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HVAC

Target Delta T Calculator

Read return dry-bulb, return wet-bulb, and supply dry-bulb. Get the target evaporator split for those conditions and a verdict that points to airflow or charge.

°F

Plain thermometer in the return, before the coil.

°F

Sling reading in the return. Sets the target split.

°F

In the supply plenum, past the coil.

Evaporator split
Split too narrow — suspect charge / airflow
19°F actual
Target
22°F
Actual
19°F
Difference
-3°F
ƒShow the math
  1. 1.Target temperature splitper industry field chart

    From a published field chart of target evaporator split vs. entering conditions (interpolated on return dry-bulb and wet-bulb). Split falls as indoor humidity rises.

    return 75 °F DB / 63 °F WB
    = 22 °F
  2. 2.Actual split
    return DB − supply DB
    75 − 56
    = 19 °F
  3. 3.Difference vs target
    actual − target
    19 − 22
    = -3 °F
Before you rely on this
  • A narrower-than-target split usually means an undercharge, too much airflow, or a weak compressor. Verify charge with superheat (fixed-orifice) or subcooling (TXV).
  • Temperature split is a screening check, not a charge measurement. Let the system run 15+ minutes and confirm with superheat/subcooling.

How to read the split

The evaporator temperature split is return dry-bulb minus supply dry-bulb. On its own a number like “18 °F” means nothing until you compare it to what the entering conditions call for.

  • The target comes from a field chart of expected split versus entering wet-bulb and dry-bulb — higher humidity, smaller target.
  • Within ±3 °F of target, the split is normal for those conditions.
  • Wide → low airflow (or low load). Narrow → charge, high airflow, or a weak compressor.

The target split here is reconstructed from generic field charts and is a screening aid. It is not a substitute for a superheat or subcooling charge check, and split alone cannot tell you the exact refrigerant charge.

Frequently asked

What should the delta T be on an AC?
There is no single number — the target depends on indoor humidity. A dry return (about 63 °F wet-bulb at 75 °F) targets a wider split near 21 °F, while a humid return (70–72 °F wet-bulb) targets a narrower 14–16 °F. A common rule of thumb is 16–22 °F, but this tool sets the real target from your return wet-bulb and dry-bulb.
Why does higher humidity lower the target split?
In humid return air the coil spends more of its capacity condensing moisture (latent load) and less dropping temperature (sensible), so the dry-bulb split across the coil is smaller even though total capacity is normal. That is why you need the return wet-bulb, not just dry-bulb, to judge a split.
My split is too wide — what does that mean?
A split more than about 3 °F above target usually means low airflow: a dirty filter or coil, closed dampers, undersized or leaky duct, or a slow blower. Low airflow lets the air sit on the cold coil longer, so it comes off colder. Fix airflow first — it also mimics an undercharge on a superheat check.
My split is too narrow — what does that mean?
A split more than about 3 °F below target usually means an undercharge, too much airflow, or a weak compressor. The coil is not getting cold enough to pull the normal temperature drop. Confirm charge with superheat on a fixed-orifice system or subcooling on a TXV system.
Is temperature split the same as measuring charge?
No. Split is a fast screening check that tells you whether to look at airflow or charge, not a charge measurement. Let the system run at least 15 minutes, then confirm with superheat or subcooling before adding or recovering refrigerant.

Related tools

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Superheat & Subcooling

Confirm charge when the split reads narrow.

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CFM per Ton Calculator

Check the airflow behind a wide split.

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Static Pressure Calculator

Find the duct restriction choking airflow.

Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed HVAC contractor. See our methodology, sources, and code editions.