Field note
Delta-T is a fast field clue, not a universal target.
What changes delta-T
- Airflow through the coil
- Entering dry-bulb and wet-bulb conditions
- Equipment type and staging
- Outside-air percentage and reheat
- Coil cleanliness, refrigerant performance, and sensible/latent load
Use it with other evidence
- Measure filter/airflow condition and blower performance.
- Pair air temperatures with refrigerant pressures/temperatures when the sealed system is in question.
- On WSHP/hydronic equipment, add entering/leaving water temperatures.
- On BAS/VAV systems, confirm damper position and actual airflow to the zone.
Examples from the field archive
The archive contains operating splits around 17-21°F on several comfort systems, but those are observations from specific conditions. A 17°F split on a 100% outside-air unit and a 21°F split on a conventional cooling call do not mean every system should target those values.
Trend when the complaint is intermittent
If a sensor or comfort complaint is intermittent, repeat measurements over time or use BAS trend data. A single normal delta-T during one visit may only prove the system was normal at that moment.
Safety / model checks
- Do not use one supply/return temperature split to declare charge correct or incorrect.
- Place temperature probes where they represent the air entering and leaving the equipment, not a mixed or sun-heated grille location.
Safety note
HVAC work can involve high voltage, rotating equipment, pressurized refrigerants, and combustion systems. Use this site as a reference only. Follow the equipment manual, applicable codes, PPE, lockout/tagout, and current refrigerant rules. Field cases and troubleshooting paths are case-specific and are not OEM procedures or universal targets.