Permits and Combustion air - Help!

bdl99August 20, 2006

I was quite happily completing all the paperwork to apply for permits when I hit the form "Combustion Air Worksheet". I'm in NJ but the after a few web searches the rules appear to be pretty standard.

The calculation says to take BTU ratings of all appliances divided by 1000 and times by 50 = Cubic Feet required.

I have two 82K BTU furnaces (one for each floor) and a 75K hot water heater, so:

82 + 82 + 75 = 239 x 50 = 11950 cubic feet.

There are rules about adjoining spaces but I hit a problem before that. Assuming a height of 8ft (which is unlikely in the finished state) that would mean I would need 1494 sq feet of space. My basement isn't that big to start with around 1300 sq ft or raw space.

I've managed to get hold of a few examples of other peoples paperwork and they all appear to be incorrect based on my understanding of the instructions. For example one home only included the BTU rating from one of their furnaces, the other only entered half the rating of the water heater. All the examples seem to be using a height of 8ft when I know the ceilings are lower than that.

Am I doing something wrong? I'm happy to post my plans or anything else if it will help. My plans had the furnaces and water heater in a 17' x 13.5' unfinished area of the basement.



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In our NJ town, one fills out the paper work for the permit outlining the plans to the best of their ability; then the code official makes any changes and/or corrections needed. Hopefully it is the same in your town. Before you make yourself nuts trying to figure out what would be the correct the town construction code office, as ask if they can help you with that part. IMO, they most likely will be happy to do so. the NJ Department of Community Affairs construction code division. I have found them to be extremely helpful more than once in the past on code questions with regard to our home improvement projects.

As a general rule, this is why it is never a good idea for one to advise another that it is OK to not get permits as long as one follows code. Code is not all that clear to those who are not well versed in such...this is a classic example how it can indeed be quite confusing.

    Bookmark   August 21, 2006 at 3:16PM
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I believe what the formula is trying to determine is if the mechanical room is an unconfined space. If the space is big enough then it is assumed that there will be enough leakage from little cracks for the combustion air to draw from. If your total basement was big enough then you would have to cut grilles in the wall btw the mechanical room and the finished basement. That way the equipment could draw air from the finished portion. This may not be acceptable if your equipment is noisy. If in your case the basement is not big enough then you will have to draw combustion air from the outside.
How difficult would that be for you? If you don't like that idea then you could fill out the paperwork and wait for the official to tell you what is acceptable.

    Bookmark   August 21, 2006 at 3:59PM
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funnycide is correct.

The purpose of the formula is to determine the physical area in cubic footage required to provide an adequate supply of combustion air.

For example, you have two furnaces @ 82k/btu each and a water heater at 75k/btu for a total of 239k/btu.

We then divide the btu x 1k and multiply by 50 to determine the required cubic footage of the room or structure where the combustion equipment is. In this case it would be:

239kbtu x 50 = 11,900cu.ft.

We then divide the total cubic feet by the actual ceiling height to determine the square footage. In this example bdl99 has stated that the actual ceiling height would be less than 8' so for the purpose of illustration let us assume the actual ceiling height will be 7'.

11,900cu.ft / 7' = 1,707sq.ft.

If the structure was greater than 1,707sq.ft it would be considered to be an "unconfined space" and no additional air would be required, however if the combustion equipment was in a smaller utility space we would need to make provision to allow air to transfer from the remaining portion of the structure into the combustion area.

In this example the required square footage exceeds the total square footage of the structure so it is then classified as a "Confined Space" and we need to make provison for sufficient outside makeup air to meet the needs of the combustion equipment.

Once we determine we need to provide a source of make up air we then need to determine the method that we will use to supply the air. The method could be a direct opening through and outside wall, a vertical duct from an attic space above or a crawlspace below or a horizontal duct from outside.

When making a direct opening through the wall we could have a single opening or we could have two openings, one within the top 12" of the combustion area and a second one within 12" of the floor of the combustion area.

In this case the combustion area is in a basement so it is highly unlikely that we can provide an air intake at the floor level so we would use the formula for a single opening.

For a single opening direct to the outside the formula is;

The total btu/hr was 239,000btu.

239,000btu / 4,000btu =

Using the square root of 59.75 we can then determine that the air intake opening must be 7.72" x 7.72" or equivalent.

A 9" diameter round opening in the wall would provide 63.61sq.ft which would be just slightly larger than our required opening.

    Bookmark   August 21, 2006 at 9:46PM
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I dont know much about gas appliances but quick search for filling out that form said...    *Do not include direct-vent gas appliances. Direct-vent draws combustion air from the out-doors and vents to the outdoors.

Does a gas waterheater waste indoor conditioned air to heat water ?

Here is a link that might be useful: form calculation instructions from nj town

    Bookmark   August 21, 2006 at 10:05PM
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You are correct that a gas water heater uses conditioned air for combustion. There is also losses of conditioned air to bathroom or range hood exhaust fans, however this is not a bad thing. As that slight amount of conditioned air is exhausted to atmosphere it results in an equal amount of infiltration of outside air into the structure.

If we did not have a small percentage of infiltration there would be an extreme risk of oxygen depletion or the levels of chemicals offgassing from the building materials or furnishings reaching a dangerous level.

    Bookmark   August 22, 2006 at 9:46AM
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