How to Measure Superheat & Subcooling

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Which Method Applies to Which Metering Device

The metering device decides the charging rule. Mixing them up is how you flood a piston compressor or starve a TXV coil while the gauges “look fine.”

• Fixed orifice, piston, capillary tube: charge by superheat. The orifice is a hole. Superheat moves when you add or recover refrigerant (and when indoor load / airflow changes). Target SH comes from indoor wet-bulb and outdoor dry-bulb, or a typical 8–18°F band if you do not have a chart.
• TXV / TEV / EEV: charge by subcooling. The valve holds evaporator superheat (often 8–12°F) by design. Adding gas barely moves SH until the valve is wide open and you have already overcharged. Liquid subcooling is what tells you the condenser has a real liquid seal.

Air-side temperature split (delta-T) is a different diagnostic. It does not replace SH/SC. When to grab which: Superheat/Subcooling vs Delta-T.

How to Take Superheat (Fixed Orifice)

Superheat is how many degrees the suction vapor is above saturation at the suction pressure you measured.

1. System in cooling, indoor fan on, outdoor running. Wait 10–15 minutes so pressures settle. Doors closed, filter in — a missing filter lies about load.
2. Hook the low-side hose to the true suction service port. Record running suction in psig. Pick the refrigerant on the manifold (R-410A, R-22, or R-32) so the meter’s sat temp matches the bottle, then still run the calculator as an independent check.
3. Clamp a pipe-temperature probe on the suction line 6–8 inches from the compressor, on a straight section, out of the sun. Insulate the clamp. Do not use a bead thermocouple waving in the wind, and do not use evaporator leaving-air temp — that is delta-T, not SH.
4. Saturation temperature comes from suction pressure on the PT chart. Line temp minus sat temp is actual SH.
5. Compare to target. With indoor WB and outdoor DB entered, the calculator uses a piston charging-chart band (±3°F). Without those, treat 8–18°F as the typical field band, not a license to ignore the OEM chart in the outdoor panel.

Low SH (liquid leaving the coil) and high SH (starved coil) causes: troubleshooting.

How to Take Subcooling (TXV)

Subcooling is how many degrees the liquid line is below saturation at the liquid pressure you measured. It is extra liquid stacked in the condenser — the seal the TXV wants to meter from.

1. Same stabilize rules as superheat. TXV systems still need a running, loaded condenser.
2. High-side / liquid pressure at the liquid service port (or true liquid line pressure). Discharge pressure at the compressor is not the same number if you have a pressure drop through the coil — use liquid when you can.
3. Clamp the probe on the liquid line leaving the condenser, shaded. A sun-baked liquid line reads high and understates subcooling.
4. Sat temp from liquid pressure minus line temp = SC. Target is the outdoor unit’s charging chart — commonly 8–12°F, often printed as 10°F. Put that OEM number in the calculator; do not assume 10 if the plate says 15.
5. While you are there, glance at suction SH. A healthy TXV holds it. If SC is perfect and SH is 25°F, you have a TXV / bulb / equalizer problem, not a “needs two more pounds” problem.

Gauges and Pipe-Clamp Probes That Do Not Lie

Analog gauges plus a pocket thermometer can work. Digital manifolds (Fieldpiece SMAN, Testo 550s, Yellow Jacket 46060) do the PT lookup on screen — which is exactly why you still want an independent check when the number looks impossible.

• Zero and refrigerant: Wrong refrigerant on the manifold gives a confident wrong sat temp. R-410A sat at 118 psig is ~40°F; R-22 at 118 psig is ~61°F. That is a 20°F SH error from a picker wheel.
• Clamp metal, not insulation: Scrape a clean spot. Clamp on paint and dirt reads low. Shade the clamp.
• Hoses: Short, known-good hoses. A leaking low-side hose pulls SH up. Core tools beat hanging 6 feet of hose off a shaking compressor.
• Range check: Residential R-410A suction in cooling is commonly ~100–140 psig, liquid ~300–450 depending on outdoor temp. A 40 psig “suction” on 410A is a problem or a hose on the wrong port — not a target SH of 80°F.

What to Do With the Result on the Job

1. Identify piston vs TXV. Charge that method only.
2. Enter readings in the calculator. Screenshot or PDF cheat sheet the result for the invoice.
3. If the flag is overcharge / undercharge, weigh — do not “add until it feels cold.” High SH on a piston with a history of dye in the attic is a leak search (AccuProbe class), not a bigger charge.
4. If the flag mentions airflow, stop adding gas. TESP vs nameplate, then CFM. A frozen piston coil is often 0.90 in. WC, not two pounds short.
5. Delta-T (supply vs return air) is the air-side capacity check. Use it with SH/SC, not instead of it — which diagnostic first.

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