Strange ductwork - Supply connected to return???

Paul500January 21, 2013

I have an older home that has had many previous owners, and I am trying to understand a strange run of ductwork at my furnace. There is a circular duct (near top of photo) about 10" in diameter that connects directly to the return plenum of my furnace. The strange thing about it is that is comes right off a supply duct about 5 or 6 feet away to the left. What could possibly be the purpose of this? My only guess is that either the return was starved or to heat/cool the return air before it enters the furnace. I am no HVAC professional, but those would not be legitimate reasons, right? If so, what could it be possibly for? It also has a damper on it, and I accidentally nudged it, so not sure to leave it open or closed. Any advice would be much appreciated! Thanks.

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Just a homeowner here, but it sounds like this is the bypass that houses your furnace's humidifier. In heating season, the damper would be open allowing some warm air to return to the return side picking up moisture from the humidifier on the way.

During cooling season this damper would be shut.

    Bookmark   January 22, 2013 at 9:54AM
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Some of the older homes in my part of the country have very old metal duct systems that were originally sized for heating only. When they finally install a full central system, they find they have too large of a cooling system... so they install these bypass trunks to dump the extra air back into the return. I dont have my ductulator handy but i imagine that 10" will be around 300-350cfm of air. Close the damper and see if your vents become noisy (cooling mode).

Hope that helps

    Bookmark   January 22, 2013 at 1:01PM
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I understand that bypasses are sometimes used on zoned systems.

    Bookmark   January 23, 2013 at 6:13PM
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Thanks all for all the theories. There was an old humidifier connected, but it was on the supply plenum, on the right side of the picture, near where you see the exhaust vent. I took that out awhile ago. Perhaps the system was originally sized for heating only. In that case, would I leave the damper open or closed in the winter? I have it closed now, and the vents are a little noisier, but nothing to complain about. I am not familiar with a zoned system, but my ductwork does have a lot of manual dampers around. Do people ever replace or redo their old ductwork? Mine seems poorly pieced together, dirty, and disorganized. My home also has an addition that receives weak supply so it's supplemented with electric baseboard heaters -- not to mention returns that use the basement ceiling joists instead enclosed metal ductwork. Seems redoing the ductwork may result in a more efficient, cleaner, and better designed system. Sounds expensive and not sure if it would add to the resale value. Thoughts?

    Bookmark   February 10, 2013 at 9:02PM
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Your duct work sounds very inefficient to a point where you have to supplement it with electric baseboard heating. You are wasting money with your present set up. Fixing the duct work will save money in the long term.

New duct work does not add to the resale value of the house. A buyer does expect the duct work to be in good condition.

    Bookmark   February 10, 2013 at 11:23PM
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