How to Calculate Delta-T
Delta-T is the fastest gut-check you own. Two temperatures, one subtraction, and you know whether a system is actually moving heat or just running. Here is how to measure it, what a normal split looks like, and what a bad number is really telling you.
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In This Guide
What Delta-T Actually Measures
Delta-T (written ΔT, and often called the temperature split) is the difference between the air entering the coil and the air leaving it. In cooling, that is the return air temperature minus the supply air temperature. It is a one-number snapshot of how much sensible heat the system is pulling out of the air as it passes across the evaporator.
What makes it valuable is that it links two things you cannot see directly: airflow and refrigerant capacity. Push too little air across a healthy coil and the air lingers, gets very cold, and the split climbs. Push plenty of air across a starved or undercharged coil and the split collapses. That is why a delta-T reading is the first thing many techs take after the system has had a few minutes to settle — it points you toward the airside or the refrigerant side before you ever hook up a gauge.
Sensible only. Delta-T reads a dry-bulb temperature change, so it captures the sensible heat the coil removes — not the moisture. On a humid day a working coil spends a big share of its capacity condensing water, which lowers the temperature split without meaning anything is wrong. Keep that in the back of your mind for the whole reading.
The Delta-T Formula
The calculation itself is about as simple as HVAC math gets:
Temperature Split
Both measured as dry-bulb temperatures, in °F.
The reason a split of a given size matters is that it plugs straight into the sensible heat equation, which is how you turn a temperature reading into delivered capacity:
Sensible Heat
The 1.08 constant comes from air density (0.075 lb/ft³) × 60 min/hr × the specific heat of air (0.24 BTU/lb·°F).
Rearrange that same equation and you can solve for the airflow the system is actually moving: CFM = BTU ÷ (1.08 × Delta-T). Delta-T is the hinge between the air you feel and the numbers on the nameplate.
How to Measure It in the Field
A sloppy measurement gives you a sloppy diagnosis. Take the two readings the same way every time.
Step 1 — Let it run.
Run cooling for 10–15 minutes so pressures and coil temps stabilize. A split taken 60 seconds after startup is meaningless.
Step 2 — Read the return.
Probe the return plenum or a return grille upstream of the coil with a calibrated thermometer. This is entering-air temperature.
Step 3 — Read the supply.
Read the supply plenum a foot or two downstream of the coil. Keep the probe out of the coil's direct line of sight so you measure air, not the radiantly cold coil face.
Step 4 — Subtract.
Return minus supply is your delta-T. Compare against the normal range below and against indoor humidity before you call it good or bad.
Watch your probe placement
Measuring the supply too close to the coil, near a leaky plenum, or downstream of a strip heater will skew the reading by several degrees. If the return probe is downstream of a return leak pulling in hot attic air, your split will read low for a reason that has nothing to do with the equipment. Same measurement, same spots, every visit.
What a Normal Reading Looks Like
On a properly charged residential split system with good airflow — roughly 400 CFM per ton — the cooling temperature split lands in a fairly tight band:
Low split — suspect low charge, too much airflow, or a very humid space
Normal cooling temperature split for most residential systems
High split — suspect low airflow: dirty filter, coil, or blower
That 14–22°F window is a starting point, not a pass/fail line. It shifts with indoor conditions. A useful field reference is how the expected split moves with the return-air wet-bulb (a proxy for humidity):
| Indoor condition | Return-air humidity | Expected split |
|---|---|---|
| Dry (arid climate, low RH) | ~30–40% RH | 20–25°F |
| Average comfort conditions | ~45–50% RH | 17–20°F |
| Humid (muggy day, high load) | ~55–65% RH | 14–17°F |
| Very humid / high latent load | > 65% RH | 12–16°F |
Manufacturer charging charts that pair indoor wet-bulb with outdoor dry-bulb are more precise than any single-number rule. When one is on the nameplate or in the install manual, use it.
Why Humidity Moves the Number
This is the part that trips up techs who treat delta-T as a hard number. A coil has a fixed total capacity, and it splits that capacity between sensible work (dropping air temperature) and latent work (condensing moisture out of the air). Delta-T only sees the sensible half.
On a muggy afternoon, a correctly operating system pours a large share of its capacity into wringing water out of the air. Every pound of moisture it condenses is roughly 1,000 BTU that never shows up as a temperature drop. So the very same equipment that gives you a 20°F split in dry March air might read only 15°F in humid July — and both readings are healthy. If you chase that lower summer number as a fault, you will overcharge the system and make comfort worse.
Rule of thumb: higher indoor humidity pushes the expected split down. Before you flag a low delta-T, take a wet-bulb reading (or note the space RH). A 15°F split in a 60% RH house is fine; the same 15°F split in a 35% RH house means something is off.
If you want to separate the sensible and latent halves properly, that is a psychrometric enthalpy calculation — covered in our sensible vs latent heat guide.
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Worked Example: Split to Capacity
Problem:
A 3-ton system is running in cooling. You measure 75°F in the return plenum and 57°F in the supply. The blower is rated to move 1,200 CFM. Is the system delivering its capacity?
Step 1: Calculate delta-T
Delta-T = 75 − 57 = 18°F
Step 2: Check it against the normal range
18°F sits comfortably inside the 14–22°F window. Good sign.
Step 3: Convert to sensible capacity
Sensible BTU/hr = 1.08 × 1,200 × 18 = 23,328 BTU/hr
Step 4: Compare to nameplate
A 3-ton unit is 36,000 BTU/hr total. Sensible capacity is typically ~70–80% of total, so ~25,000–29,000 BTU/hr expected. 23,328 sensible is in the neighborhood — reasonable, with the rest going to latent on a humid day.
Result: system checks out
An 18°F split at rated airflow, producing sensible capacity in line with a 3-ton nameplate, points to a system that is charged and moving air correctly. No refrigerant work needed based on this alone.
Note the dependency: the BTU math is only as good as your CFM. If you are not sure the blower is truly moving 1,200 CFM, verify airflow first — total external static pressure should be at or below about 0.50 iwc for most residential equipment. A high static reading means less air than the nameplate promises, which inflates your split.
Delta-T on the Heating Side
In heating, delta-T flips: it is supply minus return, and on a gas furnace it is called the temperature rise. The critical difference from cooling is that there is no universal 14–22°F band — every furnace has its rise range printed on the rating plate, and you must land inside it.
Furnace Temperature Rise
Typical nameplate windows run 35–75°F, often stamped as a range like "40–70°F." Always use the specific range on the unit.
Aim for the middle of the plated range. Rise that is too high means airflow is too low — the heat exchanger runs hotter than designed, which can crack it and trip the high-limit. Rise that is too low means airflow is too high, and the flue gases can drop below their dew point and condense inside a non-condensing furnace, rusting it out. You dial rise in with blower speed, not by touching the gas.
Gas safety
Never chase temperature rise by overfiring the burners past the nameplate input. Verify manifold gas pressure and firing rate with a manometer, correct airflow to bring rise into range, and if rise stays high with clean airflow, inspect the heat exchanger. A cracked exchanger is a carbon monoxide hazard — red-tag it.
Reading a Bad Delta-T
Delta-T does not tell you the fault — it tells you which direction to look. Here is how to read the two failure modes in cooling.
High split (above ~22°F): think airflow
Too little air is crossing the coil, so the air that does pass gets very cold. The usual suspects, in the order you should check them:
- Dirty or clogged air filter — the number-one cause, check it first
- Dirty evaporator coil restricting flow
- Closed or blocked supply registers, crushed or undersized ductwork
- Blower running too slow — wrong tap, failing motor, or slipping belt
- Iced-up coil (which itself is often caused by the low airflow above, or low charge)
Verify with static pressure. High total external static confirms the restriction.
Low split (below ~14°F): think capacity or humidity
The coil is not removing much sensible heat. Rule out the innocent cause first:
- High indoor humidity — the coil is doing latent work; a low split may be normal
- Low refrigerant charge — confirm with superheat and subcooling, not delta-T alone
- Airflow too high (oversized blower, tap set too aggressive) diluting the cold air
- Failing compressor not pumping to capacity
- Restricted metering device or TXV problem starving or flooding the coil
In both cases, delta-T is the trigger to grab your gauges or your manometer — never the whole diagnosis. Pair it with superheat, subcooling, and static pressure to confirm the actual fault before you add refrigerant or change a component.
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