DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (http://www.diychatroom.com/)
-   HVAC (http://www.diychatroom.com/f17/)
-   -   return air grille size (http://www.diychatroom.com/f17/return-air-grille-size-31187/)

isunshine 11-03-2008 11:58 PM

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?

Marvin Gardens 11-04-2008 01:01 AM

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

beenthere 11-04-2008 07:50 AM

It will matter if its already too small.

1610 CUB 11-04-2008 10:17 AM

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.

dac122 11-04-2008 10:22 AM

Quote:

Originally Posted by isunshine (Post 180491)
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.

Marvin Gardens 11-04-2008 10:38 AM

Quote:

Originally Posted by dac122 (Post 180609)
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.

isunshine 11-04-2008 12:57 PM

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).

Marvin Gardens 11-04-2008 01:27 PM

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.

tk03 11-04-2008 03:17 PM

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?

beenthere 11-04-2008 03:27 PM

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.

hvaclover 11-04-2008 03:45 PM

Quote:

Originally Posted by dac122 (Post 180609)
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.

1610 CUB 11-04-2008 05:59 PM

Quote:

Originally Posted by tk03 (Post 180687)
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

isunshine 11-05-2008 09:49 AM

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?

Marvin Gardens 11-05-2008 10:13 AM

Quote:

Originally Posted by isunshine (Post 180918)
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.

amonalisa 11-06-2008 09:10 PM

So what would be the outcomes depend on?

Quote:

Originally Posted by Marvin Gardens (Post 180510)
Depends on what your other returns are if you have any.



All times are GMT -5. The time now is 11:53 AM.