Hello Energyqueen: Just happens that I have a Whitehead 20 gallon heater. It has a name-tag that says the BTU input is 20,000 per hour. That was a very educated guess of yours! 😎
Unless the flue is double wall, I’d expect it to be hot anyway. Perhaps you could clock the heater by watching the gas meter for a length of time, say fifteen minutes, to get a better idea what the heater really uses when running. I’m not positive, but it looks like there is a main burner gas pressure adjustment built into the control.