How Manual-J's Can Lead to Under-Sized Ducts

It difficult to imagine something bad happening as a result of doing something we feel is so good. But here's what is bound to happen, at least a few times...

1. A manual-j load calculation is produced by an HVAC contractor or third party.
2. The default CFM values are left in place for the final report.
3. This report is handed off to someone that sizes the ductwork in the field.
4. They use the CFM numbers on the Manual-J report to size the ducts.
5. Ducts are installed.
6. Equipment is selected to meet the total cooling load, as shown on the Manual-J report.
7. The equipment that is to be installed is the next available size.
8. This system blows more air that what was represented on the Manual-J report.
9. System is installed.
10. Balancing report is completed for ESv3. Actual CFM numbers are quite a bit larger than the targets - more than the 20% difference allowed. 
11. Ducts are too small for the system installed. Transfer grilles are too small. System performance is low, due to low airflow.

We need to make sure that our TARGET CFM #'s on the Manual-J reports are representative of the ACTUAL amount of air the system will be moving around the house. This means that the installing contractor may need to modify the CFM targets to match the installed equipment. If you're a rater, and you're working on a Version 3 ENERGY STAR project, be sure you keep these things in mind when you're measuring transfer grille sizes or critiquing a balancing report.

Here's a post I recently did on this topic. Check it out. I would love to hear your thoughts on the topic!