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# Combustion-Air

## Combustion-Air

#1
12-04-00, 03:32 PM
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I am building a dividing wall against the back of the garage. Roughly a space 8X20 with 8' ceilings; 1280cu feet.

Permits mention mantining appropriate Combustion space for the gas furnace and water heater; but are fairly abuguous as to what is required. I found some source that says ;
"Appliance in unconfined space may rely on infiltration alone" and "Unconfined space is >50cu.ft./1000Btu/hr".

What does this mean ???

My understanding is the garage alone (no separating wall) is considered unconfined space due to infiltration... I understand that the space the water heater and the furnace in will be a smaller space, but is it about 1280 cu ft. in size.

Do I need to provide outside vention or not ???

#2
12-04-00, 07:15 PM
Join Date: Feb 1998
Location: The Shake and Bake State USA
Posts: 9,927

Hi:smarer

To directly answer your question, "Do I need to provide outside ventilation or not ???" to the answer is YES.

How much should be provided often isn't all that clear in many area's. However, the most widely excepted rule of thumb is to provide 1 square inch for every 1,000 BTU's.

The positive way to be sure there is enough combustion air to meet any code, is to allow 2 square inches for every 1,000 BTU's.

The locations of these air vents should be as follows.
Half the amount of combustion intake air up high and the other half down low, if at all possible.

The way to figure the amount needed is to add up the intake rated BTU's of both appliances and convert that amount into square inches.

Example:
Furnace at 100,000 BTU's
Water heater at 50,000 BTU's
Total is 150,000 BTU's
Air space needed is 150 square inches.
Put half this amount down low and the other half up high.

The surest way to meet any code, is to double the amount.
In the above example, use 300 square inches.

Good Luck,
TomBartco
Natural Gas Energy Technician and Consultant.

[Edited by Tom_Bartco on 12-05-00 at 10:45]

#3
12-05-00, 11:45 AM
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Tom,

I have a question. Not to question the math but for my understanding... Your example :

Example:
Furnace at 100,000 BTU's
Water heater at 50,000 BTU's
Total is 150,000 BTU's
Air space needed is 150 square inches.
150 divided by 12 = 12.5 square inches.
Then put half that amount down low and the other half up high.

I have a 66,000 BTU furnace and 34,000 BTU H2O Heater, so 100 square inches of air space is needed. Why did you divide by 12 ? I was reading in one code book where the min size of the opening must be 100 sq inches, is this the combined upper and lower or each opening ?

#4
12-05-00, 08:03 PM
Join Date: Feb 1998
Location: The Shake and Bake State USA
Posts: 9,927

Hi:smarer

I had meant divide the amount by 2.
Then divide that amount by two again.
Here's why.

In your example of 100 divided by 2 equals 50.

Of that 50, you could put up two 25 square inch vents up high and two 25 square inch vents low. As long as the totals are equal to the amount needed.

This makes finding vents of the size needed easier and locating them on the walls or doors easier too.

YES. The total amount needed is 100 square inches.