Westinghouse WGRGH20**75F Series Installation, Start-Up, Maintenance, Parts, Warranty page 32

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Air Supply from Outside Building
When combustion air is supplied directly through an outside wall,
such as intake louver openings into the dwelling, each opening
should give a minimum free area of one square inch per 4,000 BTU/
hour on the total input ratings of all appliances in the enclosed area.
Air Supply from Inside Building
When combustion air is supplied from inside the building, each
opening should give a minimum free area of one square inch per
1,000 BTU/hr on the total input ratings of all appliances in the
enclosed area. These openings should never be less than 40 sq. in.
NOTE: The minimum required total space volume without air vents
is 50 cu. ft. per every 1,000 BTU/hr.
Table 15 - Power Venting Installation Details
How to Configure the Unit for Power Venting
Leave the air intake open to configure the water heater for power
vent.
Table 16 - Configuring the Unit for Power Venting
whl-605 Rev. 002 Rel. 000 Date 1.23.19
Figure 24 - Air Supply from Outside Building
Figure 25 - Air Supply from Inside Building
Figure 26 - Configuring the Unit for Power Venting