As inspired by this previous question, is there a method or device used to measure the output of a single HVAC vent or the output of a single room?

I'm imagining some sort of hood device that could be taped or magnetically sealed to a vent that would have a CFM (cubic feet per minute) meter on it to make sure each vent is putting out a certain amount. Just to make up numbers, you could then verify that a 6" vent was putting out 1000CFM, and the 8" duct in the next room was putting out 1300CFM, but a 6" vent in the guest room was only putting out 500CFM.

If such a device doesn't exist, how do installers verify that each vent is working as designed? Is it just a matter of a temperature sensor and the fact that the air coming out "feels like the right amount"? How would the poster of the linked question "prove" that one vent wasn't putting out a sufficient volume of air for the room?

2 Answers 2


A hand held anemometer can be used to see the flow then multiply by area to get the total flow. There are quite a few for under 20$ .

  • They even make models now that plug into an iPhone or Android device.
    – Tester101
    Feb 8, 2016 at 19:37
  • When you say "multiply by area" do you mean: The anemometer has an area of 5 sq.in., and the vent area is 20 sq.in. so multiply the reading by 4? As simple as that?
    – JPhi1618
    Feb 8, 2016 at 19:52
  • That is the way we check chemical exhaust hoods. We usually take several readings at the edge, center and use the average. Multiply the total square inches (the unit I use is 1 square inch) total square inches x flow gives a very accurate total.
    – Ed Beal
    Feb 8, 2016 at 20:13
  • @EdBeal, Thanks, that's great. I was making it much more complicated than it is. Do you know if this is common or done at all in residential HVAC?
    – JPhi1618
    Feb 8, 2016 at 20:19
  • @JPhi1618 It's done by home inspectors sometimes, and HVAC technicians sometimes (more common when troubleshooting an issue). I'm not sure I'd call it a "common" practice, especially during initial setup. Maybe more in higher end more energy conscious installs.
    – Tester101
    Feb 8, 2016 at 22:48

You actually take out the register, check the size of the pipe feeding it the buy one boot the same size feeding the room and place it over the existing register boot without the register and place the Anemometer over top. It should be close in size and will give you the CFM of the air output. Then go to your building department and get a copy of the heat loss calculations which should give you the design cfm for the room you're checking, if it's low then you need to increase it. Since the amount of cfm times the temperature rise will give the btu's needed to get the right heat loss calculation, if done correctly and the buildr did his job as per the drawings. In most cases turning the fan switch on your thermostat to "ON" will bring the fan on continuous operation on high speed which is how you want it anyways to keep constant air flow in your home, equalizing the temperature in virtually all of your house. Unless of course you still don't have enough air flow still. Maybe buying that piece for your iPhone as tester 101 sugested will fix everything for you instead though! Don't forget to copy my answer Tester101 so you can paste elsewhere!

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