Most households in the U.S. use natural gas to heat water. Other fuel types include electricity, propane, and heating oil. A typical gas storage water heater has an Energy Factor (efficiency rating) of about 0.6, while a typical electric storage water heater will be rated about 0.9. Based on these Energy Factors it would seem an electric water heater uses less energy. Actually the opposite is true, here’s why:
It takes about three times as much source energy (this includes the energy needed to generate and distribute a fuel) to deliver electricity to a home compared to natural gas. This is because only about 1/3 of the fuel energy burned at the utility’s power plant actually reaches a home in the form of electricity. The rest is lost due to inefficiency at the power plant and over power lines. Therefore, an electric water heater that appears to be 50% “better” than a gas one (0.9 Energy Factor versus 0.6 Energy Factor) actually uses much more (source) energy than the gas water heater. It is for this reason that when performance modeling a new electric water heater for Title 24 compliance there is a significant penalty.
Storage water heaters range in size from 20 to 75 gallons (or larger) and are fueled by natural gas, propane, electricity, or oil. Because heat is lost through the flue (except in electric models) and through the walls of the storage tank, energy is consumed even when no hot water is being used. New energy-efficient gas-fired storage water heaters are a good cost-effective replacement option if there is an existing gas line in the home. With more insulation around the tank and one-way valves where pipes connect to the tank, standby heat loss is substantially reduced. Prices are dropping for promising new super-efficient “condensing” and “near-condensing” gas water heaters. With a condensing gas water heater, before the combustion gases are vented outside, the heat in those gases is captured and used to help heat the water in the tank. If there is no access to natural gas or propane, rather than choosing an electric storage water heater a homeowner may want to consider a heat pump water heater, which is more efficient, but also more expensive. Conventional electric storage gas water heaters will still comply with Title 24 standards when using the performance calculation method.
Today many people are choosing tankless (or instantaneous) gas water heaters. These are very compact and generally wall-hung. Their rated efficiency is higher than that of tank units, usually around 0.82. However, they can be very expensive to install in retrofit applications, requiring special ductwork and upsizing of gas lines. Before rushing out to buy an on-demand water heater be aware that they are not appropriate for every situation. Consider the water distribution system. If the hot water use-points in a home are relatively close together a tankless system may work well. In many homes, water use-points are widely spaced at opposite ends of the house. If this is the case, a single tankless system with long distances between the system and the points-of-use can create frustration. The user will have to wait through a long “slug” of cold water before receiving hot water. With regard to electric units, residential wiring generally will not support a tankless electric water heater with large enough capacity to serve multiple uses. An small electric “point-of-use” unit may be appropriate for some applications, such as a remote vanity or half-bath.
The good news for electricity users is that heat pump water heaters are becoming more common. Heat pump water heaters take energy from the air to heat water. Compared to a standard electric resistance water heater, heat pump models are more efficient because the electricity is used for moving heat from one place to another rather than generating the heat directly. The heat source is outside air or air in the room where the unit is located. They 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 storage water heater. Electric heat pump water heaters are very Title 24 friendly.
If you use a boiler for space heating it may be feasible to install an indirect water heater. An indirect water heater uses a separate boiler as the heat source. Hot water is circulated from the boiler through a heat exchanger inside a well-insulated water heater tank. An indirect water heater is an excellent option because it eliminates the tremendous flue losses associated with gas-fired storage water heaters and the hassles and extra costs of tankless gas water heaters. These systems can be purchased in an integrated form, incorporating the boiler and water heater with controls, or as separate components.
These combined units feature a powerful water heater that provides space heating as a supplemental end-use. Heated water from the water heater tank (or tankless unit) passes through a heat exchanger in a central handler to heat air which is then blown into the home’s duct system.
Solar water heaters can be a great investment because they offer a virtually cost-free and renewable energy source for one of the home’s top energy-users. But because the feasibility and benefits of a solar water heater depend on a number of variables including where you live, roof orientation, and how many people live in the house, it takes extra savvy to know what the costs and savings will be. The initial cost of a solar water heater is still much higher than other competing technologies, but if one can make the upfront investment (made easier by tax breaks and rebates), it can save 50–75% of the water heating energy over the long term.
Michael Kunz is a Certified Energy Plans Examiner (CEPE) and owner of Energy Performance Services, a company specializing in California Title 24 energy compliance. For more information call (888)828-9488 or visit their websiteat http://www.title24express.com