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: 137

Comment

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

Join Home Energy Pros

Videos

  • Add Videos
  • View All

Twitter

Latest Activity

Eric Kjelshus replied to Beverly Lerch's discussion Fireplace drafts
"A brick chimney will take some 70,000 BTU's to heat up to draft on a fireplace.   A…"
3 hours ago
Ed Voytovich replied to Beverly Lerch's discussion Fireplace drafts
"Our solution was to install a Lopi Answer wood burning insert that vents directly through a metal…"
10 hours ago
kevin mack is now a member of Home Energy Pros
14 hours ago
Jim Gunshinan's blog post was featured

A Healthy Home and a Healthy Bottom Line

There has been a lot of interest of late in the weatherization and broader home performance…See More
yesterday
Jonathan Beers commented on Home Energy Magazine's blog post Natural Gas is Becoming Less Attractive
"The carbon intensity of electricity use (lbs. of CO2/MWh) varies a lot from region to region. For…"
yesterday
Diane Chojnowski posted events
yesterday
Home Energy Magazine posted a blog post

Natural Gas is Becoming Less Attractive

The United States and Canada have been fortunate to have access to natural gas for space heating,…See More
yesterday
Jim Gunshinan posted a blog post

A Healthy Home and a Healthy Bottom Line

There has been a lot of interest of late in the weatherization and broader home performance…See More
yesterday

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service