An Evolutionary Water Heater

The legendary biologist E.O. Wilson coined the phrase “punctuated equilibrium” to describe evolutionary development of living organisms. It seems that most of the time, evolution is a slow process, leaving some organisms unchanged for eons (witness the cockroach, or more appealingly, the ginkgo tree) Dramatic evolution, when it happens, progresses quickly as a result of some significant change in the environment or a completely new type of creature showing up there.

Could something similar happen in energy-efficient product design? We should hope so. Equipment going into a building lasts a long time. If it's not efficient, it wastes energy for a long time. According to the U.S. Energy Information Agency, in 2005 21% of natural gas in the U.S. was used in residences, about 4.8 trillion ft3. The EIA tells us that 20% of all residential energy is used to heat water, and 53% of households use natural gas for that purpose. Any way you slice it, a lot of natural gas goes into heating water.

The typical gas water heater scores about 62% efficiency based on the federal energy factor test. This test basically consists of energy use over a 24-hour period, with 6 draws of 10 gallons each, taken one hour apart. There's evidence to suggest that's not really how people use hot water, which could make a difference in the as-used efficiency. But for now, the energy factor test is what we have. One of the biggest shortcomings of traditional storage water heaters is the standby loss. The heat exchanger of a typical water heater is at the bottom of the tank, where the burner sits, and a tube through the center of the tank which carries the flue gases up and away. In gas heaters with pilot lights, the pilot light generates enough heat to compensate for conductive losses and keep the main burner from turning on frequently, but that constantly burning pilot light still represents a huge waste of natural gas.

Tankless water heaters avoid this standby loss problem by heating water only as it's used, and so look a lot better on the energy factor test. There are some issues with tankless water heaters though, and one of them is that they typically require 0.75-in gas lines to supply the large burners needed to heat water as quickly as it's used. This is not such a problem with new construction, but most water heaters are installed as replacements. Replacing the half-inch gas line can be an expensive proposition, which, when added to the higher cost of the tankless water heater itself, can make the costs difficult to justify on gas savings.

It turns out most homeowners replace their water heaters when the old ones fail. And when they're in a bind, they tend to take what's on the plumber's truck, which is most likely to be a 0.62 energy factor storage water heater. Unfortunately, there it will sit, wasting gas for the next 15 years or so. Is there a way to break the cycle?

An energy agency of the California state government convened a group of water heating experts to see if there could be. The group included representatives from manufacturers, plumbers, wholesalers, and energy-efficiency experts. The task was to define the requirements for a water heater which would be more efficient, but would also be something a plumber might carry on his or her truck. The requirements turned out to be:

  • It had to be cheap, competitive in price with storage water heaters.

  • It had to be no larger than a storage water heater, and come in a single box.

  • It had to require no more than a half-inch gas line

  • It had to work with existing flue vents. Condensing water heaters are much more efficient, but run the risk of having water accumulate in them, and for that reason must be made of non-corroding materials.

The efficiency experts came up with a hypothetical design, ran simulations, and built a prototype. Their design used the largest burner you could run on a half-inch gas line, about 75,000 Btu/hour. It held up to 20 gallons, so hot water would be available instantly, without waiting for the purge and warm-up associated with tankless water heaters. Also included was an efficient but non-condensing external heat exchanger to avoid standby losses. A pump circulated water through the heat exchanger and tank. Simulations and testing indicated that a 15% savings would be possible.

Could this be an evolutionary step, or an revolutionary leap? If the savings to be had are anywhere close to 15% of the possibly 1 trillion cubic feet of natural gas used to heat water in residences, its a leap and not a step.

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