I have a 2 year old Gas Hot Water Heater in my house 50 gallon I believe. It runs out of water after about 5 minutes and the water does not seem as hot as it should be (It's turned to 3 on a max scale of 4 on the heater). The pilot light is on. Can you tell me what might cause the hot water to run out so fast. Nothing else is drawing the hot water.