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Delta T Calculator

Temperature split between return and supply air, assessed.

Your measurementsEvery assumption editable01

Measured in the return duct near the air handler.

Measured in the supply plenum, past the coil.

Temperature splitCalculated live02

Result

19°F split

Normal — within the usual 16–22 range

Return air
75 °F
Supply air
56 °F
Temperature split
19 °F
Assessment
Normal — within the usual 16–22 range

A split of roughly 16 to 22 °F is normal for residential air conditioning at typical indoor humidity. The exact target moves with humidity: drier indoor air produces a larger split for the same system because less of the coil's capacity goes into condensing moisture.

A low split usually means low refrigerant charge, a dirty coil, or too much airflow. A high split usually means too little airflow — a blocked filter, closed registers or undersized return ducts — rather than a system working unusually well.

This is a screening check, not a diagnosis. Charge should be verified by superheat or subcooling against the manufacturer's table, and airflow measured, before anyone adds refrigerant.

Measure the return and supply air temperatures and this tells you whether the split is normal. It is the fastest screening check there is on a cooling system — and the direction it fails in points at very different causes.

Why use this tool?What it does differently03

Why use this tool?

Assessed, not just calculated

The result says whether the split is low, normal or high rather than leaving you to look it up.

Points at the cause

Low usually means charge or a dirty coil. High usually means restricted airflow — not a system working unusually well.

The fastest screening check

Two thermometer readings tell you more about a struggling system than almost anything else you can do without gauges.

Humidity context

The target moves with indoor humidity, and the notes explain why.

How this worksThe method04

How this delta t calculator works

The split is simply return air temperature minus supply air temperature, measured in the ductwork either side of the evaporator coil. Sixteen to twenty-two degrees Fahrenheit is normal for residential cooling at typical indoor humidity.

The target moves with humidity. Drier indoor air produces a larger split for the same system, because less of the coil's capacity goes into condensing moisture and more into lowering temperature.

Direction of failure matters. A low split usually indicates low refrigerant charge, a dirty coil or excessive airflow. A high split usually indicates too little airflow — a blocked filter, closed registers or undersized returns — rather than exceptional performance.

How to use itStep by step05

How to use it

  1. Step 1: Let it run

    Fifteen minutes of continuous operation before measuring, so the system has stabilised.

  2. Step 2: Measure the return

    In the return duct near the air handler, not at a wall grille where room air mixes in.

  3. Step 3: Measure the supply

    In the supply plenum past the coil, out of line of sight of the cold coil surface.

  4. Step 4: Read the assessment

    Then investigate airflow or charge depending on which direction it failed.

Example usageWorked figures06

Example usage

A healthy system
Return 75 °F, supply 56 °F — a 19 degree split, comfortably within the normal range.
Restricted airflow
Return 75 °F, supply 49 °F is a 26 degree split. That reads impressive and is not: too little air is crossing the coil, and the coil is at risk of freezing.
Low charge
Return 75 °F, supply 65 °F is a 10 degree split. The system runs constantly and never satisfies the thermostat.
Frequently asked questionsCommon questions07

Frequently asked questions

What is a normal delta T for air conditioning?

Sixteen to twenty-two degrees Fahrenheit between return and supply air is normal for residential cooling. The exact target shifts with indoor humidity.

What does a low temperature split mean?

Usually low refrigerant charge, a dirty evaporator coil, or too much airflow. It means the system is not removing as much heat as it should.

What does a high temperature split mean?

Usually restricted airflow — a blocked filter, closed registers, or undersized return ducts. It is not a sign of a system performing well, and it risks freezing the coil.

Should I add refrigerant if the split is low?

Not on this reading alone. Charge should be verified by superheat or subcooling against the manufacturer's table, and airflow measured first. Adding refrigerant to a system with an airflow problem makes it worse.

Where exactly do I measure?

In the return duct near the air handler and in the supply plenum past the coil. Measuring at room grilles introduces mixing and duct losses that make the reading meaningless.

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