: I have a 40 US. gallon natural gas water tank. My question is, should I get 40 gallons of straight hot water out of this tank. The tank is approx. 3 yrs. old. If I turn on the hot water mixed with cold water I might get half a tub full of warm water. This is just a standard size tub. So there is no way I am getting 40 gal. of hot water if Im lucky Im getting 20. Should I be getting more? If so, what could be the problem.a 40 gal heater will yield about 38 gal at the temp setting Is the thermostate set real low? You want to have a plumber check the cold water dip tube, it may be bad causing water to mix at top of heater.