The most common problem in the hot water system is the loss of heating capacity. The water gets as hot as it ever did, but there just is not as much water available. This is most often caused by a condition called liming.
          Liming is the formation of a hard, insulating layer of 'lime' found inside the heating tank or coil. Liming is heat dependent. Minerals dissolve in the water and deposit on the hottest portion of the metal that the water contacts. The rate of deposition depends on the temperature and water mineral content.
          You can draw a little more hot water out of limed equipment by raising the temperature to which the tank or furnace thermostat is set. You can adjust the mixing valve, if there is one, to pass more hot water and less cold.
          At best, these efforts are short lived and to some extent self defeating. The rate of liming increases with temperature, so when you turn up the temperature of your hot water you also increase the rate of liming. Also, as the temperature increases you reach a point where the hot water emerging from the faucet is dangerous.
          Reprinted in part from Popular Science, "Designing a Hot-Water System". 1975
          When GMX units are installed, for the most part, the liming is reversed. This is not done overnight. However, there is often a decline in buildup on dishes, showers, faucets, etc.. Most users can tell the difference within a few days. Here it is fair to say that different people notice different things!