Before you go to the expense of replacing your water heater, be sure to check your faucet. Temperature-controlled and pressure-balanced shower faucets use a spring to automatically adjust the hot water whenever there is a change in water temperature. Over time, these springs start to function less efficiently, so you notice a decrease in hot water pressure. If replacing the faucet does not correct your problem, you should contact a professional water heater repair service to determine if the problem can be fixed or if you need to replace the entire unit.