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?
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.
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.
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).
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?
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?
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?
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?
If there isn't any doors to block the flow of return air then there will be little difference in the overall performance.
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