# How tall should I make my supply plenum box?



## super77 (Aug 1, 2008)

Can someone explain to me how to determine the size of a plenum box on a gas forced air furnace? Other than having the necessary surface area to mount all the supply ducts, is there a certain height that I need to be above the heat exchanger? I would like to put the furnace up on a platform as high as possible, so I am looking to keep the sheet metal above as short as possible. I am thinking about 16-1/2" wide x 20" deep x 20" high for a 75,000 btu furnace.


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## 8 Ball (May 17, 2008)

Keep in mind that the shorter you make it, the more difficult it will be to add an AC coil in the future. If you can, instal the coil now, and add the rest later.

For a gas furnace, the top of the plenum needs to be a minimum of 1" from the bottom of the floor trusses. If the manufacturer has a requirement, it will be in the IOM that came with the furnace.

20" tall will be fine.


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## super77 (Aug 1, 2008)

8 Ball said:


> Keep in mind that the shorter you make it, the more difficult it will be to add an AC coil in the future. If you can, instal the coil now, and add the rest later.
> 
> For a gas furnace, the top of the plenum needs to be a minimum of 1" from the bottom of the floor trusses. If the manufacturer has a requirement, it will be in the IOM that came with the furnace.
> 
> 20" tall will be fine.


Thanks. I will not be adding A/C now or in the future, so that's not a concern.


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