HVAC Pro Blog
Please, Make the Transition!

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 large restrictions in airlfow, raising static pressure and lowering the cubic feet per minute (CFM). Or, with ECM motors, raising the amperage 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!

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 size without a tapered transitions raise 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.


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!






Fig.1 No Transition: Filter Box

Fig. 2 Return Drop

Fig. 3 Supply Transition(s)?



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