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: 85

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 posted a discussion
2 minutes ago
Todd St.Clair 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
2 hours ago
Todd St.Clair replied to Jim Falakos's discussion Treat Software in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"How long ago did you buy this, if it was good for 5 months? And how much would you take for this."
2 hours ago
Bud Poll replied to Bud Poll's discussion Basing air exchange on house volume??
"Hi Sean, yes I am struggling with this and that is why I opened the discussion, so a definite…"
4 hours ago
Lynda Wright is now a member of Home Energy Pros
5 hours ago
Sean Lintow Sr replied to Bud Poll's discussion Basing air exchange on house volume??
"Sorry Bud but I think you need to clarify your issues with using volumes to calculate ACH As for…"
16 hours ago
Profile IconWes Anderson, Jean-Paul and Bryan joined Home Energy Pros
Friday
Dennis Heidner commented on Dale Stephens's blog post LED Lighting 24-month update 18,240 hours & counting
"Dale, good write up. For the landscaping bulbs,  if they had been T3 12V wedge bulbs,  I…"
Friday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service