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: 109

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

Profile IconLarry Kinney and teplie_poly joined Home Energy Pros
1 hour ago
Dan Antonioli replied to Dan Antonioli's discussion Net Zero Energy Hot Water
"Okay, a couple of things. The 1.0 kW per square meter reference is only under the most ideal lab…"
12 hours ago
Hans Joachim Preiss replied to Dan Antonioli's discussion Net Zero Energy Hot Water
"Dan, Let me attempt to compare apples with apples: My solar thermal system is located in…"
13 hours ago
Dan Antonioli replied to Dan Antonioli's discussion Net Zero Energy Hot Water
"Hi Hans, I don't know where you're located but where I am a 2.5 kw pv system costs closer…"
15 hours ago
Hans Joachim Preiss replied to Dan Antonioli's discussion Net Zero Energy Hot Water
"I use the combination of solar PV and heatpump water heaters in the vast majority of net-zero…"
17 hours ago
Amber Vignieri posted a blog post
21 hours ago
William Zwack added a discussion to the group Job Board
Thumbnail

Residental Energy Specialist: Washington, DC (or possibly telework)

Position Summary: CSRA Incorporated is searching for a Sr. Residential Building Energy Efficiency…See More
22 hours ago
Shaun posted a discussion

Commercial duct testing

Hello all,I am a BPI BA, HERS rater and QC inspector. I cant find a ton of information on this…See More
22 hours ago

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service