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Forced hot air control question

lostedge | Posted in General Questions on


I have a specific quesion on damper controls for multiple zones, but first the home background:

We are in the process of renovating our 3rd floor into a master bedroom. This is in addition to the existing 1,600 sq/ft house that has two existing zones: 1st floor (kitchen, bathroom, living room, dining room…7 vents total) and the 2nd floor (3 bedrooms and a bathroom…7 vents total).

The existing system is a 2009 install of a goodman furnace and goodman central AC unit. There is a split out of the furnace with a damper on each air path. The 1st floor feeds from floor. The 2nd floor feeds from ceiling. The air travels up the center of the house (where pre-existing chimney was.) In the attic it splits to the ceiling vents for the 1st floor.

We have a therm for each zone. When the second floor is calling for air, the damper coming out of the system opens up, and when the first floor is calling for air, that damper opens. If they are both calling for air, then both dampers are open. No issues, it works great.

When finishing the attic, our HVAC guy told us that we would likely be OK to service the additional 600 square feet of living space with our existing furnace and central AC unit. He also felt we have enough air flow to the attic to handle both zones.

We live in the Northeast and during this renovation added blown insulation to the walls, and will be using spray foam insulation in the new attic.

We are able to bring an air return from the new renovated space into the existing 2nd floor air return.

The question:
My ask of the HVAC tech was to create a unique third zone in the attic master. I imagined this would be done by adding a damper in the attic, which would control whether air flowed to the 3rd floor or the 2nd floor. Once my HVAC guy started working, he told me that actually the 3rd and 2nd floor had to be on the same zone unless they bring a new feed of air up from the basement. He also keeps stressing that with all the new insulation, we won’t have any issues or real need for a isolated 3rd zone in the attic master.

Essentially, what he is saying is that in the original split at the furnace, he can’t wire the damper that leads to the attic to open if EITHER of the 2nd OR 3rd floor zones are calling for air.

So, can one damper (located right at the furnace) that serves two different t-stat zones?

In other words, can we have 2 dampers leaving the furnace and then another damper in the attic? With 3 t-stats each controlling a different floor of the house?

Thanks in advance

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  1. lostedge | | #1

    edit: sorry for this typ-o. "In the attic it splits to the ceiling vents for the 1st floor." SHOULD read "...vents for 2nd floor"

    [Editor's note: I fixed it.]

  2. GBA Editor
    Martin Holladay | | #2

    I would trust your HVAC contractor on this one. Zoning forced air systems is tricky. Every time a damper is added, air flow through the furnace or air handler is reduced -- and very low air flow rates can damage a furnace or cause air conditioning coils to freeze. There are various ways to handle this problem, including a deliberate strategy to allow some unnecessary air to be dumped or bled where heat or cooling isn't required -- in order to ensure enough air flow through the furnace or air handler.

    Any advice I give on this issue may lead to problems, so your HVAC contractor is the person to listen to. Good luck.

  3. JC72 | | #3

    I have a zoned system and hate it.

    He makes sense because usually the dampers are located just off from the main plenum (mine are) and your unit is located in the basement so he'd have to install another damper (located near the existing dampers) and run dedicated supply line from the new damper up to your third floor.

    As long as the system is balanced and existing ducts are sized appropriately ya you should be able to run two floors off one zone. will it be perfect? I don't know but I do know that zoned systems are a compromise for sure.

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