Wet-Bulb Temperature, Dry-Bulb, Relative Humidity & Target Superheat — Field Guide for HVAC Techs

If you've ever charged a fixed-orifice AC with just suction pressure and a "it feels right," this guide is for you. The three readings that drive an accurate charge are dry-bulb temperature, wet-bulb temperature, and relative humidity. Once you understand what those three numbers actually mean and how they relate, the manufacturer's target superheat chart stops being magic — you'll know exactly why it gives you the number it does.

The Three Psychrometric Readings, Plain English

1. Dry-Bulb Temperature (DB)

Dry-bulb is what a normal thermometer reads in shaded air — the "regular" temperature your thermostat shows. It tells you the sensible heat content of the air. When the homeowner says "it's 78°F in here," they're quoting dry-bulb. It says nothing about how much moisture the air is carrying.

2. Relative Humidity (RH)

Relative humidity is the percentage of moisture the air is holding compared to the maximum it could hold at that dry-bulb temperature. Warm air can hold more moisture than cold air, so RH is always tied to a temperature. 50% RH at 75°F is a very different moisture load than 50% RH at 95°F.

RH alone is not enough for charging. Two houses both reading 50% RH at different dry-bulb temperatures have different actual moisture loads — and the AC will need to remove different amounts of latent heat.

3. Wet-Bulb Temperature (WB)

Wet-bulb is the temperature a thermometer reads when its bulb is wrapped in a wet wick and air is moving across it. The water evaporating off the wick pulls heat out of the bulb. The drier the air, the more evaporation, the lower the reading.

That means wet-bulb is a single number that captures both heat AND moisture in the air. If wet-bulb is low, the air is dry, the evaporator will run mostly sensible. If wet-bulb is high, the air is humid, the evaporator will run a much larger latent load. This is why HVAC manufacturers chose wet-bulb — not RH — as the indoor reading on every target superheat chart.

How to Measure Wet-Bulb in the Field

• Digital psychrometer — fastest and most reliable. Brands like Fieldpiece, Testo, and UEi give DB, WB, and RH on one screen. Place the probe directly in the return air stream, 4–6″ into the return grille, away from the filter face.
• Sling psychrometer — old-school but rock solid. Wet the wick with distilled water, sling it for 60–90 seconds, read the lower (wet) thermometer immediately.
• App + DB thermometer — if you only have dry-bulb and RH, plug them into a psychrometric calculator (most service apps include one) and let it compute wet-bulb.

Where you measure matters. Measure return-air wet-bulb at the return grille, not at the air handler after the blower picks up motor heat. A 1°F wet-bulb error can shift your target superheat by 2–4°F, which is enough to push you from a correct charge to an undercharge or overcharge.

Why Target Superheat Exists

On a fixed-orifice (piston) system, the metering device cannot adapt to load. It just sprays a fixed amount of refrigerant into the coil. If you set the wrong refrigerant charge, the coil will either flood (low superheat, liquid back to the compressor) or starve (high superheat, evaporator can't absorb heat, capacity drops).

The manufacturer calculates a target superheat based on the actual load the coil is seeing, then publishes a chart so you can set the charge to land on that target. The two inputs to that chart are:

• Indoor wet-bulb – tells the chart how much heat (sensible + latent) the evaporator must absorb.
• Outdoor dry-bulb – tells the chart how hard the condenser has to reject that heat.

Together, they describe the load the system is fighting in this exact moment. That's why target superheat moves throughout the day even when the equipment hasn't changed.

How Target Superheat Is Calculated — The Logic

The relationship the chart encodes is:

1. Higher indoor wet-bulb ⇒ higher target superheat. A more loaded evaporator boils off liquid refrigerant faster, so the vapor leaving the coil has more time to absorb extra (superheat) heat before it leaves.
2. Higher outdoor dry-bulb ⇒ lower target superheat. A harder-working condenser pushes liquid into the evaporator more aggressively, leaving less room for superheat.

That's why every manufacturer chart has indoor WB across the top, outdoor DB down the side, and target superheat in the cells.

The Field Approximation Formula

Most HVAC training programs teach the ACCA-style piston superheat approximation:

Target SH ≈ (3 × Indoor WB − 80 − Outdoor DB) ÷ 2   (all temperatures in °F)

It's an approximation — always verify against the manufacturer chart when one is available — but it lines up closely with most R-410A residential charts and is what your field calculator should match. Floor at 0°F (the chart is invalid below that).

Quick-Reference Target Superheat Table (computed from the formula)

"—" means formula returns a value at or below 0 — the chart is invalid; recheck conditions or use manufacturer literature. Generic R-410A residential split; always defer to the unit's data plate or installation manual.

Worked Example: Charging A Piston System

Conditions on the truck:

• Indoor return air dry-bulb: 78°F
• Indoor return air relative humidity: 55%
• Indoor return air wet-bulb: 65°F (measured directly with a digital psychrometer)
• Outdoor dry-bulb: 95°F

Step 1. Apply the formula: Target SH = (3 × 65 − 80 − 95) ÷ 2 = (195 − 175) ÷ 2 = 10°F target superheat. (Cross-check the unit's chart — it should be within a few degrees.)

Step 2. Measure actual superheat at the suction line near the service valve: Actual SH = Suction line temperature − Saturation temperature at suction pressure.

Step 3. Compare:

• Actual SH > target → system is undercharged. Add refrigerant slowly, watch SH drop.
• Actual SH < target → system is overcharged. Recover slowly, watch SH rise.
• Actual SH ≈ target (±3°F) → charge is correct for this load.

Charge in small steps. 2–4 oz at a time on a residential split, then let the system stabilize for 10–15 minutes before re-reading. Refrigerant moves around the system, and a quick reading right after charging will lie to you.

How Wet-Bulb Changes The Game On Humid Calls

Pull two real-world examples on the same 85°F outdoor day:

• Dry climate call — indoor 78°F DB, 20% RH → wet-bulb ≈ 57°F → (3×57−80−85)÷2 = 3°F target SH.
• Humid coastal call — indoor 78°F DB, 70% RH → wet-bulb ≈ 70°F → (3×70−80−85)÷2 = 22°F target SH.

Same equipment, same outdoor temperature, same thermostat setting — but the correct charge target differs by 19°F. If you don't read wet-bulb, you can't possibly hit the right number.

Common Field Mistakes

• Charging a piston system to subcooling. Subcooling charging is for TXV systems. Pistons charge to superheat.
• Using RH instead of wet-bulb. RH alone is ambiguous. Always convert RH + DB → WB before using the chart.
• Measuring WB at the air handler. Motor heat and duct losses skew the reading. Measure at the return grille.
• Charging when indoor load is too low. If indoor WB is below ~57°F (e.g. cold weather charging), the chart is invalid — raise indoor load with heat strips or recheck on a warmer day.
• Forgetting airflow. Target SH assumes proper CFM across the coil (~400 CFM/ton). Dirty filter, dirty coil, restrictive duct → your SH numbers will lie.

Quick Decision Card

Putting It Together — Field Workflow

1. Verify it's a fixed-orifice system. If it's a TXV, switch to subcooling charging.
2. Confirm airflow first — filter, blower wheel, coil cleanliness, static pressure.
3. Measure indoor return-air wet-bulb at the return grille. Record indoor DB and RH too.
4. Measure outdoor dry-bulb in shade, away from the condenser discharge.
5. Look up target superheat on the unit's chart.
6. Measure actual superheat at the outdoor suction service valve.
7. Adjust charge in small steps, wait 10–15 minutes between readings, until actual SH = target ±3°F.
8. Document final readings on the invoice or work order so the next tech (or the warranty audit) has the proof.

Related Tools On HVAC IQ Pro

• Superheat & Subcooling — The Complete Calculation Guide
• TXV Troubleshooting — Stuck Open vs Stuck Closed
• HVAC Age Decoder — verify the unit you're charging