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What temperature split actually measures, normal ranges by humidity, and how to place the probes
Delta-T (temperature split) is the air-side capacity check: how many degrees the coil or heat exchanger changed the air that actually went through it. It is not superheat. It is not CFM. It is two dry-bulbs, same mode, after the blower has been running long enough to settle.
Need the subtraction and the humidity-adjusted band done for you? Open the Delta-T Calculator.
Have return and supply already?Split vs a 16–22°F cooling band, shifted for outdoor humidity and temp.
Open Calculator →Jump to what you need
What Delta-T Actually Measures
The coil (or heat exchanger) changes the temperature of the air that crosses it. Delta-T is that change:
Cooling ΔT = T_return − T_supply | Heating ΔT = T_supply − T_return
It is a sensible check. On a wet cooling coil, a large share of capacity can go to dehumidification (latent) instead of dropping dry-bulb. That is why a 15°F split on a sticky 75% RH afternoon is not automatically “low charge,” and why a 22°F split in Phoenix dry air is not automatically “restricted duct.” The calculator shifts the expected band for outdoor humidity and outdoor temperature for that reason.
Delta-T cannot see refrigerant saturation. Charge lives on superheat / subcooling. Delta-T cannot see blower restriction. That is ESP and CFM. When to grab which: Delta-T vs SH/SC.
Normal Ranges by System Type and Humidity
The meme “20°F split” is a midpoint, not a spec. Typical residential direct-expansion cooling at about 400 CFM/ton:
| Condition | Expected cooling ΔT | Why |
|---|---|---|
| Typical outdoor RH ~40–55%, ~95°F design day | 16–22°F | Textbook sensible band |
| Humid (RH ~55–70%) | 14–20°F | More latent, less dry-bulb drop |
| Very humid (RH ≥ 70%) | 12–18°F | Wet coil working as a dehumidifier |
| Dry / arid (RH < 40%) | 18–24°F | Almost all capacity is sensible |
| Outdoor ≥ 100–105°F | Band shifts down ~2–3°F | Condenser capacity falls |
| Gas furnace temperature rise | Nameplate, often 30–70°F | Not 20°F. Use the plate, not a cooling meme |
Heat pumps in heat: Leaving-air is often cooler than a gas furnace. Do not flag a 25°F heat-pump rise as “low” against a 50°F gas nameplate. Use the heat-pump’s published range or treat 20–35°F as a common air-source ballpark until you have the OEM sheet.
There is no honest one-page “delta-T by humidity” chart that replaces indoor wet-bulb, CFM/ton, and coil condition. The bands above are the same ones the calculator uses — field windows, not Manual J.
How to Take an Accurate Reading
- Mode and runtime. Cooling call or heating call, blower on the speed you are diagnosing, 10–15 minutes so the coil is wet (cooling) or the exchanger is hot (heat). Filter in, doors on.
- Return probe in the moving return stream before the coil. A thermostat on the wall is mixed room air, not coil entering air. A probe jammed in insulation in the corner of the cabinet will read low and inflate cooling split.
- Supply probe in the leaving airstream after the coil / exchanger, before the first long trunk run. A register at the end of a 40-foot flex is leftover temperature, not coil leaving-air.
- A dual-temp meter (Fieldpiece ST4 class) is the right tool: two bead or clip probes, one delta on the screen. IR on painted metal is a scout, not the number you enter.
- Record outdoor dry-bulb and outdoor RH (or a psychrometer at the return for a better latent story). Enter them in the calculator so a 15°F split on a 75% day is not flagged as a 16–22 failure.
Field example — coolingReturn 75°F, supply 55°F, outdoor 95°F, RH 50%. ΔT = 20°F, inside 16–22. Same 20°F at 80% RH is toward the high end of a 12–18 band — not “perfect charge,” just a different latent mix. If the split were 10°F at 50% RH, you have a capacity or charge story, not a duct story.
What to Do With the Result on the Job
- Compute split vs the humidity-adjusted band in the calculator. Screenshot or PDF the cheat sheet.
- High cooling split → airflow until proven otherwise. Measure ESP, then CFM. Do not add refrigerant to a 28°F drop.
- Low cooling split → capacity / charge / coil / extra CFM. Hook gauges. Measure SH or SC for the metering device you actually have.
- Causes that look alike until you isolate them: high & low delta-T troubleshooting.
Two dry-bulbsReturn vs supply, with humidity context so 15°F is not always “broken.”
Open Delta-T Calculator →FAQ
Is 20°F the spec?
No. It is the middle of a typical sensible band. Humidity, outdoor temp, and CFM/ton move it. The calculator’s window is the field rule; the OEM data plate wins on furnaces.
Can I use a supply register?
Only as a room-comfort check. Coil split is at the equipment. Duct gain/loss between the plenum and the boot will fake a low cooling ΔT.
Does outdoor humidity equal indoor latent load?
It is a proxy, not a psychrometric chart. Return wet-bulb is better if you have it. Outdoor RH is what most techs actually log on a humid-day call, so the tool uses that as context.
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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.
View on Amazon →
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.
View on Amazon →
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.
View on Amazon →
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.
View on Amazon →
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.
View on Amazon →
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.
View on Amazon →As an Amazon Associate, TestTalkHQ earns from qualifying purchases.