skip to main content

Heating & Air Conditioning

question about required cold air returns

16 years ago

I am building a new house and its a total DIY project. I already have a 3 ton split system (gas) that I will have a profesional re-install, but I am doing most of the duct work myself. Because of the layout of the house, I will need 3 cold air returns. The question is.....what size? I am thinking that I need three 15" pipes. More? Less? thanks for the help!

Sort by:Oldest

Comments (4)

daddo
16 years ago
At 400 cfm per ton, 1,200 cfm is needed at a minimum. But for returns, it is always recomended to go larger. I use an old formula which is simple and always works for me.
Take the total system required cfm X .36, then add that to the total cfm.
Example= 1,200 cfm X .36= 432, 432 + 1,200= 1632 cfm.
An 18" round duct, or three 12" round ducts will work. Then the return grill area in the cieling or wall should be- 14 X 14, 12 X 16, ect for the sake of noise the bigger the better.
The first number in the grill size is the width, the second is the hiegth- so a 14 X 20 would be 14" wide and 20" tall- the stamps in the grill running the 14" width.
ccoombs1
Original Author
16 years ago
thank you! that is exactly what I needed to know. One more question, if you don't mind. This house is being finished in stages. When we move in, one of the 3 cold air returns will not be in use....it will be covered over since that area of the house is unfinished. It may be 9 months or so before all 3 ducts are in use. To plan for this, should I install two 15" ducts in the area of the house that will be in use? I can install a 12" or even maybe a 10" in the other area (the master bedroom) and cover it with plastic until I am ready for it.
Related Discussions
Cold Air Return Connection Furnace Question
Q
Comments (5)
Interesting,point, but I've seen the Contractor's humidifier installations in the subdivision, so I doubt it. They hook one the humidifier pipe into the main duct above the furnace. The two pipes I am referring to are actually hooked up to the bottom of furnace unit it self, similar to the main cold air return duct. Sorry I don't have photos at this point.
...See More
Cold air return vent questions
Q
Comments (4)
Are there dampers near the supply plenum on the duct runs to balance airflow to the rooms? From what source are the 3 diffusers fed; from one duct run or more? The 3 diffusers maybe sized correctly when all three are used, otherwise the static drop may be too high. We would only be guessing from here. - udarrell Here is a link that might be useful: Air Side of Air Conditioning
...See More
Cold Air Return duct question - what is this? (pics)
Q
Comments (6)
Thanks for the reply, tinman. Yes, there are two joist pockets, because the CA duct in the floor above straddles the middle joist, and there are two openings in the ductwork (one for each joist pocket). I'm leaning towards tidying this up - I can't imagine it's helping the efficiency at all. Only thing I could think was if for some reason there needed to be a mixing/balancing of air temperature from upstairs with downstairs before reaching the furnace - but that seems a stretch.
...See More
Cold-Air Return Question
Q
Comments (6)
Mike, Sounds like you really need to do some up-dating. A little styrofoam in the air return would be like putting a Band-Aid on a decapitation. It's a lost cause. What you need is whole-house insulation and a new heating system. Your exisiting nearly 80-year-old furnace is probably sending more heat up the flue than you're getting into your living space. First thing to do is check with your utility company to see if there are any rebates available for adding insulation, replacing windows, or updating your antique heating plant and ductwork. Then get an analysis to find out where you'll get the most immediate bang for your buck. Insulation may be the best bet, but your old furnace (and ducting) is probably so inefficient, that it might be your most urgent consideration. From what you've described, no matter what you do, it will probably repay your investment in the very near future. I'm glad I don't have your heating bill.
...See More
daddo
16 years ago
The sizes I gave you are the minimum. You can run 15" on all three- even 20" if you wanted. A larger duct won't hurt anything but the pocketbook.
If you want to use just two temporarily, you can run two 14" with no problems.
Ducting usually comes in even sizes, I've never seen a 15" round duct.? I've also noticed that register sizes are different up north than they are here in the south- we use sizes such as 12 X 6, 10 x 6, and so on- I notice up there you use 12 x 4, 10 x 4 and so on- strange.
ccoombs1
Original Author
16 years ago
I am not sure what size round pipes are available...someone told me to get 15" ones. I never actually shopped for them yet. I will go with two 14" returns in the main part of the house and at lease a 12" one for the bedroom then. That should work great. Thanks for all your assistance. This should be an interesting project! LOL!

Related Stories

FUN HOUZZ Everything I Need to Know About Decorating I Learned from Downton Abbey

Mind your manors with these 10 decorating tips from the PBS series, returning on January 5

MOST POPULAR What to Know About Adding a Deck

By Mitchell Parker

Want to increase your living space outside? Learn the requirements, costs and other considerations for building a deck

ARCHITECTURE The Truth About 'Simple' Modern Details

By Bud Dietrich, AIA

They may look less costly and easier to create, but modern reveals, slab doors and more require an exacting hand

HEALTHY HOME A Guide to Indoor Air Purifiers

By Laura Gaskill

Get the lowdown on air filtration systems for your house and the important ratings to look out for

LIGHTING 5 Questions to Ask for the Best Room Lighting

By Karen LeBlanc Design TV Host & Writer/Blogger

Get your overhead, task and accent lighting right for decorative beauty, less eyestrain and a focus exactly where you want

MOST POPULAR 8 Questions to Ask Yourself Before Meeting With Your Designer

By Kelli Kaufer Designs

Thinking in advance about how you use your space will get your first design consultation off to its best start

REMODELING GUIDES Considering a Fixer-Upper? 15 Questions to Ask First

By Sarah Phipps Design

Learn about the hidden costs and treasures of older homes to avoid budget surprises and accidentally tossing valuable features

CONTRACTOR TIPS Building Permits: What to Know About Green Building and Energy Codes

By Matt Clawson

In Part 4 of our series examining the residential permit process, we review typical green building and energy code requirements

CONTRACTOR TIPS Building Permits: When a Permit Is Required and When It's Not

By Matt Clawson

In this article, the first in a series exploring permit processes and requirements, learn why and when you might need one

CONTRACTOR TIPS Building Permits: 10 Critical Code Requirements for Every Project

By Matt Clawson

In Part 3 of our series examining the building permit process, we highlight 10 code requirements you should never ignore

Sponsored

Columbus Premier Design-Build and General Contractor

More Discussions

Mount range hood to wall or ceiling?

19

Engineered vs. Solid Hardwood over radiant floor heating

8

Replace rusted swamp cooler panels with aluminum sheeting?

2

is this leak due to condensation?

21

HVAC condensate pump issue

7