I removed all of my existing duct work that was designed as a radial system with a sheet metal rectangular plenum with very long flex runs poorly attached. I am replacing my system with all round sheet metal duct as an extended plenum design. Assuming proper duct sizing calculations, I am wondering if it is reasonable to attach a flat piece of sheet metal to the air handler to make the connection to the round duct instead of installing a new sheet metal rectangular plenum first to connect the supply round sheet metal duct.

Resources: Duct System Design Considerations

Manual: Model: chpf3636b6ca

This picture illustrates what the end result would look like: A round duct directly connected to a rectangular air handler. This picture is not of my existing system.

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1 Answer 1


Such a connection introduces more friction to the system, reducing the available flow. If the system is sized adequately to accommodate this, then yes, it is OK. If I understand you correctly, this large round duct you are connecting to the air handler is serving as an extended plenum. Partly depending on the relative size of the duct and air handler, this may be the one place where it would be worth constructing a proper transition fitting instead of a simple flat adapter.

In a way, the flat adapter is crippling the entire system before it even gets a chance to get going, just my opinion, based on my innate desire for efficiency. As stated earlier, if the system can accommodate an inefficient transition, then it is OK to use it. I just happen to not like it.

  • Thanks for the response. Can you explain a little further the best way to make the connection? Should I use something like this? nordfab.com/nordfab-products/hood/… Commented Nov 20, 2013 at 14:43
  • 1
    @JasonRikard that seems like exactly what you should be using. You can also get a local sheet metal shop to build something, so long as you provide the dimensions and what types of connections you're making. This is especially useful if you have space restrictions, or want it to include a turn etc.
    – gregmac
    Commented Nov 20, 2013 at 18:39
  • FWIW, I agree completely with gregmac and am pleased he was able to give you a timely answer.
    – bcworkz
    Commented Nov 20, 2013 at 23:21

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