Calculating free area for louvered grille


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Old 09-08-15, 02:29 PM
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Calculating free area for louvered grille

I bought some louvered return grilles at the blue store, 24x8 and 14x14 for the purpose of providing combustion air to 140,000 btu/h input worth of appliances in a storage room adjacent to a new finished area. I'm finding it difficult to get any info on the percentage of free area those grilles provide. It looks like each has to be 140 in^2 respectively. Any help on how to get that percentage of free area?
 
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Old 09-08-15, 03:25 PM
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Here's a chart that estimates NFA for some common types of vent screens. You probably want the 1/8 inch mesh with rain louver entry, or maybe the 1/4 inch mesh with rain louver entry.

Ventilation Requirements (scroll down a bit)

Hope this helps.
 
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Old 09-08-15, 03:46 PM
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Without the mfg stating the NFA, we are left to guess and that isn't good. If these grills have a bug screen along with louvers you are looking at about a 50% reduction (my guess). I recently searched for gable vents and those louvers were closer to 25% NFA. The little round pop-in vents are only 19% NFA. So you see the numbers vary considerably and may not be as high as you need.

In addition, vents to the outside may have height requirements along with needing more than one.

A few years ago I had a 280K btu boiler installed and my options were a powered air supply or a 3' x 3' grill. For security purposes I took the powered option.

I did a little searching, but nothing came up. Some of the HVAC guys will have some suggestions.

Bud

CT, Wow am I slow,
 
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Old 09-09-15, 02:42 PM
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Thanks for your answers. I think I need to ask in a different way. My question is much simpler (I think...).

I have these two grilles and am looking to find out how to calculate the free area of each one. The back says 1/2 in fin spacing.

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Old 09-09-15, 05:36 PM
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There is no single formula for what you are asking. Each mfg should have tested each product and have those specifications available.

Combustion air ultimately comes from the outside. If you are providing vents to other conditioned spaces then those areas must be leaky enough to the outside to provide the needed air.

Since these vents will be inside your home, they should not need bug screens which are part of the limitation you are trying to determine.

First question, since I can't read it, what is the brand name and have you check the mfg to see what they list for NFA?

Bud
 
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Old 09-09-15, 06:05 PM
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To a rough approximation, the net free area would be calculated by taking the total area of the vent and subtracting the area where air can't flow through, like the louvers and the solid area around the edge. If there is insect screening covering the vent, that reduces the NFA further. As Bud said, manufacturer usually provides this info so you could try to track that down.

But I can tell you, if you need 140 SQ in NFA for each, you are nowhere near that.

For example, 14x14 is 196 sq in. Just looking at the vent one can see the openings are less than 50% of the vent, so NFA is certainly less than 100 sq in. I would bet closer to 80.

If you are going to get this inspected, the inspector will likely ask for proof of the NFA, if he has any doubt at all that you have enough vent area, so you may want to look for a vent that provides the NFA data. I think some people use a louvered door to the furnace room to provide the makeup air.
 
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Old 09-09-15, 06:36 PM
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Maybe I am confused?? I have a home built in 1996. I have seen tons of finished basements and have never seen return grilles the sizes that are being suggested. Or maybe I have but never noticed them. I was talking with the inspector (no inspection yet, this is the last thing before) about a full louvered door 30x80 and apparently it still didn't provide enough nfa. But you are saying there is the potential to be able to supplement the grilles and the door and have them considered together?

The brand of the grilles pictured is "Accord" and I couldn't find that info on the manufacturer website or on the seller's website.

Thanks again for your replies.
 
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Old 09-09-15, 06:47 PM
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Would this be a reliable graph to use?

Air Return Intakes - Sizes and Capacities
 
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Old 09-09-15, 07:26 PM
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I have been looking and as you found, these mfgs are not helping at all. As for the graph in the link you provided I'm not familiar with air flow rates for combustion air.

However, I did find a related link with this statement
"When the total input Btu/h rating of all enclosed appliances/equipment exceeds 100,000 Btu/h (29.3 kW), the combined net free area of all combustion air openings shall be increased by not less than one (1) additional square inch (645 mm 2) for each 1,000 Btu/h (293 W) in excess of 100,000 Btu/h (29.3 kW)."

If that applies you would need an additional 40 inē of total NFA, plus, if there is another combustion appliance in this room, like a gas water heater, its btu rating must be included. Here is the link I quoted from.
http://www2.iccsafe.org/states/phoen...hapter%207.pdf

You could send a question to Accord to see if they can provide a number.

I agree with CT that without a bug screen you are probably looking at about a 50% NFA.

You might also check with your furnace service people to see if your unit can be fitted with a combustion air supply. I know Beckett makes a combustion air kit for my oil furnace.

Bud
 
 

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