A vacuum valve must be installed on the cold water inlet in either a vertical or horizontal position to assure warranty coverage a Marathon water heater. Without a vacuum valve properly installed, a break in the main supply line can create a vacuum within the water heater and irreparably damage the inner tank.
Marathon water heater models changed in March of 2009 to require a different vacuum relief valve. Now there are two different kinds of valves available depending on when your water heater was manufactured. Please see model numbers and descriptions below:
Part # SP623335
Use this valve on all Marathon water heater models manufactured in April 2009 or later.
Part # SP623336
Use this replacement vacuum valve for all Marathon water heater models manufactured in March 2009 or earlier.