SOLAR TODAY - September/October 2010

You have decided to install a solar water-heatingsystem. Therearemany questions you needtoanswer, but for purposes of this two-part article we’ll focus on four: What size does the solar-heating portion of the system need to be? Will you have a supplemental water heater? What size supplemental water heater do you need? What is the “best” supplemental heater to install? This article will focus on answering these questions for single-family residential applications, but the principles apply to all situations.

ately after they open the tap, which is possible but rather expensive. Well over 90 percent say they want the time to tap to be between 2 and 3 seconds!

In the first article, we’ll examine the first two questions. Specifically, we’ll discuss how to squeeze the water and energy waste from your hot water distribution system, which might enable you to depend on smaller water heaters. Look for a discussion on sizing your supplemental heater and a recommendation for the best type in the November/December SOLAR TODAY.

taking a Whole-System Approach

First, some background. I installed my first solar thermal system in 1974 and my first photovoltaic system in 1979. The thermosyphon solar thermal system that I installed in 1981 at my off-grid house in California is still working, although it is time to reapply the black absorber coating. That system has no integrated hot water backup. The few days a year when some combination of use and lack of sun causes us to run out, we heat water on the stove, do what I call “dry cleaning” — wash up using very little water — and wait for the sun to return the next day.

The speed of delivery, which can be stated in terms of volume-to-hot as well as time-to-tap, depends on the method of heating the water and the location of the water heater relative to the hot water outlets. If you want hot water to arrive “instantaneously” (say, in less than 3 seconds), two things must be true: You need hot water in the water heater before the start of each hot water event, and the water heater needs to be close to where it is needed. An instantaneous water heater is one in which hot water leaves the water heater immediately when one calls for hot water. Contrary to the definition in federal statutes, this describes a water heater that has some amount of storage that is hot. With only a couple of exceptions, tankless water heaters, which I would define as on-demand, continuous heaters, have no such storage and are therefore not instantaneous.

Since 1993, I have focused on hot water as a system, something building science has tended to ignore. I have interviewed more than 20,000 people nationwide and in several other countries to determine what people want and expect from their hot water systems. (For a discussion of these expectations, see Klein and McCabe, “The Future of Hot Water and Solar Thermal,” presented at SOLAR 2010 and published in Solar@Work: tinyurl.com/28skedd.) The recommendations I present in this article come from this background and from my studies of how the components of a hot water system interact with each other and the desires of the people who use them.

You can make the use phase — washing dishes, taking showers or whatever application — more efficient. This can be achieved through water-efficient equipment (aerators, showerheads, dishwashers and washing machines) and energy- and water-efficient behaviors (taking shorter showers or using cold water to wash clothes). However, making the use phase more water-efficient may result in unhappy customers if we ignore the impact of the other system components, such as the hot water-distribution system’s effect on delivery times or the fact that tankless gas water heaters do not fire at low flow rates. By the way, the typical household generally has an observed peak hot water flow rate of less than 3 gallons per minute, or gpm ( 11. 4 liters per minute); there is more diversity and less overlap

FIgURE 1: TYPICAL HOT WATER EVENT

Figure 1, at right, shows a typical hot water event: There’s a delivery phase, a use phase and a cool-down phase. People would like the delivery phase to be short. According to those interviewed, a few want hot water to arrive immedi-

WAterfilm energy, gfxteCHnology.Com

Installing a drain-water heat-recovery system on the showers can capture roughly half of the temperature rise from incoming cold water to the mix-point temperature of the shower. Showers represent 75 to 80 percent of a household’s hot water use.

of uses than one might suspect.

How much hot water do we use? While there is clearly a large standard deviation, we can reasonably estimate that an adult in the United States uses about 20 gallons ( 76 liters) of hot water per day. For a typical family of three, that means 60 gallons per day. Assuming that the temperature rise is 90°F (from 50°F to 140°F), it takes about 275 therms, or 5,750 kilowatt-hours, to heat this water every year, factoring in the efficiency of the water heater. Then there is the cost for the water itself. Nationally, the average cost is

$0.006 per gallon for water and sewer combined, or $130 annually for this family’s water. Assuming that natural gas costs $1 per therm and electricity costs $0.10 per kilowatt-hour, the total cost for hot water is about $400 per year for the family that heats with natural gas or about $650 per year for that family that heats with electricity. These numbers give us an idea of how much we can afford to spend on hot water systems for water and energy savings: We cannot save more than we spend!