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Old 10-01-2012, 08:59 AM   #1
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Supply Plenum Size


I am having our 2.5 ton heat pump and air handler replaced in our house. The house also needs substantial improvement to the ductwork, which I am doing myself before the HVAC contractor comes to install the unit. I have been studying ductwork design and theory, and feel I have a good handle on it.

My one question is what size should the supply plenum off the top of the air handler should be? I have read and been told conflicting answers. One is that the supply plenum should be no larger than the blower opening off the top of the air handler, which seems small, at only 16x11". But most opinion, particularly from the HVAC supplier, is that the plenum should be the same size as the overall A.H. cabinet size, which is 17.5 x 21".

That's how the current one is. The problem is that the new AH is smaller than the old one, and the main trunk line is 18" wide, which is 1/2" wider than the AH cabinet.

So what to do? I suppose the supply plenum could be made an inch or so wider to allow for the trunk to connect, and would just hang off the side of the AH cabinet a bit. But that seems a bit half-baked, and would make the sheet metal work more complicated.

Suggestions? Thanks!

David

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Old 10-01-2012, 03:12 PM   #2
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Supply Plenum Size


You need to design the supply system for the cfm you need, the outlet coming out of the air handler is just a basic starting point and is not related to the size of everything that comes after it, it usually is higher velocity than what you need, so the duct usually gets wider after the opening, but not always. It has nothing to do with the size of the cabinet.

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Old 10-01-2012, 03:19 PM   #3
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Supply Plenum Size


All you have to do is call the supplier or have your hvac contractor call them and ask for a matching plenum for the unit, giving them the model and serial numbers. That's it.
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Old 10-01-2012, 04:52 PM   #4
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Supply Plenum Size


after getting your sizes on main and return duct...we transistion from ah opening to main duct..that small tapper should not hurt a thing...and it looks great ...ben
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Old 10-01-2012, 05:00 PM   #5
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Supply Plenum Size


Thanks for the replies so far.

Matching the supply plenum size to the actual air outlet seems to be the favored method. It just seems a bit narrow. I understood this was to be a large pressurized mixing chamber.

As far as asking the distributor for the standard plenum, I did that, and they indicated plenums are field-fabricated, and should fit the size of the entire AH cabinet. (This contradicts other conventional wisdom on sizing.)

The main issue, it seems, is that the trunk coming off it is 18" wide, and the plenum off the AH will only be 16" wide. So I need to transition it 2" wider somehow. Any approved methods? After getting so deep into getting my air flow right with the rest of the system, I want to make sure I get it started off the optimal way.

Thanks!

David
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Old 10-01-2012, 05:04 PM   #6
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Supply Plenum Size


Quote:
Originally Posted by db4570 View Post
Thanks for the replies so far.

Matching the supply plenum size to the actual air outlet seems to be the favored method. It just seems a bit narrow. I understood this was to be a large pressurized mixing chamber.

As far as asking the distributor for the standard plenum, I did that, and they indicated plenums are field-fabricated, and should fit the size of the entire AH cabinet. (This contradicts other conventional wisdom on sizing.)

The main issue, it seems, is that the trunk coming off it is 18" wide, and the plenum off the AH will only be 16" wide. So I need to transition it 2" wider somehow. Any approved methods? After getting so deep into getting my air flow right with the rest of the system, I want to make sure I get it started off the optimal way.

Thanks!

David
hey dave see your concern but ithink your over thinking it we taper this connection all the time....ben
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Old 10-01-2012, 05:51 PM   #7
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Supply Plenum Size


Taper the plenum down from 18 to 16" wide.

11X16 at 1,000 CFM is roughly 820FPM, which is more then ok.
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Old 10-01-2012, 11:15 PM   #8
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Supply Plenum Size


Quote:
hey dave see your concern but ithink your over thinking it we taper this connection all the time....ben
You are correct, Ben! Thanks for pulling me back from the edge of the Cliff of Obsession!

The trunk has a tapered takeoff that angles down the plenum. So I need to figure out how to do a double-taper transition.

David
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Old 10-02-2012, 01:30 AM   #9
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Supply Plenum Size


Just put a plentum on the Damn thing. If it doesn't fit put some metal tape and duct seal to fill the gaps. A half inch doesn't mean Crap. All the air will be forced into the ducts either way. Just make sure it is insulated and your ducts are properly sized and you will be fine...
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Old 10-02-2012, 09:50 AM   #10
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Supply Plenum Size


Taking a look at it again, and measuring things, helped me realize that all I need to do is have the plenum built a little wider so it hangs over the edge of the AH a couple inches, and I'm good to go. Easier than I thought.

Thanks for the advice!

David
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Old 10-02-2012, 05:42 PM   #11
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Supply Plenum Size


I recommend that you get a transition piece made to transition from the size of the AH outlet to the size of the plenum, of it's more than a small difference. It will look a lot more professional.

Last edited by hvac benny; 10-02-2012 at 05:46 PM.
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Old 10-02-2012, 08:57 PM   #12
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Supply Plenum Size


agree with above don't just let it hang over the edge ...transistion...taper//sheet metal s slips 18 to 16...beauty in in the install...
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Old 10-04-2012, 07:47 AM   #13
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Supply Plenum Size


You don't need to make a transition to make it pretty. You can snip the corners, bend it in, screw and metal tape it together. Once you put duct seal and insulation on the plentum it looks like nothing out of the norm.
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Old 10-09-2012, 08:16 AM   #14
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Supply Plenum Size


Thanks for the help, guys. I think I have decided to have the sheet metal outfit to make the plenum flare out at an angle a couple inches wider from it's mounting flange on the AH. This will make it wide enough to easily attach my existing 18" wide trunk.

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