Static Pressure

Notes

• Every duct can be correctly sized and the system still fail. Static pressure is a budget, and it is spent by the filter, the coil, the grilles and the duct together. Here the total is 0.65 against a blower rated for 0.5. That is over, and airflow suffers by roughly 15%.
• The filter is usually the biggest single line, and the easiest to fix. A 1″ MERV 13 costs 0.45 in w.c. on its own — almost the entire budget before the air has gone anywhere. A 4″ media cabinet filters as well or better at 0.15, because more filter area means lower velocity through the media. It is the rare upgrade that improves two things at once.
• Equivalent length is not tape-measure length. Every elbow, boot, takeoff and transition adds equivalent feet — a hard 90 can be worth 40 ft of straight duct. The 150 ft used here is the equivalent figure, and a system that measures 60 ft on a drawing often runs 200 ft equivalent.
• Measure it, do not estimate it. Two static pressure ports and a manometer, one before the coil and one after the blower, take ten minutes. Every number on this page is a planning estimate; the manometer is the truth. A contractor who has never measured static on your system does not know whether it is working.
• Fixing it in the right order: filter first (cheapest and biggest), then return air (usually the starved side), then supply. Turning the blower speed up is the wrong fix — it moves the noise, raises the power draw, and leaves the restriction where it is.