I have a water heater in my home that seems to be performing erratically. There are 3 people in our house, and we all take hot showers at different times of day, but mostly in the morning. There is never enough hot water for more than a 5 minute shower. However, the other day we had 2 guests staying at our house and all 5 of us took showers in the morning (probably 30-45 min apart) and we never ran out of hot water. I would guess that we each got at least a 10 minute shower. Is there something wrong with the water heater or a part in it? The house is about 15 years old, and the water heater runs on gas.