Delta T Calculator — Free Online Calculator
Delta T — the temperature split across the coil — is the fastest first check of whether an HVAC system is moving and conditioning air properly. Enter the return and supply air temperatures and this tool gives your split, whether it's in the healthy range, and the most likely causes when it isn't.
Enter your air temperatures
Temperature split
Reading
Healthy
Ideal target
18–20°F
Likely causes
A 16–22°F split with average humidity means good airflow and a proper refrigerant charge — no action needed.
See the breakdown
A split is a screening check, not a charging method. Verify charge by superheat/subcooling and airflow per manufacturer specs.
What your reading means
Cooling mode, average indoor humidity. Targets shift with humidity and altitude.
| Temperature split | Reading | Most likely cause |
|---|---|---|
| Under 14°F | Low | Low refrigerant/leak, dirty coil, or too much airflow |
| 14–16°F | Low-normal | Often OK in humid conditions; verify charge |
| 16–22°F | Healthy | Good airflow and charge |
| 22–25°F | High | Restricted airflow — filter, vents, ducts, low blower speed |
| Over 25°F | Very high | Severe airflow restriction — do not ignore |
Sources & standards: HVAC School — "20° ΔT, A Lazy Rule of Thumb" · ENERGY STAR — HVAC Quality Installation (airflow & charge verification) · Furnace temperature rise per equipment nameplate / AHRI.
The formula, explained in plain English
Delta T is just the temperature difference the system creates between the air going in and the air coming out. Which way you subtract depends on whether you're cooling or heating.
What Delta T tells you
How much heat the coil is adding or removing as air passes through — a quick health check on airflow and capacity together.
Where to measure
Return a few feet before the coil, supply a few feet after — away from line-of-sight to the coil so radiant heat doesn't skew the probe.
Humidity shifts the target
In humid air more energy goes to removing moisture, so the cooling split runs lower — 15–18°F can be normal at high RH.
Not a charging method
Confirm refrigerant charge with superheat or subcooling, not split alone — Delta T only flags that something is off.
Worked examples
Three readings showing a healthy split, a classic high-split fault, and a furnace in heating mode.
Healthy AC — 75°F return, 56°F supply
Result: a 19°F split sits right in the 16–22°F band — good airflow and a proper charge.
Dirty filter — 76°F return, 51°F supply
Result: restricted airflow is overcooling slow-moving air. Change the filter and re-check.
Furnace rise — 130°F supply, 68°F return (heating mode)
Result: a 62°F rise is within the typical 40–70°F nameplate range — airflow and firing are well matched.
Frequently asked questions
Common questions about HVAC temperature split and furnace rise.
What is a good Delta T for an AC?
A healthy cooling temperature split is 16–22°F between return and supply air, with ~18–20°F typical at average humidity. Outside that range points to an airflow or refrigerant problem.
What does a high Delta T mean?
Above ~22°F in cooling almost always means restricted airflow — dirty filter, closed/blocked vents, undersized or leaky ducts, or blower speed set too low. Air moves too slowly and gets overcooled.
What does a low Delta T mean?
Below ~16°F means the coil isn't removing enough heat: low refrigerant/leak, a dirty evaporator coil, or too much airflow. It can also be normal briefly right after start-up.
Does humidity affect Delta T?
Yes. In humid air more of the system's capacity goes to removing moisture (latent heat), so the measured split runs lower — 15–18°F can be perfectly normal at high indoor humidity. Find the dew point for your return-air conditions with our Dew Point Calculator.
What is a normal furnace temperature rise?
Most furnaces specify a rise of 40–70°F between return and supply, listed on the nameplate. Too high means airflow is restricted (overheating risk); too low means too much airflow or too little heat.
Can I charge refrigerant using Delta T?
No. Delta T is a screening check. Set and verify charge with superheat (fixed-orifice) or subcooling (TXV) and confirm airflow per the manufacturer — split alone can't tell you the exact charge.
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