Anemometer vs Flow Hood — Which CFM Is More Accurate

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They Are Not the Same Number

In-duct CFM is velocity × duct area at a plane in the metal. It includes air that will later leak out of takeoffs, and it excludes air that already leaked in upstream of a return plane. A proper Log-Tchebycheff traverse with a calibrated hot-wire is the gold-standard duct measurement when you do it right.

Flow-hood CFM is what left (or entered) that grille into the room. The hood captures the jet, the meter integrates it, and you do not multiply by a guessed free area. It will not match duct CFM if the boot leaks, a damper is cracked, or you are hooding one of six supplies on a trunk.

Accuracy is “accurate to which question?” Trunk balance and blower delivery → duct traverse. Room comfort and TAB at the diffuser → hood.

When the Anemometer (In-Duct) Is More Accurate

• You cannot seal a hood — huge returns, irregular grilles, floor registers in a tight hallway, or a diffuser the skirt will not fit.
• You need trunk or leaving-coil CFM for a blower chart or to split a plenum. There is no “hood” on a 20×8 trunk in a joist bay. That is the STA2-class traverse the duct CFM calculator is built for.
• You already have a good plane — long straight duct, known inside dimensions, liner accounted for, 6+ points averaged. Then the formula is honest physics.

It is less accurate when you center-stab, use outside dimensions, stand in an elbow, or convert grille FPM with duct area. Those are operator errors, not a reason to throw the probe away. How to do it: duct CFM guide.

When the Flow Hood Is More Accurate

• Delivered room CFM. What the occupant feels. Leaks between trunk and boot show up as hood < duct.
• Supply diffusers with complex faces where a vane AK factor is a guess.
• TAB / commissioning against a room schedule. Hoods are what most balancers sign.

Hoods lie when the skirt leaks, the grille is so jetty the hood backpressures and cuts real flow (some meters backpressure-compensate; cheap ones do not), or you hood a return that is pulling from a door undercut as much as from the duct. They are also bulky. There is no capturing-hood SKU in the TestTalkHQ affiliate CSV at publish time — so this article does not pretend to sell one. Borrow, rent, or buy off-catalog; do not substitute a vane and call it a hood.

Where Handheld Vane Meters Fit

HoldPeak / BTMETER-class vanes at a register are a third, weaker CFM. You must apply the manufacturer’s free-area factor. Using duct metal area with face FPM will overstate. Use them for a rough “is this register dead?” check, then hood or traverse for a number you will write on a report.

If CFM at the room is low, do not argue anemometer vs hood for an hour before you measure cabinet static. Restriction vs delivery is the other article: static vs airflow and the ESP calculator.

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