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Zone dampers and bypass duct.


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I am see a few zone system on new two story homes. Some have a bypass duct and damper. Some do not. I am writing up the lack of the bypass duct and damper.

The question I have is there a proper way to use a zone system with out a bypass duct. Something like slowing the fan down.

Thanks guys

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Sorry for the late responce Phillip.

First off I'd call the manufacture to discuss. The VVT/VAV systems I've seen without bypass dampers have an air flow design that follows the fan curve (or rides the fan curve). It may be acceptable not to have a bypass damper depending on the design of the system.

Do you know who the t/c system manufacture is?

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I recently installed a damper and zoned system with a barometric bypass duct from the supply plenum to the return plenum. The bypass duct functions basically on the static pressure in the supply plenum. I took the static pressure and got .55"WC. That was my goal - around .5. I installed 3 dampers and when one zone is called for it goes full open and the other 2 still remain 50% open. So I think the bypass duct may depend on what the dampers do when that zone that is not the primary zone. You also have to ensure that the bypass duct does not put too much cold air back into the return system. By the way I used 800 KwHrs less this July than last (temps about the same - high 90s - 100 all month).

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Have you checked the static pressure in all configurations (different dampers opened and closed)? It may very well be that in the worst case the static pressure is approximately .5"WC.


The items you are talking about is beyond a normal home inspection. It would be something an HVAC Technician would do.

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  • 1 month later...

I just had a new construction today with one unit and two zones, upstairs on one zone, downstairs on the other. Most times I also see barometric zone dampers on a bypass duct but not on this one. Each zone damper is either full on or off which means the remaining zone gets the full flow from the blower. The entire system whistled at the return all the time (one central return grill) and the active zone whistled at all registers when the other was off.

Just as a general note, I always check zoned systems in all possible configurations to check for this situation. Basically, the builder here tried to cheap out and skip the bypass and got caught. I am calling it out and recommending the installation of a bypass duct and damper.

As alluded to by RSpearmo, check your TD to make sure they don't bypass too much air and start making ice on the coil.

They could design the duct system large enough to handle all the air on either zone and not have to have a bypass but that is a bit trickier to have it large enough for full volume but still have enough static pressure in all modes.

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