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!

Tags: duct, manual-j, sizing

Views: 419

Photos

Loading…
  • Add Photos
  • View All

Latest Activity

Duncan Cumming posted a photo
11 minutes ago
Profile Iconmnar taha, Boilers Prices, Duncan Cumming and 4 more joined Home Energy Pros
16 minutes ago
Anthony Adams posted a status
"Trying to figure heat load for HVAC on bonus room over a garage. Lots of wall space exposed to arttic."
2 hours ago
Hamed Ghanbari replied to Brian's discussion Everything you need to become a energy auditor for sale in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Hello, I am interested to buy a Minneapolis Blower Door DG700. How much is your unit? Thanks. "
14 hours ago
Hamed Ghanbari joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
14 hours ago
Colin Genge updated an event
Thumbnail

Measuring HVAC Performance webinar at Online

February 23, 2016 from 11am to 12pm
If you install, service or test HVAC systems you will want be there for this special webinar.…See More
18 hours ago
Colin Genge updated an event
Thumbnail

Testing for IECC Code Compliance for duct and house leakage webinar at Online

February 16, 2016 from 11am to 12pm
The International Energy Conservation Code (IECC) is in use or adopted in 47 states, the District…See More
18 hours ago
Jan Green posted a status
"I'm honored to be selected to speak at the 2016 RESNET Conference right here in sunny Scottsdale 2/29-3/2/16 with SRP & others!"
19 hours ago

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service