Delta T
The temperature difference between the air entering and leaving the indoor coil, sometimes called split. A delta T well outside the manufacturer's stated range points to an airflow or refrigerant problem, and tells you which direction to look.
Also called temperature split, ΔT
What is delta T?
Delta T is the temperature difference between the air entering the indoor coil and the air leaving it. It is the quickest field indicator of whether a system is moving heat as intended.
Delta T: how it works
Air gives up heat as it crosses the coil, so the drop between return and supply reflects how much heat the coil is absorbing per unit of air. Too little air across the coil gives a large drop on a small volume; too little refrigerant gives a small drop on a normal volume. That is why the number, read against the manufacturer's range, points in one direction or the other.
Where you meet the term
- As one of the first field measurements taken on a cooling complaint.
- On a commissioning sheet, where the split achieved at handover becomes the later comparison point.
- In a homeowner conversation, where a rough reading gives an early indication before a technician arrives.
How to check it
- Let the system run before measuring. Readings taken in the first minutes of a run are meaningless. Give the system time to reach steady conditions. Owner check
- Measure at the return grille and at a supply register. On a ductless head, measure at the intake at the top and the outlet at the front. Keep the probes out of direct sun and away from other heat sources. Owner check
- Read the result against the manufacturer range. The acceptable range is published per model and it shifts with indoor humidity, so a single universal target does not exist. Technician
Delta T: what it is not
- Delta T is not a pass or fail number on its own. It narrows the search and is read alongside airflow and refrigerant measurements.
- The target moves with indoor humidity. A humid space produces a smaller air temperature drop because part of the coil's work is condensing moisture.
- Measuring at the wrong points, or too soon after start-up, gives a number that looks authoritative and is not.
Delta T questions
What is a good delta T?
The manufacturer publishes a range for the model, and it depends on indoor humidity as well as on the equipment. A figure well below or well above that range is what matters, not hitting a specific number.
Does high delta T mean the system is working well?
Not necessarily. A large drop across a starved coil moves less total heat than a moderate drop at full airflow, and it is a signature of restricted airflow rather than of good performance.
Can I measure delta T on a mini-split?
Yes, roughly, using a thermometer at the intake grille on top and another at the outlet louvre. Let it run first, and treat the result as an indication rather than a commissioning measurement.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Why Is My Mini-Split Not Cooling?Trane ductless cooling checks, airflow limits, frozen coil clues, and professional refrigerant boundary.