In the world of ultra-low-energy homes, water heating is often the largest energy user. It’s time to pay attention to this significant energy load.
Selecting a water heater involves decisions about the type of fuel used, how and where the water is heated (especially the issue of storage versus tankless water heaters), and whether water heating can be combined with space heating. Here’s a review of the most efficient products on the market to suit your particular water-heating needs.
The vast majority of water heaters in North America are storage-type—an insulated tank with either an integrated gas burner, or one or two electric-resistance elements. The advantage of storage water heaters is that the water can be heated with a relatively small burner or heating element, while still providing ample hot water. Even when much of the hot water is used up, the tank remains stratified, with remaining hot water staying at the top where it is drawn off.
Recovery time depends on the capacity of the gas burner or electric elements. Hot water recovery is an important property of a storage water heater; it is generally presented as the “first-hour rating” in gallons. This is the amount of hot water that can be delivered by a water heater during one hour. With storage water heaters, this assumes the tank starts fully charged with hot water, so the first-hour rating is the volume of the tank, plus the volume of water that can be heated in an hour (assuming standard assumptions for temperature rise).
To calculate hot water demand for a family, you should figure out the peak usage period, then use rules for hot water use for those needs. The U.S. Department of Energy (DOE) assumes 10 gallons of hot water for a shower, 2 gallons for shaving, 4 gallons for hand dish-washing and food prep, 6 gallons for a dishwasher, and 7 gallons for a clothes washer. So for the morning period, one-hour water use might total three showers, one shave, and one hand dish-washing, for a total of 36 gallons. The first-hour rating would have to meet that.
Disadvantages of storage-type water heaters include the potential to run out of hot water and the standby heat loss from the large temperature difference between the outside and inside (often 60°F or more). Inexpensive storage water heaters may have just an inch of polyurethane insulation, providing as little as R-6. The best have R-12 to R-15 in foam insulation. With gas storage water heaters that have pilot lights, the pilot itself throws out a lot of heat (typically about 300 Btu/hour), which can contribute some heat to the tank, though most of the pilot light heat actually escapes up the central flue. Depending on water heater location in a building, the climate, and the time of year, standby losses may either reduce a building’s heating load or increase its cooling load, though the impact of this is fairly modest.
There can also be losses from storage water heaters via the supply and discharge pipes. Storage water heaters should be installed with “heat traps” that prevent the thermosiphoning (natural circulation from hot water rising) that can occur if the water heater is located below a home’s plumbing. Adequate pipe insulation should also be a part of water heater installation (see “Reduce Demand—Improve Efficiency” article in this issue).