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richard_conley63

How do I fix this undersize duct system?

Richard Conley
2 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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