SOLAR TODAY - September/October 2010

get started with efficient water heating

PHotoS By dAvid SCHumACHer, tAken during tHe 2010 AnnuAl Convention of tHe floridA ASSoCiAtion of PlumBing, HeAting And Cooling ContrACtorS.

Above, minimizing the volume in piping between the source of hot water and the hot water outlets reduces waste. Depending on diameter, pipes as small as 6 inches and as long as 25 feet can have the same volume. Each of these pipe lengths holds 1 cup of water. Right, in addition to reducing piping volume, we should also insulate pipes. For equal heat loss per foot, half-inch pipe gets half-inch wall thickness, three-quarter-inch pipe gets three-quarter-inch wall thickness and so on.

The cool-down phase begins when you turn off the tap, with the temperature of the water in the pipe cooling from the water heater all the way to the hot water outlet. It takes on the order of 10 to 15 minutes for the water in uninsulated pipes to cool from 120°F to 105°F when the pipes are located in air at a temperature between 65°F and

Wringing out the wastes and using water more efficiently can reduce overall hot water demand by 25 to

50 percent. It also reduces

70°F. The water cools more quickly when the surrounding temperature is colder, such as in a basement or a crawl space or when the pipes are located in or under a concrete slab. Likewise, it cools more slowly when the pipes are insulated or in a hot attic.

The water heater must be set to a higher temperature than the mix-point temperature, and the useful hot water temperature needs to be less than the point at which you mix it. Why? You need to have some headroom from the mixing point down to the useful hot water temperature point because of variations in desired temperature for any given application on any given day.

I would also observe that as solar practitioners, we can do very little to effect behaviors directly. What we can do is install systems that reduce the structural wastes so that we can encourage water-and energy-efficient behaviors.