There are two showers in my house and the one on the second floor produces hot water for about 30 seconds, unless we turn the heat up for about ten minutes and then take a shower. We have been told to replace the symmons valve including the housing unit. We tried just replacing the mixing valve but that did nothing. Why would we have to replace the housing unit? What would be wrong with it that would cause the problem we have? The other shower works fine and has just recently been installed and we get very hot water everywhere else. Could it have something to do with the boiler, which is only three years old?