Airflow | United States
Cooling Delta-T and Split Temperature Sanity Check
A quick field reference for using return-to-supply temperature difference without over-reading one number.
Built for United States field work where techs bounce between split systems, furnaces, heat pumps, package units, and mixed local code adoption.
Ticket note prompts
- Write down return and supply locations so the next tech knows whether the split was measured at the cabinet or at the duct runs.
- Note indoor and outdoor conditions when the split was taken, especially after pull-down or high humidity complaints.
- Record what follow-up measurement proved the diagnosis: airflow, charge, compressor performance, or latent-load explanation.
Comeback prevention
- Confirm the system is stabilized before comparing the final split with the first reading.
- Make sure airflow was verified if the split improved after filter or blower corrections.
- Call out any high latent-load conditions so a future revisit does not misread the same number.
How to use the number correctly
- Take return and supply readings as close to the equipment as practical, not at the farthest register.
- Let the system stabilize before trusting the number, especially after recent door openings or thermostat changes.
- Use wet conditions and airflow context; delta-T alone is not a charging method.
Quick interpretation
| Observed split | What it can mean | First checks |
|---|---|---|
| Below normal | High latent load, low charge, bypass, or weak compressor | Airflow, filter, blower speed, charge method |
| In normal band | System may be fine | Confirm airflow and customer comfort complaint |
| Above normal | Low airflow or restriction | Filter, coil, blower wheel, supply/return pressures |
Do not miss these traps
- A perfect split with terrible airflow can still leave rooms uncomfortable.
- A low split during pull-down after a hot start may be normal for the moment.
- Furnace temperature rise belongs to the nameplate range, not to cooling split rules.