Here’s how to satisfy ASHRAE 62.2 when using a supply-only system. The following are two examples of how to set things up, depending on what you’re trying to achieve and where you are in the installation process.
- You know how often you want the system to run per hour, but you’re not sure what size duct to use so that you get enough air.
- The ductwork is already installed. You just need to tell the system how often it needs to run.
[hr] 
SCENARIO ONE – You want to size the duct that brings in the ventilation air, so that the system only runs for a specific period of time to satisfy the ASHRAE 62.2-2010 requirement:
If the calculation says you need 50cfm, but you only want to run the fan for 20 minutes per hour, here’s how you’d figure out how much air you’d need to pull in for those 20 minutes:
- 60 / # OF MINUTES YOU WANT TO RUN THE FAN x CFM OF AIR YOU NEED
- So, in this case 60 / 20 x 50 = 150 cfm
- Size the duct to deliver 150 cfm
[hr] 
SCENARIO TWO – Let’s say the ductwork is already installed, so you just want to adjust the TIME that the system will run so that you satisfy the volume requirements for ASHRAE 62.2-2010.
If you have the damper open and the system on, and it’s sucking in 90 cfm. And, you know that your house requires 50 cfm/hour, here’s how you’d calculate it:
- WHAT YOU NEED / WHAT YOU HAVE x 60
- So, in this case, 50 / 90 x 60 = 33 minutes
- Adjust the settings so that the system will ventilate for 33 minutes per hour