Air intakes vary by brand and model.
Product manual has complete information for air intake, and yearly burner cleaning and maintenance.
1) You posted under Richmond heaters which is Rheemhttp://www.rheem.com/products/tank_water_heaters/gas_water_heaters/
For a manual, click on your product and next page has documents that download to your computer, including care and use manual.http://globalimageserver.com/fetchDocument.aspx?id=01091171-3bac-4cb9-a0a8-a11aac3bd668
Sample manual shows air intake is a grille located on side of tank.
Intake should be kept clean, and water heater never placed in room with dust, link or chemicals. Even trace chemicals will react with burner and prematurely corrode burner parts.
2) If you have Bradford White water heater, air intake is on side of tank through louvers.
3) If your gas water heater stands on 3-4 legs, then air intake is located on bottom of tank, making it difficult to clean unless tank is sitting on bricks.
Here is photo of air-intake screen on bottom of Whirlpool tank.
Tomorrow this heater is scheduled for installation, and it will be put on bricks.
On this specific model the front cover can be taken off, the burner cleaned, and cover put back on using same gasket, and without buying parts.
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