With energy-consuming equipment, such as water heaters and refrigerators, we have good data on energy consumption and can set clear standards accordingly. In some product categories—clothes washers, for example—Energy Star standards were adopted because those standards provide a high enough threshold to represent just the very top segment of the product market (less than 10%). In other product categories—e.g., refrigerators and dishwashers—we set a higher threshold than ENERGY STAR: for example, exceeding those standards by 10% or 20%. With lighting and lighting control equipment, certain generic products qualify, such as compact fluorescent lamps and occupancy/daylighting controls, while in other categories only a subset of products qualify. In some cases, products that meet the energy efficiency requirements are excluded, because of evidence of poor performance or durability. Microturbines are included here because of the potential for cogeneration (combined heat and power) that they offer.
The S, ST, L, and LT domestic hot water tanks allow water heating by a gas- or oil-fired boiler. Unlike most domestic indirect water heater tanks, the Buderus uses non-ozone-depleting rigid foam. Because the unit is made in Germany, there are environmental impacts of shipping to consider.
The most efficient fuel-fired residential water heaters include electronic-ignition gas-fired tankless or on-demand models, heat-pump water heaters, and advanced combination space- and water-heating systems. All electric-resistance water heaters have the inefficiencies and fuel-source pollution concerns inherent to electric power generation, so are not included here. In addition, tankless electric water heaters require very large current feeds—often 40 or even 60 amps, which necessitates heavy wiring and can create capacity challenges for utility companies.
Gas-fired condensing storage-tank type water heaters have fuel efficiencies greater than 90% and use a variety of types of insulation. Tankless water heaters have no standby losses or storage tank insulation concerns, and some models have sealed combustion and no pilot lights. Heat-pump water heaters have at least double the efficiency of electric-resistance water heaters, but these are still very uncommon. In combined or integrated systems, efficiencies are boosted by uniting space heating and/or cooling into a single system that includes water heating. Some boilers also provide domestic hot water, and some electronic-ignition on-demand water heaters can be used as boilers for heating, particularly in very-low-energy buildings. Including a tankless coil in a regular boiler, however, is less efficient than storage-tank type water heaters. Other factors to consider include indoor air quality (in terms of combustion gases), particularly whether the system has sealed combustion with direct venting to the outdoors and/or enhanced combustion emissions controls (for NOx, particulates, etc.), the ozone-depletion impacts associated with refrigerants for heat pumps and blowing agents for storage tank insulation, and whether it is capable of continued operation in the event of a power outage. In almost every type of high-efficiency water heater there are issues of rate-of-use, climate, and maintenance that require consideration to make the appropriate selection for optimal results. Storage water heaters listed here have Energy Factors (EF) of at least 0.67 reflecting Energy Star criteria for 2010 and CEE’s Tier I standard. Models with inputs greater than 75,000 Btu are not required to have energy factors, so for those units we require thermal efficiency over 90%. Tankless water heaters listed here have Energy Factors (EF) of at least 0.82 reflecting Energy Star criteria or have a high EF along with additional environmental features such as sealed combustion.
Water heating accounts for about 17% of the energy consumed in homes, and some businesses—such as hotels and restaurants—have very high hot water demands. Providing energy-efficient hot water for these different uses can be accomplished using a variety of technologies. The most efficient fuel-fired water heaters include electronic-ignition gas-fired tankless or on-demand models, advanced combination space- and water-heating systems, and gas-fired condensing storage-tank type water heaters.
Tankless water heaters have no standby losses, and some models have sealed combustion and use condensing technology. Some models can be used as boilers for heating in very-low-energy buildings. In combined or integrated systems, efficiencies are boosted by uniting space heating and/or cooling into a single system that includes water heating, and some boilers provide domestic hot water.
A variety of heat exchangers are also used to heat water. Desuperheaters capture waste heat created by refrigeration and air-conditioning equipment in commercial, and some residential applications, but there have to be significant cooling loads for these products to be cost effective. And electric heat-pump water heaters pull heat from the air using compressors to evaporate and condense refrigerant in a closed loop, transferring that heat into water stored in an insulated tank. These units also cool and dehumidify surrounding air, so they need to be located appropriately. Heat can even be recovered from drain lines using gravity film heat exchanger (GFX) and other technologies.
Choice of water heater can depend on fuel, space, rate-of-use, climate, maintenance, and other factors. Note: combustion gases generated by some water heaters can impact indoor air quality, and refrigerants used in some heat pumps can impact the ozone. Electric-resistance water heaters are not included here because of the inefficiencies and fuel-source pollution inherent to electric power generation.
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