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- Duct check
Two thermometers, five minutes
Before you spend anything, measure. Three temperatures tell you whether your system is cooling properly and whether the attic is eating the difference on the way to the far rooms.
Your three readings
Take the readings with the system running for at least fifteen minutes on a hot afternoon. Degrees Fahrenheit.
Temperature split
—Return minus the closest register. On most systems in cooling this sits somewhere around 16–22°F.
Loss along the ducts
—How much warmer the far register is than the close one. A few degrees is normal; much more usually means attic heat is getting in.
How to take the readings
- Run the system in cooling for fifteen minutes with doors open and windows shut.
- Hold the thermometer in the return airflow — not against the filter — until the number settles.
- Do the same at the register closest to the indoor unit.
- Then at the register farthest away from it.
- Write all three down with the outdoor temperature.
These are field numbers, not a diagnosis. A low split can be low airflow, a dirty coil or refrigerant; a big loss along the ducts can be leaks, crushed flex or missing insulation. We measure the rest with instruments.
What we do about ducts
- SealMastic at the boots, plenums and joints — not tape that lets go in the heat.
- InsulateBring the runs up to a proper jacket, and bury them in insulation where we can.
- Re-runShorten the path, take out the kinks and support the flex so it stops sagging.
- BalanceSet the dampers so the far rooms finally get their share.
