Water Heaters FAQ
Next to heating or cooling, water heating is typically the largest energy user in the home. As homes have become more and more energy efficient during the past 20 years, the fraction of energy used for water heating has steadily increased. This chapter takes a look at the high-efficiency water heaters available and how you can reduce water heating costs with your present water heater.
TYPES OF WATER HEATERS
Storage water heaters are by far the most common type of water heater in the U.S. today. Ranging in size from 20 to 80 gallons (or larger) and fueled by electricity, natural gas, propane, or oil, storage water heaters heat water in an insulated tank. When you turn on the hot water tap, hot water is pulled out of the top of the water heater and cold water flows into the bottom (through a "dip tube" from the top) to replace it. Because heat is lost through the flue and the walls of the storage tank (this is called standby heat loss), energy is consumed even when no hot water is being used. New energy-efficient storage water heaters contain higher levels of insulation around the tank and one-way valves where pipes connect to the tank, substantially reducing standby heat loss.
Demand (or instantaneous) water heaters eliminate the storage tank by heating water directly when there is a call for hot water. These units are growing in popularity in the U.S. The energy consumption of these units is generally lower since standby losses from the storage tank are eliminated. Demand water heaters with enough capacity to meet household needs are gas- or propane-fired. They have three significant drawbacks for some applications: Large simultaneous uses (two showers and the clothes washer, for example) may challenge their capacity, particularly in winter,when the inlet water is coldest. They will not turn on unless the hot water flow is ½ - 3/4 gallon/minutes. Retrofit installation can be very expensive. Finally, because the efficiency tests were not developed with these designs being considered, it is not known if the "EF" accurately estimates energy consumption. If you choose a tankless unit, look for one eligible for 2006-2007 federal tax credits (EF levels).
Heat pump water heater s are more efficient than electric resistance models because the electricity is used for moving heat from one place to another rather than for generating the heat directly. The heat source is the outside air or air in the basement where the unit is located. Refrigerant fluid and compressors are used to transfer heat into an insulated storage tank. Heat pump water heaters are available with built-in water tanks called integral units, or as add-ons to existing hot water tanks. A heat pump water heater uses one-third to one-half as much electricity as a conventional electric resistance water heater. In warm climates they may do even better, but there are few sources for these products.
Indirect water heaters use the home’s boiler or furnace as the heat source. In boiler systems, hot water from the boiler is circulated through a heat exchanger in a separate insulated tank. In the less common furnace-based systems, water in a heat exchanger coil circulates through the furnace to be heated, then through the water storage tank. Since hot water is stored in an insulated storage tank, the boiler or furnace does not have to turn on and off as frequently, improving its fuel economy. Indirect water heaters, when used in combination with new, high-efficiency boilers or furnaces, are usually the least expensive way to provide hot water. These systems can be purchased in an integrated form, incorporating the boiler or furnace and water heater with controls, or as separate components. Gas, oil, and propane-fired systems are available.
Solar water heaters use energy from the sun to heat water. Solar water heaters are designed to serve as preheaters for conventional storage or demand water heaters. While the initial cost of a solar water heater is high, it can save a lot of money over the long term. Solar water heaters are much less common than they were during the 1970s and early 1980s when they were supported by tax credits, but the units available today tend to be considerably less expensive and more reliable. At today’s prices, solar water heaters compete very well with electric and propane water heaters on a life-cycle cost basis, though they are still usually more expensive than natural gas.
WATER HEATER EFFICIENCY
The energy efficiency of a storage water heater is indicated by its energy factor (EF), an overall efficiency based on the use of 64 gallons of hot water per day. The first national appliance efficiency standards for water heaters took effect in 1990. New standards, which took effect in January 2004, increased the minimum efficiency levels of water heaters.
The most efficient gas-fired storage water heaters have energy factors ranging from 0.60 to 0.65, corresponding to estimated gas use below 250 therms/year. Condensing water heaters have energy factors as high as 0.86. The most efficient electric storage water heaters have energy factors ranging between 0.93 and 0.95, resulting in estimated annual energy use below 4,725 kWh/year. There is little difference between the most efficient electric resistance storage water heaters and the minimum efficiency standard. Fortunately, heat pump water heaters using less than half as much electricity as conventional electric resistance water heaters are becoming commercially available. If you use electricity for water heating, consider installing a heat pump water heater. Otherwise, look for the most efficient electric resistance unit in your size range.
With demand water heaters, the manufacturers provide different specifications: the energy input (Btu/hour for gas, kilowatts [kW] for electric); the temperature rise achievable at the rated flow; the flow rate at the listed temperature rise; and so on. In comparing different models, be aware that you aren’t always looking at direct comparisons, especially with temperature rise and flow rate. For example, while one model might list the flow rate at a 100°F temperature rise, another might list the flow rate at 70°. Until there are industry-standard ratings for temperature rise and flow rates, it will be difficult to compare the performance of products from different companies. Some companies are beginning to publish energy factor ratings for these products and this information should make for easier comparisons.
UPGRADING YOUR EXISTING WATER HEATER
Even if you aren’t going to buy a new water heater, you can save a lot of energy and money with your existing system by following a few simple suggestions.
- Conserve Water: Your biggest opportunity for savings is to use less hot water. In addition to saving energy (and money), cutting down on hot water use helps conserve dwindling water supplies, which in some parts of the country is a critical problem. A family of four each showering five minutes a day can use about 700 gallons per week—a three-year drinking water supply for one person! Water-conserving showerheads and faucet aerators can cut hot water use in half. That family of four can save 14,000 gallons of water a year and the energy required to heat it.
- Insulate Your Existing Water Heater: Installing an insulating jacket on your existing water heater is one of the most effective do-it-yourself energy-saving projects, especially if your water heater is in an unheated basement or space. The insulating jacket will reduce standby heat loss—heat lost through the walls of the tank—by 25–40%, saving 4–9% on your water heating bills. Water heater insulation jackets are widely available for around $10. Some newer water heaters come with fairly high insulation levels, reducing (though not eliminating) the economic advantages of adding additional insulation. In fact, some manufacturers recommend against installing insulating jackets on their energy-efficient models. Always follow directions carefully when installing an insulation jacket. Leave the thermostat(s) accessible. With conventional gas- and oil-fired water heaters, you need to be careful not to restrict the air inlet(s) at the bottom or the draft hood at the top.
- Insulate Hot Water Pipes: Insulating your hot water pipes will reduce losses as the hot water is flowing to your faucet and, more importantly, it will reduce standby losses when the tap is turned off and then back on within an hour or so. A great deal of energy and water is wasted waiting for the hot water to reach the tap. Even when pipes are insulated, the water in the pipes will eventually cool, but it stays warmer much longer than it would if the pipes weren’t insulated.
- Lower the Water Heater Temperature: Keep your water heater thermostat set at the lowest temperature that provides you with sufficient hot water. For most households, 120°F water is fine (about midway between the “low” and “medium” setting). Each 10°F reduction in water temperature will generally save 3–5% on your water heating costs. When you are going away on vacation, you can turn the thermostat down to the lowest possible setting, or turn the water heater off altogether for additional savings. With a gas water heater, make sure you know how to relight the pilot if you’re going to turn it off while away.
SELECTING A NEW WATER HEATER
Whether you’re replacing a worn-out existing water heater or looking for the best model for a new house you’re building, it pays to choose carefully. Look for a water heater that satisfies your hot water needs and uses as little energy as possible. Often you can substantially reduce your hot water needs through water conservation efforts.
- Think About a Replacement Now: If you're like most people, you’re unlikely to go out looking for a water heater until your existing one fails, leaving little time to look for a water heater that most appropriately fits your needs and offers the highest level of energy efficiency. A much better approach is to do some research now. Figure out what type of water heater you want—gas or electric, storage or demand, stand-alone or integrated with your heating system, etc. Then, figure out the proper size for your household.
- Sizing a Water Heater: The capacity of a water heater is an important consideration. The water heater should provide enough hot water at the busiest time of the day. The ability of a storage water heater to meet peak demands for hot water is indicated by its "first hour rating." This rating accounts for the effects of tank size and the speed by which cold water is heated. To determine your family’s peak-hour demand, use this worksheet. Demand water heaters should be sized according to the required gallons per minute (gpm) flow rate and temperature rise required for your largest expected hot water fixture (usually a shower).With solar water heaters, you should discuss your requirements carefully with the solar water heating salesperson. You will need to size both the solar hot water system itself and the back-up electric or gas water heater. It generally makes the most sense to size a solar water heater to provide two-thirds to three-fourths of your total demand, and provide the rest with a back-up system.
- Fuel Options: What type of fuel makes the most sense for your water heater? If you currently have an electric water heater and natural gas is available in your area, a switch might save you a lot of money. Oil- and propane-fired water heaters are also usually less expensive to operate than electric models. Before you rule out electricity, though, check with your utility company. It may offer special off-peak rates that make electricity a more attractive option.
- Look for Sealed Combustion or Power-Vented Systems: For safety as well as energy efficiency, look for gas- or oil-fired water heaters with sealed combustion or power venting. Sealed combustion means that outside air is brought in directly to the water heater and exhaust gases are vented directly outside. The combustion is totally separated from the house air. Power-vented equipment can use house air for combustion, but flue gases are vented to the outside with the aid of a fan. In very tight houses, drawing combustion air from the house and passively venting flue gases up the chimney can sometimes result in back-drafting of dangerous combustion gases into the house.
COMPARING THE TRUE COSTS OF WATER HEATERS
When comparing the cost of various water heating options, keep in mind that there are two types of cost you need to look at: purchase cost and operating cost. Life-cycle costs, which take into account both the initial costs and operating costs of different water heaters, provide a much more accurate representation of the true costs of the water heater than the purchase price alone. Life-cycle costs for the most common types of water heaters under typical operating conditions are shown in the table here. When both purchase and operating costs are taken into account, one of the least expensive systems to buy (conventional electric storage) is one of the most costly to operate over a 13-year period. An electric heat pump water heater, though expensive to purchase, has a much lower cost over the long term. A solar water heating system, which costs the most to buy, has the lowest yearly operating cost among electric systems.