The burner comes on only when the water cools down enough to
cool the sensor. That's because of

1) use of hot water by opening a faucet
2) heat loss through the walls of the tank
3) heat loss up the chimney

The difference between heat loss of a 30 or 40 is minimal.
The 40 is cheaper to buy and install, since it's a more
common size. I'd go with the 40.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"desgnr" wrote in message
...
Would there be much difference in operating costs for a 40
gallon gas water
heater vs. a 30 gallon ?

--
Dell Inspiron
Pentium dual-core 2.2 GHz
2 GB DDR2 SDRAM
Windows Vista Home Premium SP1