Tech Tip: Do you have enough make-up air to support proper combustion?

Posted by Chris Bernier on Mar 9, 2015 1:24:00 PM

Combustion Problems? Do you have enough make-up air to support proper combustion?

Determining if a space is confined:

1. Find the total BTU being fired in the space in question.

2. Multiply that by 50. This will total the number of cubic feet of space that is needed to support combustion.

3. Find the volume of space in question using this formula: Length x Height x Width = Volume in cubic feet.

4. If the number from step 2 is larger than the number from step 3 the space is considered confined.

Verifying if a space is confined: Example: A 100,000 BTU gas fired boiler and a 50,000 BTU gas fired water heater are being installed in a room that is 30 feet long, 29 feet wide and has 8 foot high ceilings.

1. What you need: Find the total cubic feet of space needed to support combustion and ventilation.

Formula:

Total Cu. Ft. = Total BTU Load x 50

Total Cu. Ft. = 140 x 50

Total Cu. Ft. = 7000 Cu. Ft.

2. What you have: Find the total cubic feet of space where the gas fired equipment is to be installed.

Formula:

Cu. Ft. = Length x Width x Height

Cu. Ft. = 30’ x 29’ x 8’

Cu. Ft. = 6960 Cu. Ft.

3. Solution: The total cubic feet needed is greater than the total cubic feet available and the space is considered confined. 7000 Cu. Ft > 6960 Cu. Ft. Combustion air must be acquired from another space.


Methods of Getting Air

Here are some guidelines to help.

Sizing the opening for outside air (NFPA Interpretation)

One opening, within 12 inches of the ceiling, shall be installed to the outside of the building and must have 1 square inch of free air for every 3000 BTU’s fired in that space.

Example: A boiler room has a 50,000 BTU gas fired water heater and a 100,000 BTU gas fired boiler. How big must the free air space opening to the outside be?

Answer: The free air space must be 50 square inches or a 10 x 5 inch opening to the outside of the building. (100,000 BTU’s + 50,000 BTU’s = 150,000 BTU’s; 150,000 BTU’s/3000 BTU’s = 50 square inches)

Complications: The above formula for free air space does not account for grills or louvers made of wood, plastic or metal. Some grills/louvers have the total of free air space pertaining to that grill or louver. If no information is present, the following sizing percentages should be used: Wood/Plastic = 25% free area; Metal = 75% free area.

Louver Sizing Using Wood/Plastic:

A 10 x 10 inch unmarked wood or plastic louver has a total of 100 Sq. In. The percentage of free air is 25%. 100 Sq. In. x .25 = 25 Sq. In. Free Air This type would not work for the example given.

Louver Sizing Using Metal:

A 10 x 10 inch unmarked metal louver has a total of 100 Sq. In. The percentage of free air is 75%.

100 Sq. In. x .75 = 75 Sq. In. Free Air. This type would work for the example given.

Advantages of Direct Vent Water Heaters:

Unlike non-direct vent heaters, this type of water heater is connected directly to the outside by two pipes. This means the water heater uses outside air for combustion and not air used to heat or cool the building. Direct vent water heaters always have access to an adequate supply of combustion air and will never create negative air pressure or function inefficiently. Direct vent water heaters can be vented either horizontally or vertically offering more flexibility during installation.

Topics: Tech Tips

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