AMTROL has introduced its Dual "O" Ring Sealing System for the WH-7 and WH-9 Series BoilerMate Classic water heaters. The Dual "O" Ring system provides a second seal that acts as a backup for the protection of the interior section of the water heaters. This dual "O" ring system offers immediate confirmation that a fully functioning seal has been made and guards against future leaks should the heater be removed for servicing. Should the seal not be secure, the second "O" ring will inhibit internal water heater damage caused by internal leaks. AMTROL. Circle 2.