6 gallon hot water heater only delivers 4 gallons

ehstars wrote:
Hello, I have a 6 year old Suburban SW6DE water heater which only gives me 4 gallons of hot water. This short change just started recently. The water gets heated to 135 degrees on either propane or electric, or both, but just doesn't deliver the whole 6 gallons. I have looked endlessly for leaks but no joy. The only thing that I see which might contribute slightly to this is my pressure relief valve is dripping a bit, about one drop every two seconds, more or less. But I doubt that would add up to two gallons in the 15 minutes it takes for the heating cycle to end, would it? Any advice would be great. Thanks.