TXV Systems: Charge by Superheat or Subcooling?

The rule is simple once you see the logic: charge by the reading your metering device does NOT control.

A TXV is a modulating valve. The sensing bulb on the suction line (opening force) balances against spring pressure and equalizer pressure (closing forces) so the valve feeds exactly enough refrigerant to hold a set superheat at the evaporator outlet — most residential valves control around 8–12°F. Add half a pound of charge and the valve pinches down. Recover some and it opens up. Superheat stays parked either way, so on a TXV system it tells you almost nothing about charge level.

What does move with charge on a TXV system is subcooling. Since the valve throttles the feed, extra refrigerant has nowhere to go but the condenser — liquid stacks up, the condensed refrigerant spends more time rejecting heat, and subcooling climbs. Pull charge out and subcooling drops. That direct, honest relationship is why subcooling is the charging metric for TXV systems.

A fixed orifice — piston, flowrator, cap tube — is just a calibrated hole. Feed rate is set by the pressure difference across it, period. Overcharge the system and head pressure pushes more refrigerant through, feeding liquid deeper into the evaporator: superheat drops. Undercharge it and the coil starves: superheat climbs. The orifice can't compensate, so superheat is the reading that tracks charge.

The formula, per the subcooling charging method documented by AC Service Tech:

Subcooling (°F) = liquid line saturation temperature (°F) − actual liquid line temperature (°F)

Saturation temperature comes from your high-side gauge pressure converted on the P-T chart for that refrigerant (digital manifolds and probe apps display it directly). Line temperature comes from a clamp probe on the liquid line at the outdoor unit.

1. Airflow first. Verify roughly 350–425 CFM per 12,000 BTU/h of capacity, clean filter, clean coils. Charging against bad airflow bakes the airflow problem into the charge.
2. Get to steady state. Run the system 5–10 minutes minimum before trusting readings. AC Service Tech's method also calls for both indoor and outdoor temperatures above 70°F so there's enough load to read against.
3. Read liquid pressure at the liquid service valve and note the saturation temperature for your refrigerant.
4. Clamp the temperature probe on the liquid line near the same point. Strap it tight and insulate it from sun and ambient air.
5. Subtract. Example: manifold shows a liquid saturation temperature of 105°F, the liquid line measures 95°F — that's 10°F of subcooling.
6. Compare to the target on the data plate — often printed as "TXV Subcooling" on the outdoor unit's rating plate or inside the corner panel. Within ±3°F is the accepted tolerance; closer is better. No plate? 10°F ±3°F is the standard field fallback, but confirm against manufacturer literature when you can.

Adjust in small amounts. Add charge to raise subcooling, recover to lower it, and give the system 5–10 minutes to settle between every adjustment. Chasing a moving number wastes refrigerant and time.

On a piston or cap tube system the charging metric is superheat:

Superheat (°F) = actual suction line temperature (°F) − suction saturation temperature (°F)

Suction saturation comes from low-side gauge pressure on the P-T chart; line temperature from a clamp probe on the suction line near the outdoor unit.

The critical part: target superheat is not a fixed number. It moves with the load on the coil. The manufacturer's charging chart sets the target from two field measurements — indoor wet bulb (measured in the return, close to the evaporator inlet) and outdoor dry bulb (measured near the condenser inlet, in the shade). A correctly charged system reads high superheat on a mild day and low superheat on a hot, humid one. Charging every system to "10°F of superheat" is how coils get flooded.

When the OEM chart isn't on the panel, AC Service Tech publishes this approximation:

Target superheat = [(3 × indoor WB) − 80 − outdoor DB] ÷ 2

Example: indoor WB 64°F, outdoor DB 96°F → (192 − 80 − 96) ÷ 2 = 8°F target superheat.

Values below computed from that formula — confirm against the OEM chart before charging:

Indoor WB → / Outdoor DB ↓	60°F WB	64°F WB	68°F WB	72°F WB
85°F DB	7.5°F	13.5°F	19.5°F	25.5°F
95°F DB	2.5°F	8.5°F	14.5°F	20.5°F
105°F DB	—	3.5°F	9.5°F	15.5°F

Where the math lands at roughly 5°F or below (marked —, or close to it), conditions are wrong for the superheat method — the margin against flooding the compressor is too thin. Weigh the charge in per the nameplate instead and verify superheat when conditions allow.

Charging method answers "how much refrigerant." Diagnosis needs both numbers. Superheat tells you what's happening in the evaporator; subcooling tells you what's happening in the condenser. Read as a pair (the approach HVAC School teaches as part of its five-pillar refrigerant circuit diagnosis), they separate charge problems from airflow problems and restrictions:

Subcooling	Superheat	Likely condition
Low	High	Undercharge (or a leak — find it before topping off)
High	Low (fixed orifice) / normal (TXV)	Overcharge
High	High	Liquid line restriction — plugged filter-drier, kinked line, or an underfeeding TXV
Normal	Low	Low evaporator load — dirty filter, iced coil, weak blower — or an overfeeding TXV

Two TXV-specific reads worth memorizing:

• Normal subcooling + high superheat on a TXV system means the charge is fine but the valve isn't feeding — restricted inlet screen, lost bulb charge, or a bulb that's loose or poorly insulated on the suction line. Adding refrigerant "because superheat is high" just overcharges it.
• Superheat pinned in the 8–12°F band while subcooling is off target is the system working as designed: the valve is doing its job, and subcooling is telling you the truth about charge.

One gauge reading never diagnoses a system. The pair, plus airflow verified, almost always does.