The Number That Matters Is The Difference
Vent air temperature alone misleads — it depends on how warm the house is. The diagnostic is the split: return-air temperature (at the big intake grille) minus supply-air temperature (at a vent near the air handler). A system in good health holds that split around 18-22°F under normal Florida conditions. The split is the same logic techs apply with better instruments; your kitchen thermometer gets a usable first read.
How To Measure Yours In Ten Minutes
Run cooling at least fifteen minutes. Hold a probe or instant-read thermometer in the return grille airflow until the reading settles; note it. Then hold it in the airstream of a supply vent near the air handler; note that. Subtract. Do it on a hot afternoon for the fairest test, and avoid the vent farthest down the duct run — long runs in hot attics lose a few degrees on the way and will slander a healthy system.
Reading The Result: Split Too Small
A split under about 15 degrees with everything running says the coil is not pulling enough heat: low refrigerant from a leak, a dirty coil, or a unit losing capacity. Pair it with symptoms — longer runtimes, rising bills, ice sightings — and you have a diagnosis worth booking. The warm-air and refrigerant-leak guides linked below carry that path.
Reading The Result: Split Too Big
A split over about 22-25 degrees usually means weak airflow across the coil — loaded filter, matted coil, duct restriction, or a struggling blower. The air gets very cold because too little of it is moving; comfort and coil-freezing problems follow. Check the filter first, then the airflow guide linked below.
What This Measurement Cannot Tell You
Delta-T is a screen, not a verdict — humidity load, duct losses, and measurement spots move it a few degrees honestly. Use it to decide whether to book a visit and to sanity-check what you are told. A tech whose diagnosis contradicts a sane split should be able to explain why with gauge readings; "it needs a pound of refrigerant" against a healthy 20-degree split is a sentence that deserves a second opinion.
Reading Your Temperature Split
Fifteen minutes of runtime, two readings, one subtraction.
| Your split | What it suggests | Next move |
|---|---|---|
| 18-22°F | Healthy heat removal | Enjoy; recheck seasonally |
| 15-18°F | Marginal — early decline or measurement noise | Re-test on a hot afternoon; watch trends |
| Under 15°F | Low refrigerant, dirty coil, lost capacity | Book a diagnosis |
| Over 22-25°F | Airflow starvation — filter, coil, ducts, blower | Check filter; then airflow diagnosis |
The Delta-T Self-Check
- Run cooling 15+ minutes before measuring.
- Measure return-grille air, then a supply vent near the air handler.
- Subtract: target roughly 18-22°F.
- Low split + weak cooling? Book refrigerant-side diagnosis.
- High split + feeble airflow? Filter first, then the airflow path.
Authoritative Sources
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Schedule ServiceFAQs
What temperature should air from AC vents be?
There is no single right vent temperature — it should be about 18-22°F colder than your return air. A 78-degree house blowing 58-degree air is healthy; the same vent temperature in a 72-degree house is a different story.
Is 65-degree vent air bad?
It depends on the return: from 78-degree return air, 65 at the vent is a weak 13-degree split worth investigating. From 82-degree return air on a brutal afternoon, it is within range. Always measure both ends.
Why is my AC blowing cold but the house never cools?
A healthy split with a losing house points past the equipment: duct leakage dumping cold air in the attic, badly undersized equipment, or envelope gains the system cannot outrun. The duct guide is usually the next read.
Does Florida humidity change the numbers?
Yes — heavy moisture loads eat some of the split (the coil spends capacity condensing water), so a degree or two of seasonal slack is honest. The screening bands above already assume Florida conditions.
What thermometer do I need?
Any decent instant-read or probe kitchen thermometer beats guessing. Infrared surface guns read vent metal rather than airstream — usable, but probe-in-the-airflow is the cleaner method.
My split is fine but a tech says I need refrigerant — who is right?
Refrigerant verdicts should come with gauge readings — superheat and subcooling — that you can ask to see. A normal split with no symptoms makes "needs a top-off" worth a free second opinion before money changes hands.
