Hello, I have an Apollo Hydroswirl Super 5050 75 Gallon water heater--15 years old. The problem is that some mornings the water is barely warm--next morning it's nice and hot. Seems to go through this pattern repeatedly- Its just two of us in the house. I'm wondering if the thermostat is bad, and IS there a way to change it? I'm reasonably handy... Thanks very muchy!