Why is a plumber installing a hot water heater concerned with density?
Well… hot water is less dense than cold water. If the plumber reverses the input with the output, cold will go in the top, the hot water will sit on top of the tank and the cold bottom will feed the output. At the tap, the water will be cold (or at best lukewarm). The cold-water input is either at the bottom of the tank or through a long tube to the bottom of the tank from the top outlet. This lets the cold into the bottom where it is heated, gradually rising to the top at its hottest. And, of course, the hot water outlet is at the top. Knowing about ‘density’ helps the plumber make the correct connections even if the inlet and outlet are not labeled. Pretty simple, once you think about it.
