About this tool
The HVAC Delta-T Health Check is a free, in-browser troubleshooting tool that compares the temperature of the air entering your system (return) with the air leaving it (supply) and tells you whether the split is normal. In cooling mode the delta-T is simply return air temperature minus supply air temperature; a healthy central air conditioner typically drops the air 16–22°F, with 14–25°F considered acceptable depending on indoor humidity.
All of the arithmetic runs locally in your browser, so the temperatures you type are never uploaded. A delta-T below 14°F usually points to an overcharged system, excessive airflow, or a low cooling load, while a delta-T above 22–25°F usually means restricted airflow — a dirty filter or evaporator coil, a blocked return, or an undercharge. In gas-heating mode the tool instead computes the temperature RISE (supply minus return) and checks it against the furnace nameplate rise range (commonly 40–70°F): a low rise suggests overfiring or too much airflow, a high rise suggests too little airflow.
This is an informational maintenance guide, not a diagnosis of a specific failure — humidity, duct losses, and where you take the readings all shift the numbers. Measure supply and return air a foot or two from the equipment, away from register throw, after the system has run for 10–15 minutes, and use the verdict as a starting point for a professional charge or airflow check.