Maintenance and Efficiency Upgrades
The vast majority of homes in the United States have stand-alone water heaters. These are independent hot-water storage tanks with their own heat source, with the sole function of providing domestic hot water. A home that is heated by a boiler is likely to have a tankless coil or an indirect-fired storage tank. Standard gas water heaters are cheap and inefficient. They are typically expected to last only 5 to 15 years, so most people do not invest much money in them. Replacing a water heater tends to be an emergency event, full of stress because the old water heater has suddenly failed. Proper maintenance can extend the life of a water heater, and some efficiency improvements can be made to existing tanks with minimal investment; but if you have a standard water heater made before 2010, it may make more sense to upgrade to a very high-efficiency unit and not try to make the old one last longer. If your present water heater appears to be near the end of its useful life, now is the time to do research on the right type of replacement unit. Then you will be well prepared, whether you decide to wait until the old one goes or to replace the tank at your convenience.
Two types of tanks: Gas and electric
Setting the temperature
The temperature of many hot-water heaters can be lowered without any noticeable reduction in the hot-water supply. Lowering the temperature of stored hot water saves energy by reducing conductive heat loss through the tank surfaces, called standby loss. Standby losses depend on the location of the tank (how cold it is outside the tank), how well the tank is insulated, and where the tank is located (in a cold climate, most losses in the winter just contribute to heating the house). The better insulated the tank and the greater your hot-water use, the less savings a temperature setback will provide.
Reducing the temperature setting by 10°F may save 2 percent to 5 percent of your water heating costs. The investment is $0, and it’s easy to readjust if you turn it down too much. In addition to energy savings, a lower tank temperature reduces scaling and corrosion and increases the life of the tank. The thermostat knob on a gas water heater is usually located at the front of the gas valve. Look carefully at the markings; on many gas water heaters, counterclockwise rotation increases the temperature setting. Electric tanks have two thermostats—one for each element—located behind metal covers near the bottom and the top of the tank. Because there is exposed wiring inside, shut off the circuit breaker before unscrewing the covers. Set both elements to about the same temperature, or set the top element just slightly cooler so the bottom element activates first.
Hot water stored below 120°F may increase the risk of Legionella pneumophili bacteria, which can cause pneumonia. But Legionella infections in single-family homes are quite rare compared to the risk of scalding, which increases rapidly at tank temperatures over 120°F. The U.S. Consumer Product Safety Commission recommends all home water heaters be set at 120°F, and even at that temperature third degree burns can occur with exposures of five minutes or more. Antiscald mixing valves can help keep children and elders, who are at higher risk for burns, safe.
Insulating hot-water pipes
Hot-water pipes lose energy in two ways: through conductive losses in pipe walls while hot water is running and through thermosiphon effects. Pipe insulation is readily available at most home centers and hardware stores and is easy to install. The biggest benefit from pipe insulation occurs within the first 10 ft. of the tank. Be sure to use the right diameter of insulation; most homes have a mixture of 1⁄2-in. and 3⁄4-in. pipes, so it’s a good idea to measure how much of each you will need.
Replacing Hot-Water Systems
Tank-type water heaters
Most standard gas water heaters have UEF ratings in the high 50s—that means over 40 percent of the energy is wasted. Until recently, the highest efficiency tank-type units you could buy were in the low 60s; most had efficient burners, electronic pilots, and high insulation levels. There is a new generation of high-efficiency tank-type water heaters emerging that have higher uniform energy factors. For example, the Kenmore® Elite uses a special air intake and other features to achieve an UEF of 0.7. Both the Kenmore and the Rheem® Fury (0.67 UEF) use electronic vent dampers. Although not the highest efficiency available, these products provide a sizable performance boost at a modest price increase over a standard gas water heater.
The most efficient units on the market are condensing gas water heaters; they typically have UEF ratings of 0.90 or higher. The best ones have stainless-steel tanks and high firing rates—typically around 90,000 to 120,000 Btu per hour, two to three times that of a typical tank. Because of their high cost—roughly $1,500 to $2,500—condensing water heaters may be more appropriate for supplying whole-house heating as well as hot water. Their high heat output makes them well suited for supplying a hydro-air system. A hydro-air handler used in conjunction with a condensing gas water heater is probably one of the most efficient replacements for an old, inefficient gas furnace and a standard electric or gas water-heating tank, especially if you have piped natural gas and live in a cold or mixed climate. Standard electric tank-type water heaters appear to be more efficient than gas units, with UEFs ranging from about 0.86 to 0.95.
The uniform energy factor, or UEF rating, is based on a standardized test that takes into account burner efficiency, pilot usage, and standby losses for “typical” hot-water usage. UEF ratings for gas, oil, and electric water
heaters can be found at www.ahridirectory.org under “residential water heaters.” The directory also shows first-hour ratings and the burner efficiency, which is called “recovery efficiency.”
Tankless water heaters
Indirect-fired tank (with boiler)
Heat pump water heaters
Heat pump water heaters (or HPWHs) have come a long way. Now considered a mature technology, today’s HPWHs offer better efficiency and reliability at a price range of about $1,300 to $2,000. HPWHs have UEFs in the range of 2.0 to 3.5, which means that between half and three-quarters of the heat comes from the environment around the heater, rather than from the electric meter. There are a number of factors that affect the real installed efficiency. First, controls in most units allow for resistance heat to turn on when demand is high or surrounding air temperatures are low; this can really cut into the overall efficiency, depending on how the control is set and how you use hot water. The most efficient units have smaller electric backup heaters (less than 2,500 watts) and run the heat pump even while the resistance heat is on. Larger tanks also tend to run more efficiently than small tanks. Also, the heat comes from the air around the tank. If you live in a cold climate, that heat is not entirely free: If the tank is in a conditioned space, you’ve already paid to heat it up. If it’s in an unconditioned basement or crawlspace, the air may get too cold and efficiency can suffer. HPWHs need space, especially in cold climates; if the space available to install the water heater is in a closet or other restricted area, a HPWH is not a good choice. Some units allow for a duct to divert the chilled air output outdoors in cold weather. Also, the standard UEF ratings for HPWHs are based on relatively warm climates; the Northwest Energy Efficiency Alliance has a listing of units that achieve higher efficiencies in cold climates, which can be found at neea.org/img/documents/qualified-products-list.pdf. The bottom line is this: If you are looking to replace an existing electric or LP-gas water heater, consider getting a HPWH. Average savings should be around $300 per year, so payback on the $1,000 to $1,500 increased cost is fairly reasonable, especially if there are incentives available to bring the cost down. Savings may be higher where electric rates are higher than average, even in colder climates. Savings will tend to be largest when hot-water use is high.
Material adapted from Build Like a Pro: Insulate & Weatherize, published by The Taunton Press, 2012; used by permission.