Adding outside make-up air.

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Old 12-26-11, 05:58 AM
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Adding outside make-up air.

Even though this article (How to Provide Makeup Air for a Wood Stove | GreenBuildingAdvisor.com) isn't specific to boilers, I thought it had some good information pertaining to make up air. Using a P-trap setup to prevent cold air from coming in has been talked about several times here. The article addresses the myth of that type of setup and I thought it would be helpful to share.
 
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Old 12-26-11, 07:55 AM
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Hi drooplug,
I've seen the GBA article before, but didn't follow all of the referenced links, so will do so as the holiday slows down. To put my interest in perspective, I have been working on the forces that actually move air into and out of all homes and combustion air (I'll avoid the word make-up which can be misleading) is something very important as we move into the heating season.

As the GBA article states, a hole is a hole, and other than the added length/resistance, the only other consideration is the weight of the cold air, which I will take a look at.

Bud
 
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Old 12-26-11, 08:11 AM
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Don't forget pressure differential. With wind blowing across the outside of the house in the right way, the weight of the cold air in the trap isn't going to be enough to keep it in place.
 
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Old 12-26-11, 12:03 PM
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You could run a duct that opens close to the furnace so as the furnace draws air, the incoming cold air doesn't commingle as much with warmer air elsewhere in the basement.
 
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Old 12-26-11, 01:19 PM
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NFPA 54, National Fuel Gas Code, contains requirements for providing minimum combustion air. It's primarily a safety issue, not economic - since the outside air, whether provided directly through a deliberate scheme or through natural infiltation, has to be heated.

Anyway, NFPA 54, part 9.3 contains the requirements - which are a bit complicated. But for most residential installations, natural infliltration is sufficient. There are exceptions, of course - e.g., if the burner is tightly enclosed in a boiler room without sufficient louvers to connect the boiler room to the rest of the house. The code calls for a minimum volume of 50 cu ft (effectively communicating with the boiler) per 1000 Btu/hr heat input.

Like other such codes, NFPA 54 may be trumped by local requirements or by manufacturers' installation instructions.
 
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Old 12-26-11, 02:19 PM
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For gas and oil equipment of power vented or chimney vented equipment see this link.
Calculating Combustion Air
I don't think this applies to solid fuel.
 
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Old 12-26-11, 03:12 PM
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Thanks, rbeck. That link seems to follow the requirements of NFPA 54 for combustion air, except: A door doesn't necessarily stop communication between areas of a house to get the required 50 cu ft of volume per 1000 Btu/hr - if there is a sufficiently large permanent opening through the door. A louver wouldn't be acceptable, assuming it can be closed (like a typical supply register for a forced-air system). A cold-air return register, that can't be closed, would be OK, but it needs to be 1 sq in of free area per 1000 Btu/hr.

Also, you can take credit for open areas between joists that run between rooms in a basement. And, exposed open areas above drop ceilings.

I did a little playing around the the dimensions of my house a few years ago. I pretty much concluded that it would be unlikely, but not impossible, for most houses to be able to rely solely on infiltration, even with typically oversized boilers - assuming there is not a door on the boiler room or, if there is a door, that it has an adequate register installed in the door.
 
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Old 12-26-11, 04:26 PM
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Bottom line is what the room can't deliver must be gotten from some where else. A louvered door is not part of the calculation until you determine how much more you need. then the question becomes can the louvered door supply enough.
Where the code is a bit gray in my eyes is it states that if you get your combustion air from vents to other parts of the building you need a high and low diffuser and each one must be able to supply 100% of combustion air. So one parts talks about suppling a grille in a door to halls etc and the other talks about 100% if utilizing other areas connected by high and low grilles.
And don't get combustion air confused with ventilation air. All rooms whether the boiler/furnace is chimney vented, power vented or sealed combustion you need a high and low grille, each 6" from floor and ceiling and not less than 100 in. of free air.
 
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