PB Heat MIH manual Air Openings All Air from Outdoors through Vertical Ducts

Vavail

Vreq

PREINSTALLATION

Figure 1.4: Air Openings – All Air from Outdoors through Vertical Ducts

ii.Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 in2 per 2000 Btu/hr (22 cm2 per 2000 W) of total rated input for all appliances in the space. See Figure 1.5.

Figure 1.5: Air Openings – All Air from Outdoors through Horizontal Ducts

b.One Permanent Opening Method: Provide one permanent opening beginning within 12 inches (305 mm) of the top of the space. The opening shall communicate directly with the outdoors, communicate through a vertical or horizontal duct, or communicate with a space that freely communicates with the outdoors. The opening shall have a minimum free area of 1 in2 per 3000 Btu/hr of total rated input for all appliances in the space and not less than the sum of the cross-sectional areas of all vent connectors in the space. The gas-fired equipment shall have clearances of at least 1 inch (25 mm) from the sides and back and 6 inches (150 mm) from the front of the appliance. See Figure 1.6 for this arrangement.

Figure 1.6: Air Openings – All Air from Outdoors through One Opening

5.Combination Indoor and Outdoor Combustion Air: If the required volume of indoor air exceeds the available indoor air volume, outdoor air openings or ducts may be used to supplement the available indoor air provided:

a.The size and location of the indoor openings comply with Subsection 3.

b.The outdoor openings are to be located in accordance with Subsection 4.

c.The size of the outdoor openings are to be sized as follows:

⎛	Vavail	⎝
Areq = Afull x ⎜1 –
		⎜
	Vreq
⎝
		⎛

where:

Areq = minimum area of outdoor openings. Afull = full size of outdoor openings calculated

in accordance with Subsection 4. = available indoor air volume

= required indoor air volume

6.Engineered Installations: Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air and shall be approved by the authority having jurisdiction.

7.Mechanical Combustion Air Supply:

a.In installations where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from the outdoors at the minimum rate of 0.35 ft3/min per 1000 Btu/hr (0.034 m3/min per 1000 W) of the total rated input of all appliances in the space.

b.In installations where exhaust fans are installed, additional air shall be provided to replace the exhaust air.

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