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return air grille size

6K views 17 replies 9 participants last post by  Marvin Gardens 
#1 ·
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?
 
#7 ·
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).
 
#10 ·
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.
 
#13 ·
I found my furnace manual (model: Carrier 58MCA 060-12: http://xpedio.carrier.com/idc/groups/public/documents/techlit/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?
 
#14 · (Edited)
I found my furnace manual (model: Carrier 58MCA 060-12: http://xpedio.carrier.com/idc/groups/public/documents/techlit/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.
 
#17 ·
Should his ratio of floor 1 to floor 2 return size be the same as the ratio of floor 1 to floor 2 heat demand? If he changes the balance and starts pulling a greater percentage of return air from the basement won't that also pull a greater percentage of supply air to the basement?

Or will it tolerate a lot of variability in the balance?

Trying to learn something today.
 
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