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

Sean Lintow Sr replied to Tony Rigdon's discussion Placement of 5/8" firewall, over or under foam board insulation?
"I am pretty sure he is talking about using commercial drywall designed for this type of purpose and…"
11 minutes ago
Hal Skinner replied to Tony Rigdon's discussion Placement of 5/8" firewall, over or under foam board insulation?
"Found an article that might help …"
8 hours ago
Bud Poll replied to Tony Rigdon's discussion Placement of 5/8" firewall, over or under foam board insulation?
"Hi Tony, what/where is your climate?  Also, how is the rest of the wall constructed? Adding…"
9 hours ago
Sean Lintow Sr replied to Tony Rigdon's discussion Placement of 5/8" firewall, over or under foam board insulation?
"contact the local AHJ for sure - but for "fire" reasons it should be on the outside…"
11 hours ago
Tony Rigdon posted a discussion

Placement of 5/8" firewall, over or under foam board insulation?

I'm adding onto my personal residence and since the house is within five feet of the property line,…See More
12 hours ago
Hal Skinner replied to Hal Skinner's discussion Rooftop A/C units and exposed ductwork in the group The RCC Classroom (Radiant Control Coatings)
"Hi Tom.   As far as media advertising goes for the good RCCs, it really doesnt work well at…"
14 hours ago
Sean Lintow Sr commented on Chris Laumer-Giddens's blog post Mini-Split Heat Pumps are Quirky, Especially their Capacity
"Quick question Chris - the set & forget (for temperature setbacks) is that with any or just…"
16 hours ago
Martin Smith joined Hal Skinner's group
Thumbnail

The RCC Classroom (Radiant Control Coatings)

A group where energy professionals can learn about all the different energy saving aspects of…See More
18 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service