AC Vent Temperature: Check Your Temperature Split (Calculator)

How the temperature split works

The temperature split, also called delta T, is the drop in air temperature across the indoor coil. Warm room air enters at the return; cold air leaves at the supply registers. The calculator subtracts one from the other and compares the result with a target range that shifts with indoor humidity.

Temperature split (ΔT) = Return air temperature − Supply air temperature
Target range: dry air 18–22 °F · moderate 16–20 °F · humid 14–18 °F

Humidity moves the target because the coil splits its work between cooling the air (sensible heat) and condensing moisture out of it (latent heat). In humid air more of the capacity goes into removing water, so the air temperature falls less. In dry air nearly all of the capacity lowers the temperature, so the split is larger. Across all conditions, 14–22 °F is the commonly quoted normal band for standard residential systems.

The split is a screening test, not a charge test. A technician confirms refrigerant charge with superheat and subcooling and checks airflow directly. The split tells you whether that visit is needed.

How to measure it properly

1. Set the thermostat at least 5 °F below room temperature so the system runs continuously, and let it run for at least 15 minutes.
2. Use a probe or digital thermometer. Infrared guns read the surface of the grille, not the air, and can be several degrees off.
3. Measure the return at the main return grille or at the filter slot near the air handler.
4. Measure the supply at the register closest to the indoor unit, with the probe pushed a few inches into the airflow. Distant registers pick up heat from ducts in attics and read higher.
5. Take two or three readings a few minutes apart and use the average.

Measurements taken just after start-up, with a heavily loaded filter or with half the registers closed will not reflect how the system normally runs.

Worked examples

Example 1, the calculator's defaults. Return air 75 °F, supply air 57 °F, moderate humidity.

ΔT = 75 − 57 = 18 °F
Target for moderate humidity = 16–20 °F → Normal
Expected supply air = 75 − 20 to 75 − 16 = 55–59 °F

Example 2, a weak system on a humid day. Return 76 °F, supply 66 °F, humid (above 55% RH).

ΔT = 76 − 66 = 10 °F
Target for humid air = 14–18 °F; 10 °F is more than 3 °F below the bottom of the range → Low

With good airflow, a 10 °F split usually means the coil is not getting cold enough: a low refrigerant charge from a leak, a compressor that is not pumping properly, or an outdoor unit that has stopped.

What a low or high split points to

A high split can feel good at the register, but low airflow reduces total capacity and can freeze the coil. A low split goes with an AC that is not blowing cold. Supply air that is very cold can also make registers sweat; see condensation on AC vents.

Diagnostic sequence after an abnormal reading

1. Replace the air filter and open every supply register, then remeasure after 15 minutes.
2. Confirm the outdoor unit's fan and compressor are both running. If not, see outside AC unit not running.
3. Look for ice on the suction line or indoor coil. If present, switch cooling off and run the fan to thaw it before testing again.
4. With the power off at the disconnect, rinse the outdoor coil with a garden hose.
5. If the split is still outside the range, book a technician to check charge, airflow and the compressor. Refrigerant handling requires EPA Section 608 certification.

Do not remove panels on the air handler or outdoor unit to take readings closer to the coil. Both contain line-voltage wiring, and the outdoor unit's capacitor can hold a charge after the power is off.

Why some systems read differently

Variable-speed and two-stage systems running at low capacity can show smaller splits than the table suggests, especially in mild weather, because they deliberately move air more slowly with less capacity. Heat pumps in cooling mode behave like any AC. Ductless mini-splits are measured at the indoor head and usually show a larger split because their airflow per ton is lower. For any of these, use the reading as a trend: a split that has dropped compared with last season is more telling than a single number.

Frequently asked questions