Conventional Water Heating Efficiency
Most commercial water heating is done with storage water heaters that use gas, oil or electricity. Ranging in size from 20 to 80 gallons (75.7 to 302.8 liters), storage water heaters remain the most popular type for residential heating needs in the United States. A storage heater operates by releasing hot water from the top of the tank when the hot water tap is turned on. To replace that hot water, cold water enters the bottom of the tank, ensuring that the tank is always full.
Because the water is constantly heated in the tank, energy can be wasted even when no faucet is on. This is called standby heat loss. Newer, more energy efficient storage models can significantly reduce the amount of standby heat loss, making them much less expensive to operate. To determine the most energy efficient model, consult the Energy Guide label required on storage water heaters. Energy Guide labels indicate either the annual estimated cost of operating the system or energy efficiency ratings.
The efficiency of most gas water heaters currently in use is about 65% and standby losses are about 6.5% of stored capacity per hour. An accepted measure of the energy performance of water heaters is the energy factor (EF), which takes into account thermal losses from the tank. The National Appliance Energy Conservation Act (NAECA) has established minimum energy factors for water heating equipment used in residential applications, and since many commercial applications use the same type of water heating equipment, water heaters in commercial applications tend to fall under the NAECA minimum performance standards.
Criteria for Selecting a Water Heater: As with any purchase, balance the pros and cons of the different water heaters in light of your particular needs. There are numerous factors to consider when choosing a new water heater: fuel type, capacity, efficiency, and cost.
Determining Capacity: Although some consumers base their purchases on the size of the storage tank, the peak hour demand capacity, referred to as the first-hour rating (FHR) on the Energy Guide label, is actually the more important figure. The FHR is a measure of how much hot water the heater will deliver during a busy hour, and it is required by law to appear on the unit's Energy Guide label. Therefore, before you shop, estimate your household's peak hour demand and look for a unit with an FHR in that range.
Gas water heaters have higher FHRs than electric water heaters of the same storage capacity. Therefore, it may be possible to meet your water-heating needs with a gas unit that has a smaller storage tank than an electric unit with the same FHR. More efficient gas water heaters use various nonconventional arrangements for combustion air intake and exhaust. These features, however, can increase installation costs.
Rating Efficiency: Once you have decided what type of water heater best suits your needs, determine which water heater in that category is the most fuel efficient. The best indicator of a heater's efficiency is its Energy Factor (EF), which is based on recovery efficiency (i.e., how efficiently the heat from the energy source is transferred to the water), standby losses (i.e., the percentage of heat lost per hour from the stored water compared to the heat content of the water), and cycling losses.
The higher the EF, the more efficient the water heater. Electric resistance water heaters have an EF between 0.7 and 0.95; gas heaters have an EF between 0.5 and 0.6, with some high-efficiency models around 0.8; oil heaters range from 0.7 to 0.85; and heat pump water heaters range from 1.5 to 2.0. Product literature from manufacturers usually gives the appliance's EF rating. If it does not, you can obtain it by contacting an appliance manufacturer association.
Some other energy efficiency features to look for are tanks with at least 1.5 inches (3.8 centimeters) of foam insulation and energy efficiency ratings shown on the Energy Guide labels.
Comparing Costs: Another factor uppermost in many consumers minds is cost, which encompasses purchase price and lifetime maintenance and operation expenses. When choosing among different models, it is wise to analyze the life-cycle cost - the total of all costs and benefits associated with a purchase during its estimated lifetime. Units with longer warranties usually have higher price tags, though. Often, the least expensive water heater to purchase is the most expensive to operate.