Any idea how well something like this would work?

https://www.amazon.com/Digital-Anemometer-Handheld-Accurately-Temperature/dp/B07J64TCBQ/

Why is this necessary? If it's not functioning properly, wouldn't it be better to call the installer and let them figure out what's happening? They should have the necessary equipment and know-how.

Down to 5 CFM will be hard to measure, the pitot pressures are very low! Might be better to trap the entire flow with a flow hood, this one says it will go down to 7 CFM:

https://www.valuetesters.com/cps-abm-hood-airflow-capture-hood-balometer.html

Hot wire anemometers will go down to the low velocities you want, but then there is the issue of non-uniform flow within the duct itself that would require multiple measurements across the duct diameter. A flow hood avoids this issue.

@Elmer J Fudd, this is a research project with a couple of universities.

@formulaross20, thanks. A flow hood would be tough because some of the vent configurations include things like six 3" holes spread out across 10'. Any thoughts on how accurate hot wire anemometers are vs vane?

Theoretically, a hot wire anemometer should be more sensitive at low airflow than a vane anemometer. The vane device requires inertial energy transfer from the flowing gas to the vanes in order to turn them. There's not much kinetic energy available at low velocities to start with, and friction in the vane device has to be overcome as well. In contrast, the hot wire device relies on heat transfer from the hot wire to the moving gas. No friction and heat/energy transfer is fairly easy to measure and sensitive to gas flow. Mass air flow sensors in current vehicle engines is almost all done by hot wire anemometry these days.

Never knew that Fluke made those. I've a variety of Fluke electrical meters and love them. The 922's a bit out of budget though (like 10x :-).

Hot wires seem to all be limited to 50°c which is pretty standard with electronics like this. With some things I'm comfortable taking it in, measuring, and getting out or w/ CO2 keeping the meter lower down where temps are lower and having just the sensor up in the higher heat. This needs the temp for it's calculations though so I'm guessing it'd be useless for temps above 50°c?

I think I could take a vane anemometer in, measure, get out and be OK so long as I can find one that's somewhat accurate to something below 50 ft/m ?

I could keep a manometer down lower where it's below 50°c and just have the tubes up in the duct.