CFM at register

[Inspector 102]

On a residential construction project, is there a standard cfm that can be measure at each register in a room to determine adequate ventilation flow? Extremely long run of duct work with multiple take-off and homeowner is concerned about distribution of the system.

 

[Mech]

I do not deal with the IRC, but I skimmed through the 2021 edition. Section 1805 specifies airflow when mechanical ventilation is provided. I did not see that mechanical ventilation is actually required though. I assume there is a section somewhere that requires minimum operable window / exterior door sizes to provide ventilation air based on the room size.

If you want to verify mechanical ventilation air flows, I think you would need to dive into the IMC, chapter 4. The calculations in that chapter look at the inefficiency of delivering ventilation air to the spaces as well as a ratio of ventilation air required for each room compared to the supply air delivered to each room. The HVAC outside air rate is then increased based on those ratios to ensure the rooms theoretically are getting the minimum amount of ventilation air. I recommend a spreadsheet to do the calculations as there will most likely be a bunch.

 

[klarenbeek]

A ballpark range for a 6" round typical supply run is around 100 CFM for a properly designed duct system at full blower speed. It will be less when running at a lower speed, cooling is generally a higher speed than heating. It will also be less on modulating furnaces running on low fire, or low speed continuous circulation. That's the variables for a properly designed duct system, and that's based on 100 equivalent feet of duct. Residential duct should be designed per ACCA manual D.

More length reduces airflow. Elbows reduce airflow, and the type of elbows used in rectangular duct can make a big difference, especially if you have more than one within a short distance. A long straight supply trunk line is better than a shorter trunk with lots of bends. If the main trunk line size isn't reduced as individual runs come off it, or if it's oversized, you will lose static pressure and therefore airflow at the farther runs, particularly for longer trunk lines. Not enough return air can also be a problem, but you really can't oversize your return. Flex duct reduces airflow even when installed right, which it usually isn't. It can be a real killer to airflow if not done right.

 

[Inspector 102]

Thank you for the information to consider. As an inspector, but not on this job, the homeowner asked about determining if the system was proper before it gets covered with drywall. When I reviewed the installation, it looked good to me from an installation position but had nothing to go by on design. I did find out that the homeowner is paying to have flex duct removed and insulated pipe installed. Some jobs you just do not want to get involved with. Thanks again.

 

[Paul Sweet]

The HVAC calculations should include the CFM for each room. The unit and ducts can't be properly sized without this.

Klarenbeek's roles-of-thumb are good, but contractors often install 6" ducts for many spaces that require less than 100 CFM just so they have fewer different duct and fitting sizes to deal with.

 

[Gregg Harris]

Look at ACCA manual D for duct design. Each room requires X amount of CFM to feed the occupied zone of the room. The air velocity will dictate the throw into the space and the CFM will control the volume.