Indirect Water Heater Tanks

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.