changes
A heat pump is an air conditioner that can run its refrigerant cycle in either direction. The four-way reversing valve reroutes hot discharge gas so that, depending on the mode, either the indoor or the outdoor coil acts as the condenser. The compressor still pumps the same direction and the metering devices still meter, but the role of each coil swaps.
In cooling, the indoor coil is the evaporator (absorbing heat from the house) and the outdoor coil is the condenser (dumping it outside). In heating, that reverses:
- Outdoor coil = evaporator. It absorbs heat from the outdoor air, even at 30°F, and boils the refrigerant to a low-pressure vapor.
- Indoor coil = condenser. Hot discharge gas condenses here, rejecting heat into the supply air stream.
The single most common mistake techs make on a heat pump in heating is measuring in the wrong place out of muscle memory. Superheat is always taken at the evaporator outlet and subcooling always at the condenser outlet. In heating, that means superheat moves to the outdoor unit and subcooling moves indoors.
Where Superheat and Subcooling Live in Heating
Forget which line is labeled "liquid" and "suction" on the unit data plate; those labels are written for cooling. In heating, the small line between the coils carries subcooled liquid from the indoor condenser tothe outdoor metering device, and the large line carries cold suction vapor back from the outdoor coil.
| Measurement | Cooling mode | Heating mode |
|---|---|---|
| Superheat pressure port | Low side at outdoor unit | Low side at outdoor unit (still low side) |
| Superheat temp clamp | Suction line at indoor coil | Vapor line leaving outdoor coil |
| Subcooling pressure port | High side at outdoor unit | High side at outdoor unit (still high side) |
| Subcooling temp clamp | Liquid line at outdoor coil | Liquid line leaving indoor coil |
Notice the gauge ports do not move. The service valves at the outdoor unit still read the low side (suction) and high side (discharge) no matter the mode, because the reversing valve lives downstream of the compressor. What moves is where you take the temperature reading, since the coil doing the boiling and the coil doing the condensing have swapped ends.
Field reality: subcooling in heating means clamping a thermometer on the liquid line right where it leaves the indoor coil or air handler. That is a harder spot to reach than an outdoor liquid line, which is one reason most techs rely on the superheat reading and the manufacturer charging chart when a system is running in heating.
The Formulas Do Not Change
Reversing the cycle does not reinvent thermodynamics. Both calculations are exactly what they are in cooling; only the coil you apply them to has moved.
Superheat
Evaporator saturation temp comes from the PT chart at your measured suction pressure. In heating, that evaporator is the outdoor coil.
Subcooling
Condenser saturation temp comes from the PT chart at your high-side pressure. In heating, that condenser is the indoor coil.
Superheat still measures how many degrees the vapor has climbed above its boiling point, confirming no liquid is returning to the compressor. Subcooling still measures how far the liquid has dropped below its condensing point, confirming a full column of liquid is feeding the metering device.
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Worked Example: Reading a Heat Pump in Heating
Scenario:
An R-410A heat pump with a TXV runs in heating on a 35°F day. At the outdoor unit you read a suction pressure of about 76 psig and a high-side pressure of about 340 psig. The vapor line leaving the outdoor coil measures 30°F; the liquid line leaving the indoor coil measures 96°F.
Step 1: Find the outdoor coil (evaporator) saturation temp
76 psig on an R-410A PT chart ≈ 22°F saturation.
Step 2: Calculate superheat at the outdoor coil
SH = 30°F − 22°F = 8°F
Step 3: Find the indoor coil (condenser) saturation temp
340 psig on the R-410A PT chart ≈ 105°F saturation.
Step 4: Calculate subcooling at the indoor coil
SC = 105°F − 96°F = 9°F
Reading: reasonable, verify vs chart
8°F superheat sits at the low end of the healthy 8-14°F TXV window, and 9°F subcooling is close to the bottom of the 10-18°F range. On a cold day those numbers are plausible, but the only correct call is to compare against the manufacturer's heating charging chart for 35°F outdoor temperature. Low subcooling here could hint at a slight undercharge or be perfectly normal for this model in heating.
Target Ranges and What They Mean
The benchmark ranges are the same numbers you memorized for cooling, but interpret them knowing the evaporator is now fighting cold outdoor air.
| Measurement | Normal range | Concern if |
|---|---|---|
| Superheat (TXV) | 8–14°F | Below 5°F (flooding) or above 20°F (starved) |
| Subcooling | 10–18°F | Below 5°F (undercharge) or above 20°F (overcharge) |
| Compressor amp draw | Below RLA | Above RLA or below 50% of RLA |
Low superheat, low subcooling
Points to overcharge or liquid flooding back through the outdoor coil. Compressor floodback is a real risk in cold weather because the evaporator is already running low. Confirm before adding or removing refrigerant.
High superheat, low subcooling
Classic undercharge or a restriction. In heating this also shows up as weak supply temps and an outdoor coil that ices unevenly and struggles to clear on defrost.
Because R-454B operates within roughly 3% of R-410A pressures under identical conditions, these same target ranges carry over to newer A2L heat pumps. Just be sure you are reading the correct PT chart for the refrigerant on the nameplate.
Defrost, Cold Weather, and Bad Readings
In heating, the outdoor coil runs below the outdoor dew point and frost builds on it. Periodically the board triggers a defrost: the reversing valve momentarily flips to cooling, the outdoor fan stops, and hot gas melts the ice. During that window your superheat and subcooling numbers are meaningless.
- Never trust a reading taken during or in the first few minutes after a defrost cycle — pressures are still swinging.
- A coil that stays iced, or a unit that will not exit defrost, points to a failed defrost board, a stuck reversing valve, or faulty defrost sensors rather than a charge problem.
- The colder it gets outdoors, the lower your suction pressure and outdoor saturation temp fall — that is normal physics, not a low charge. Always read against the manufacturer chart for the actual outdoor temperature.
Safety note: On any recovery, brazing, or charge adjustment, follow EPA Section 608 refrigerant-handling rules. If the system is an A2L refrigerant such as R-454B, follow the A2L handling requirements — leak detection, ventilation, no ignition sources, and proper recovery equipment rated for mildly flammable refrigerant.
Why You Should Not Charge in Heating Mode
You can absolutely diagnose in heating, but final charge verification by subcooling belongs in cooling mode whenever the weather allows. Most manufacturers specify that subcooling charging be done in cooling with an outdoor temperature above roughly 65°F, where the numbers are stable and the charging chart is designed to apply.
- In heating, the indoor coil is the condenser and subcooling is measured at a hard to reach indoor liquid line, introducing measurement error.
- Outdoor conditions and defrost cycles make heating-mode pressures a moving target.
- When you cannot wait for warm weather, the weigh-in method — recovering and charging to the nameplate weight with adjustments for line length — is the accurate way to set charge in cold conditions.
Field workflow that holds up
Diagnose the complaint in heating using superheat at the outdoor coil, then confirm charge in cooling on a warm day using subcooling, or weigh in the factory charge if you have to work in the cold. Document your outdoor temperature with every reading so the next tech knows the conditions behind the numbers.
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