For as long as I can remember, the agreed epidemic with ductwork was always undersized return ducts.  Although we are far from perfect with duct sizing in New England, I have frequently seen great strides in fixing this issue - particularly with replacement systems.  Lets face it, you should just be properly designing the duct system on new installations.  Undersized ducts cause great restrictions in airlfow, raising static pressure and lowering the cubic feet per minute (CFM).  Or, with ECM motors, raising the amperage draw above full load.  More recently, I continue to find efforts with regards to sizing, but other rules of duct design being ignored.  I am going to concentrate on one particular rule that can have the same affect as undersized ducts:

  • On supply and return, when the trunk is wider than the plenum, a transition fitting must be used!

Fig.1  No Transition: Filter Box

   Lack of transitions create turbulence and restrictions in your duct system.  Even if the Return Duct is large enough for your desired CFM, abrupt changes in sizes without a tapered transition raises static pressure drastically above design.  Take Figure 1 for example.  Imagine the force needed to pull the same volume of air through the nice IAQ Filter installed.  At least the entire filter area is being used! I frequently find larger filter boxes than the air handler opening, a waste of filter area - but at least less of a restriction.

Fig. 2 Return Drop

  Figure 2 is an all too common mistake on replacement system, when installing a high performance filter in basement systems.  The new filter box pushes the return drop out of the range of connecting to the trunk, without an offset transition.  Most tin-knockers will do what they can to get the furnace operating.  Following this up by cutting in a grille in the return drop to either fix the undersized ducts, or lack of a transition, is not going to work when it comes to air-conditioning!  The air must come from the conditioned space in order to remove the latent heat, not from a moisture laden basement...

  Can anyone tell me what is going on in Figure 3???  I hope this wasn't you!

http://excessair.blogspot.com/2014/07/please-make-transition.html

Fig. 3 Supply Transition(s)?

Views: 87

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Hal Skinner replied to Hal Skinner's discussion 'YGRENE' super fund for energy upgrade financing
"Hi again John, If you could directly answer these questions and comment on anything else related,…"
4 hours ago
Gregory Homan commented on Tom White's video
Thumbnail

The Future of Housing - And How Airtightness Can Help

"This is a reposting. The original is out of date due to the URL of the source video having…"
5 hours ago
Profile IconRick Causo, Sean McLoughlin, PAUL JOHNSON and 1 more joined Home Energy Pros
5 hours ago
Chris Dorsi posted a group
Thumbnail

International Home Performance Forum

A group dedicated to international collaboration among home performance practitioners.All topics…See More
5 hours ago
Tom White commented on Tom White's video
Thumbnail

The Future of Housing - And How Airtightness Can Help

"This video link has been deleted.  You can now view and comment on the updated video at…"
5 hours ago
Tom White posted a video

The Future of Housing - And How Airtightness Can Help

http://www.houseplanninghelp.com/80 The search for a new home can be daunting; whether it’s your first purchase, you’re relocating, or your family is increas...
5 hours ago
Corbett Lunsford's 2 videos were featured
5 hours ago
Sean Lintow Sr posted a blog post

http://thehtrc.com/2015/illinois-the-eri-the-iecac-resnet-the-final-push

Monday April 6th (10 AM to noon) will decide whether the ERI aka as the HERS option will be part of…See More
6 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service