Q. How little difference in elevation can exist between the top of cold-water tank and hot-water faucet and maintain a good delivery at said faucet, the two being in the same room and connected through a hot-water boiler in story below?

A. This depends altogether on what is meant by a "good delivery." The discharge depends entirely on the difference of level between the cold water in the tank and the faucet. If your pipe is twenty feet long, and you want to draw two gallons a minute, your faucet must be about twenty-seven inches lower than the water, but if you want four gallons a minute it must be about seven feet lower.