Return Air Grille Size - HVAC - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 11-03-2008, 11:58 PM   #1
Newbie
 
Join Date: Nov 2008
Posts: 3
Rewards Points: 10
Default

return air grille size


I have a 18x18 return air grille on my dining room wall. I'd like to replace it to 18x16 for a cosmetic reason (so only the height will be two inch shorter). Can it bring any harm to my furnace if I make the replacement? It's only two inch difference, so I'm hoping/guessing it will be fine. Will the two inch make any noticeable difference?

Advertisement

isunshine is offline   Reply With Quote
Old 11-04-2008, 01:01 AM   #2
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

return air grille size


Depends on what your other returns are if you have any.

Advertisement

__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 11-04-2008, 07:50 AM   #3
An old Tradesmen
 
Join Date: Oct 2008
Location: PA
Posts: 26,521
Rewards Points: 4,682
Default

return air grille size


It will matter if its already too small.
beenthere is online now   Reply With Quote
Old 11-04-2008, 10:17 AM   #4
Member
 
1610 CUB's Avatar
 
Join Date: Jul 2008
Location: Keokuk,Iowa
Posts: 209
Rewards Points: 150
Default

return air grille size


If the grill is cut into a 2X4 wall, their could be as little as 60cfm between the two sizes. And that should not be a problem. Just make sure the new grill has the wide spaces so as to not reduce the air flow any more.
1610 CUB is offline   Reply With Quote
Old 11-04-2008, 10:22 AM   #5
Member
 
Join Date: Sep 2008
Location: Upstate New York
Posts: 368
Rewards Points: 250
Default

return air grille size


Quote:
Originally Posted by isunshine View Post
It's only two inch difference, so I'm hoping/guessing it will be fine. Will the two inch make any noticeable difference?
Actually its 36 sq/in difference in cross-sectional area, or 11% reduction.

As mentioned if you have more than enough return it will be okay. If you have just enough or not enough you will cause a problem or exacerbate a problem.
dac122 is offline   Reply With Quote
Old 11-04-2008, 10:38 AM   #6
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

return air grille size


Quote:
Originally Posted by dac122 View Post
Actually its 36 sq/in difference in cross-sectional area, or 11% reduction.

As mentioned if you have more than enough return it will be okay. If you have just enough or not enough you will cause a problem or exacerbate a problem.
Exactly.

We need more information first before we can say it's okay.

If there are no other returns I would say no, it's not okay. A 12% reduction is pretty big.
__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 11-04-2008, 12:57 PM   #7
Newbie
 
Join Date: Nov 2008
Posts: 3
Rewards Points: 10
Default

return air grille size


The furnace covers two floors - floor1 (basement) and floor2 (living/dining/kitchen/family room). The return air grille is located in floor2 and there's no other return air grille on that floor. Floor1, which is about the half the size of floor2, has its own and it's like 10x10 or 8x8.

How do I know whether I have enough return or not? What information do you need? If I can make up the 2-inch (or 36 sq/in) difference in floor1 (i.e. replace it with 10x12), there won't be any problem, correct? I might be flexible to do that. By the way, the house is brand new (so is the furnace).
isunshine is offline   Reply With Quote
Old 11-04-2008, 01:27 PM   #8
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

return air grille size


Find out how many CFM's your furnace uses.

If the house is brand new more than likely the returns are sized just right. They don't oversize them because of cost. To them every dime matters.
__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 11-04-2008, 03:17 PM   #9
Member
 
tk03's Avatar
 
Join Date: Sep 2008
Location: PA
Posts: 167
Rewards Points: 150
Default

return air grille size


I have been out of the warm air industry for a long time and do all boiler work but....The grille is a lot larger than that joist space can carry, correct? Or am I missing something?
tk03 is offline   Reply With Quote
Old 11-04-2008, 03:27 PM   #10
An old Tradesmen
 
Join Date: Oct 2008
Location: PA
Posts: 26,521
Rewards Points: 4,682
Default

return air grille size


If you have a door between the 2 floors. Then the air from floor 2 can't freely flow from floor 2 to floor 1, so making floor 1 bigger won't help.
beenthere is online now   Reply With Quote
Old 11-04-2008, 03:45 PM   #11
Member
 
hvaclover's Avatar
 
Join Date: Oct 2008
Location: Suburbs of Detroit Mi
Posts: 3,704
Rewards Points: 2,000
Default

return air grille size


Quote:
Originally Posted by dac122 View Post
Actually its 36 sq/in difference in cross-sectional area, or 11% reduction.

As mentioned if you have more than enough return it will be okay. If you have just enough or not enough you will cause a problem or exacerbate a problem.


LOL ......Dec122 If your exposure to new and existing construction is any thing like mine has been, than you KNOW that there are very few homes with enough cold air return, let alone more than enough.

Sorry, little off topic but it struck a chord.
__________________
Just slow, not stupid.
hvaclover is offline   Reply With Quote
Old 11-04-2008, 05:59 PM   #12
Member
 
1610 CUB's Avatar
 
Join Date: Jul 2008
Location: Keokuk,Iowa
Posts: 209
Rewards Points: 150
Default

return air grille size


Quote:
Originally Posted by tk03 View Post
I have been out of the warm air industry for a long time and do all boiler work but....The grille is a lot larger than that joist space can carry, correct? Or am I missing something?
You nailed it! You or I cant get anyone else to believe it ....but
1610 CUB is offline   Reply With Quote
Old 11-05-2008, 09:49 AM   #13
Newbie
 
Join Date: Nov 2008
Posts: 3
Rewards Points: 10
Default

return air grille size


I found my furnace manual (model: Carrier 58MCA 060-12: http://xpedio.carrier.com/idc/groups...58mca-12pd.pdf) and found a couple of things:

1. Airflow CFM -- Heating: 1065, Cooling: 1200
2. Minimum return-air opening at furnace, based on metal duct -- For 1200 CFM 20-in. round or 14-1/2 X 19-1/2 in. rectangle

I mentioned 10x10 or 8x8 for floor1 in my previous post, but I had it wrong. It's 12x12 instead. So here's my math:

Manual: 20x20 = 400
Current: 12x12 + 18x18 = 144 + 324 = 468 (so i guess the builder made it more than enough)
My attempt: 12x12 + 18x16 = 144 + 288 = 432

I'll be still more than what it says in the manual. So I guess I'm good to go, right?
isunshine is offline   Reply With Quote
Old 11-05-2008, 10:13 AM   #14
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

return air grille size


Quote:
Originally Posted by isunshine View Post
I found my furnace manual (model: Carrier 58MCA 060-12: http://xpedio.carrier.com/idc/groups...58mca-12pd.pdf) and found a couple of things:

1. Airflow CFM -- Heating: 1065, Cooling: 1200
2. Minimum return-air opening at furnace, based on metal duct -- For 1200 CFM 20-in. round or 14-1/2 X 19-1/2 in. rectangle

I mentioned 10x10 or 8x8 for floor1 in my previous post, but I had it wrong. It's 12x12 instead. So here's my math:

Manual: 20x20 = 400
Current: 12x12 + 18x18 = 144 + 324 = 468 (so i guess the builder made it more than enough)
My attempt: 12x12 + 18x16 = 144 + 288 = 432

I'll be still more than what it says in the manual. So I guess I'm good to go, right?

The minimum you need is 282 sq in rectangle according to the manual. (I am not sure where you got the 400).

My charts say that you need at least 432 so I am not sure what the manufacturer knows that I don't.

Having a vent cover reduces the air flow by 30% in most cases I have seen similar to screens on the windows that do the same thing.

So the area you have currently is 144+324*0.70 = 327

You want to go to 144 + 288 *0.7 which is 302.

So it looks like the answer is yes. You can do that providing there is not something we are missing in the configuration.
__________________
My idea of a perfect day: No where to go and all day to get there.

Last edited by Marvin Gardens; 11-05-2008 at 10:33 AM.
Marvin Gardens is offline   Reply With Quote
Old 11-06-2008, 09:10 PM   #15
Newbie
 
Join Date: Nov 2008
Location: Tallahassee, Florida
Posts: 2
Rewards Points: 10
Default

return air grille size


So what would be the outcomes depend on?

Quote:
Originally Posted by Marvin Gardens View Post
Depends on what your other returns are if you have any.

Advertisement

amonalisa is offline   Reply With Quote
Reply

Tags
grille size , return air


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with return and supply size XHornet85 HVAC 9 09-17-2008 12:13 AM
Return Air Grille Size...? ssporty21228 HVAC 4 08-17-2008 11:00 PM
Return Air Grille brucetackett HVAC 2 01-15-2008 11:12 AM
What size should cold air return grille be? AllGoNoShow HVAC 1 02-27-2007 04:48 PM
return grille farrington135 HVAC 2 07-20-2006 10:31 PM




Top of Page | View New Posts