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JPM 11-21-2008 08:48 AM

Plenum code requirement
I am new to furnaces but am attempting to install an upflow Goodman gas furnace (2nd smallest unit they sell) in the basement area of an apartment. Two questions ...
1. Going into the chimney are two hot water heaters 3in duct lines. I would like to join them then expand into 4in line then wye for the furnace then into the chimney. So I would have the small furnace (70M btu) along with the 2 40gal hot water heaters into the inch line. Is that okay?
2. The plenum given to me from the plumbing supply store has proper width and length but is too tall for the height of the basement ... can I just trim off about 1/4 or so of height to get it to fit? Also, what is the minimum clearance needed between top of plenum and the the joists for the floor above?


hvaclover 11-21-2008 10:23 AM

I think you should Google vent tables and take a look at them.

If you are adding an additional gas appliance your chimney must be the right capacity, otherwise you get monoxide backing up into home.

JPM 11-21-2008 11:46 AM

Thanks for that.

I am okay with chimney size as there are other units that have similar number of appliances.

The difference is that in the other units, the appliances are piped into the chimney in all sorts of manner ... some together, some singularly. I would like to correct this by going in with one proper size and wyeing off it as I go back towards the appliances. If that is case, then am I okay with the 4in line?

Again, thanks for your help.

kennzz05 11-21-2008 11:55 AM

doubtfull: i think your supposed to be at least 1 in larger then the largest flue going into the crock and im sure 4 in wont handle between 75,000+30,000+30,000 and most codes wont let you tie them all togeather like that your going to have to beat another hole in the chimney and install a crock for the additional appliance(not that big of a deal) you need to be familure with local codes to determine what your next step is if this is a rental than it better be up to code if something happens like carbon monioxide poisoning you will be libel if not up to code

beenthere 11-21-2008 02:22 PM

You don't want to use a 3" pipe for 2 water heaters.
You'll need a 4" wye, and 2 reducers. Or a reducing wye.
Probably need a 5" wye to hook the gas furnace into the same pipe.

You should check the size of the chimney(internal sq in).

By code. To use a common vent connector, it must be the size of the largest appliances vent plus 50% of the smallest. That means you need 16 sq in. So it would have to be atleast a 5" pipe.
Since your venting 3" breeched water heaters into it. The chimney's sq in can be no more then 49 sq in.

IFGC, chimney can be no more then 7 times the smallest breech/vent connected to it.

PS: The plenum can be 1/2" away from the floor joist. Just needs clearance so it doesn't make noise from expansion.

JPM 11-21-2008 04:48 PM


thank you for this information!! It was my first time using this site and it has been a very good experience.

Saw that Mr BeenThere is from Lancaster PA ... use to live there myself. A good town.

Looking into the chimney is causing some other questions for me ... I will post them separately.

Again, many thanks.

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