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Discussion Starter #1
First off are there any good books that I can read on HVAC
I needed advice regarding my HVAC. I bought a house over a year ago and have been slowly figuring things out.
I have 2 HVAC units , 1x 2.5 ton for the top floor (4 bedrooms), 1x 5 ton for the main floor/basements which are both badly undersized from what I've read. I'm going to separate it into separate replies to make it easier to decipher
 

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Discussion Starter #2
Upstairs
2.5 Ton for the top floor (4 bedrooms), two returns (16x16 actual grill not opening) w/8inch and 10inch flex duct). The filter is 18x20. From what I've read I should have a 20x25 filter.
What I was thinking ideally was to run
3x8" returns from 3 bedrooms
1x10" return from 1 bedroom (master)
1x10" return (Hall / original)

How do I figure out the max size return plenum to buy (that would fit my unit)? Also when connecting to the plenum can I connect on any side or only on vertical surfaces (sides)

Alternatively if I merge the 3x8" and 1x10" into a junction box and run a 14" to the existing plenum how bad of an idea is that? Will still be undersized on the return
 

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Discussion Starter #3 (Edited)
Downstairs

Downstairs
5 ton for the basement and main floor. It also has a 18x20 filter and ONE 22x22 grill (actual grill not opening). The return grill is near the stairs going to the basement with the main floor having an open floor plan. There are no returns in the basement (the plenum right before it hits the filter has a cutout and grill installed)

The plenum is half running behind drywall and connecting with a metal duct going to the upstair grill and would be somewhat of a bear to get to.

I was thinking of cutting a hole in the ductwork after the plenum and installing two returns to the basement area.
Regarding the plenum itself I can have someone try to remove and replace it with a larger one to accomodate a 20x25 filter but the problem is that would still be 50% undersized. From what I've read there should be two separate plenums coming out on either side w/a 20x25 filter on each. the other side of the HVAC is where the boiler sits. Also there isn't enough headroom to significantly raise the unit.
Googling I could find filters upto 24x36 on homedepot. Is it possible to connect a plenum of that size? Have the upstairs duct hook to it and the basements new ductwork as well

 

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Upstairs
2.5 Ton for the top floor (4 bedrooms), two returns (16x16 actual grill not opening) w/8inch and 10inch flex duct). The filter is 18x20. From what I've read I should have a 20x25 filter.
What I was thinking ideally was to run
3x8" returns from 3 bedrooms
1x10" return from 1 bedroom (master)
1x10" return (Hall / original)

How do I figure out the max size return plenum to buy (that would fit my unit)? Also when connecting to the plenum can I connect on any side or only on vertical surfaces (sides)

Alternatively if I merge the 3x8" and 1x10" into a junction box and run a 14" to the existing plenum how bad of an idea is that? Will still be undersized on the return
For the plenum the length can be anything you need. Just need to measure the height and width you need.

A 10" is not big enough for 3-8".
 

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5 tons should return through the bottom, or on both sides, or 1 side and the bottom.
 

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Discussion Starter #6 (Edited)
For the plenum the length can be anything you need. Just need to measure the height and width you need.

A 10" is not big enough for 3-8".
Regarding the plenum, it's 18 high x 20 wide x 22 long. In order to use a 20x25 filter I presumed I'd need a plenum 20 wide x 25 high x 22 long (or longer depending on how many returns).

I was suggesting merging the 3x8" +1x10" (Hall) into a junction box and pulling a 14" from the junction box. Instead of tieing all 4 directly to the plenum

Also forgot to ask but should the plenum in the attic be wrapped in insulation (The current one is not, it's made of sheet metal)
Thanks a bunch
 

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Works better if use wyes. 8X8X10, 10X8X14. 14X10X16", 14" is too small.

Your current plenum is probably lined(insulated inside). Wrapping it works better.

You can transition down from a air filter box to your air handler's return dimensions. And have a box/plenun on the inlet side of the filter to connect your duct.
 

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Discussion Starter #8 (Edited)
So basically have a larger plenum (say 20x25x30) with a air filter of 20x25 at one end. Then have a tapered edge that goes down to 18x20 and is screwed into the HVAC unit it.
If i have that I can just run the new returns straight to new larger plenum :)

Regarding the 5ton it doesn't have room below (clearance issues at the top to lift) or the opposite side (boiler), so only have 1 side to work with :(

Thanks again for all your help
 

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So basically have a larger plenum (say 20x25x30) with a air filter of 20x25 at one end. Then have a tapered edge that goes down to 18x20 and is screwed into the HVAC unit it.
If i have that I can just run the new returns straight to new larger plenum :)

Regarding the 5ton it doesn't have room below (clearance issues at the top to lift) or the opposite side (boiler), so only have 1 side to work with :(

Thanks again for all your help
Yep, make the plenum like that, longer if you need it.

Yep, lots of times space is limited.
 

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Discussion Starter #10
One more question, I'm not sure about the Plenum work (the transition seems complicated for doing it myself and) the HVAC guy I called wanted over a grand just for replacing the plenum.
I currently have a 21wx21dx19h plenum.
Of that the first 3-4 inches are where the filter is and beyond giving me a usable 17 inches.

My two current returns are on the front and side of the plenum. Is it ok to add the 3 new ducts into the top of the plenum or would that be too much air too close to the filter.



 

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That will be ok.
 
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