I have two 50 gallon Sears (A.O. Smith) hot water heaters hooked up together. Lately, my water temperature has been low despite (a) high setting(s). I took two temp. measurements at the closest sink and the readings were about 20-30 degrees too low for the settings. Sears came out and did a quick look at the tanks and concluded that the gas valves should be replaced on both tanks. He attempted no diagnostics on the valve(s). The parts are covered under warranty but they want $270. per tank to replace the valves. That seems awfully high to me. What would be an average fair price to replace a valve. The tanks are only 6 years old. Thank you