How do I fix this undersize duct system?

4 years ago

Long story short is that the 3 ton HVAC system upstairs at my house has never kept up. I replaced the unit this spring (old one was 20 years old) - and it didn't fix anything. So I've spent a good chunk of the summer fixing insulation, air gaps, reflective barrier, etc, and I finally got around to doing the math - and it looks like my ducting is way undersized. The output (supply?) seems to have been designed for a 2 ton unit, and the intake/return seems to be even a little small for that - with a single 14" flexduct and 14x20" grill/intake.

My numbers say I've got capacity for about 800 cfm out, and 700 in with the current ducting, and my turnover calculations say I need about 1300. Upstairs is about 1600sqft with 8 foot ceilings (end rooms have sloped ceilings)

So, two questions:
1. Can I just add another 14" flexduct and 14x20 grill to the return/intake side? and join it to the intake plenum? Red question mark in hallway was location I was thinking.

2. Should I just add another 8x10 trunk line and a couple more air outlets?

Any suggestions? (Sizes are consistent across a color). Air Handler is in the attic. Stairs from 1st to 2nd and 2nd to attic are above each other.

I've attached a terrible paint drawing.

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Comments (7)

klem1
4 years ago
In general what you suggest will make a positive difference.
sktn77a
4 years ago
An additional 14x20 return grill will give you 560 sq in total (3 ton should have 600) so you're close. 14" flex will give you around 700 CFM so adding a 14" flex will give you around 1400 CFM total which is fine (3 ton needs 1200).

Start with the returns. If that doesn't help, go for the supplies.
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klem1
4 years ago
A side note on air returns and air filters. Of air handlers that come from mfgr with filter rack,60% are too small and odd size. I'm a strong advocate of oversized aftermarket filter racks. Oversized returns and filters have no undesirable effect on effeciency or equipment life where as undersized causes problems with both. I can't find any advantage in having returns in every room unless doors are closed.
Richard Conley
Original Author
4 years ago
Thanks. I can add additional return pretty easy just because of attic layout. an additional supply trunk is going to take some time/work to do right. The attic has plywood on top of the joists - so I'd have to run it on top of that across the space - which would be a pain, or pull up the plywood and put it underneath. I need to add more insulation and seal a bunch of things, so I can do that all at once.

I put an aprilaire 2210 (20*25*3 filter I think) filter box on it. That causing any issues?
sktn77a
4 years ago
"I put an aprilaire 2210 (20*25*3 filter I think) filter box on it."

On what?
Richard Conley
Original Author
4 years ago
Sorry - was responding to klem's comment about most OEM filters being too small. There's an AprilAire 2210 filter box on the intake/return side of the unit right before the unit itself.

Aprilaire says .11"@1200 CFM on it, or .04"@600 for MERV11. .16@1200 and .06@600 for MERV13

They're only about $125, so if I need one for each of the two 14" returns we're talking about, instead of just the single one, I can do that.
klem1
4 years ago
20x25x3 is plenty for the 3 ton handler so you don't need added filteration. Many will give you grief over Merv 13 based on hearsay with no actual flow measurments.