Delta-T Calculator

Temperature split from return vs supply air — expected range shifted for outdoor humidity and temperature

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Dual-Temp Probes & Backup Diagnostics

From the TestTalkHQ HVAC Tools / Thermometers catalog — a dual-probe meter for return vs supply air, IR guns for a quick surface check (not a substitute for in-stream probes), plus the next meter when split says airflow or charge.

Fieldpiece ST4 Dual Temperature Meter

Two probes, one delta. Park one in the return stream and one in the supply after the coil — this calculator is the expected-range check against that display.

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Fluke 62 Max Industrial Infrared Thermometer

Fluke 62 Max IR Thermometer

Fast surface scan of a supply plenum or register. IR is not in-stream dry-bulb — use it to find a hot/cold spot, then confirm split with probes.

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Klein Tools IR5 Dual Laser Infrared Thermometer

Klein Tools IR5 Dual-Laser IR

Dual-laser IR for a quick plenum or grille surface read. Same caveat as the Fluke: emissivity and mixing air will lie. Probes own the calculator inputs.

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HoldPeak 866B Thermo-Anemometer

When split is high, you still need FPM. Vane meter for register face velocity after delta-T says the coil is seeing too little air.

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Testo 512-2 Differential Pressure Manometer

High cooling split usually means low CFM. TESP vs nameplate is the restriction check — pair with the ESP calculator, not a second guess at thermometers.

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Fieldpiece SM380V SMAN Digital Manifold

Low split with decent airflow is often charge. Hook gauges and run superheat / subcooling — delta-T is not a charging chart.

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How to Use This Delta-T Calculator

Place one probe in the return stream before the coil and one in the supply stream after the coil. Run the mode you are diagnosing for 10–15 minutes. Enter outdoor dry-bulb and outdoor RH so a humid-day 15°F split is not graded against a dry 16–22°F meme.

Cooling ΔT = T_return − T_supply | Heating ΔT = T_supply − T_return

Walkthrough: How to Measure Delta-T. Causes: high & low delta-T troubleshooting. Gauges vs thermometers: Delta-T vs Superheat/Subcooling and the live SH/SC calculator.

High split is usually airflow — ESP calculator and duct CFM calculator. This tool does not size duct (ductulator) and does not charge the system.

Delta-T Calculator