BTU: The British Thermal Unit is a measure of heat energy, used to describe the gas-burning capacity ("input rating") of tank-type and tankless water heaters, expressed as BTU per hour (BTU/hr.). It is the amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit. There are 8.33 pounds of water in one gallon; therefore, it will take 8.33 BTU to raise one gallon of water 1°F.

Combustion: Combustion takes place when gaseous, liquid or solid fuels react at an elevated temperature with oxygen by burning, thus releasing heat. For good combustion, an adequate supply of oxygen must be available, and all the carbon in the fuel will be converted to carbon dioxide and all the hydrogen to water vapor. The presence of carbon monoxide in the flue gases and/or sooting indicates imperect combustion.

Delta T, or T: The difference between the temperature of the cold water flowing into the unit for heating and the temperature of the water at the outlet of the water heater.

Efficiency: A measure of performance, expressed by dividing the results by the effort to obtain those results. Thermal efficiency in a water heater is the percentage of the potential heat content of a fuel that becomes usefully available as hot water. Thermal efficiency is a measure of performance under stated conditions, so efficiency will vary with those conditions.

Energy Factor, EF: The Energy Factor is a measure of energy efficiency in typical household use. A higher number indicates a more efficient use of energy to heat water.

Film Wrap Overheat Limiter: A Rheem exclusive, this device instantly detects an unsafe temperature in the heat exchanger of a Rheem tankless water heater and immediately shuts down the unit.

First Hour Rating (FHR): The number on the top left corner of the Energy Guide label, referring to the
amount of hot water a tank water heater can supply with a full tank. The number is stated in gallons.

Flow Rate: The amount of water in gallons flowing in a plumbing fixture or pipe over a period of time - normally, minutes. For tankless water heaters, flow rate is typically expressed in gallons per minute (gpm).

FVIR Technology: Flammable vapor ignition-resistance technology prevents the ignition of flammable vapors around the tank water heater. All gas-fired residential tank water heaters manufactured since July 1, 2003, must have this technology.

GAMA Certified: GAMA is the Gas Appliance Manufacturers Association. GAMA certification assures consumers that gas-fired water heaters and heating products meet federal standards for efficiency. Learn more at http://www.gamanet.org/gama/inforesources.nsf/vAllDocs/FAQ?OpenDocument#Q10

GPH: Stands for gallons per hour and refers to the amount of water in gallons used in a wide variety of applications per hour.

GPM: Stands for gallons per minute and refers to the amount of water flowing through a plumbing fixture or pipe.

High-Altitude Chip: Unless optimized for high-altitude use, gas appliances cannot achieve their maximum BTU capacity at mountain altitudes. Only Rheem tankless water heaters offer the High-Altitude Chip technology for efficient operation up to and above 9,000 feet.

Instant Hot Water: A description sometimes erroneously used to describe tankless hot water heaters. The speed with which hot water is delivered to the point of use is impacted by water-heater location (how close the water heater is to the point of use), not by water-heater type (tank vs. tankless).

Instantaneous Water Heater: A type of water heater in which heat energy is added only when water is flowing through the water heater; i.e., a tankless water heater, in contrast to a storage or tank-type water heater.

Inlet Water Temperature: The temperature of the outside water coming into a water heating system. When sizing water heaters, it's best to consider the coldest inlet water temperature. The rule of thumb is:
i. 40°F for the northern tier of states in winter.
ii. 50°F in most parts of the South in winter.
iii. 60°F year-round in Southern California, the Southwest and Gulf states.

Low NOx/Ultra Low NOx: A type of gas burner used on water heaters with nitrous-oxide emissions of only 10 nanograms per Joule. Low NOx and Ultra Low NOx water heaters comply with Southern California's SCAQMD (South Coast Air Quality Management District) Rule 1121.

Output: The output on a water heater is normally given in gallons per hour (gph) at a 100°F temperature rise. This quantity of water states the amount of water that the heater will deliver at 140°F when the cold start temperature is 40°F. All water heaters are stamped with this output rating in gph.

Peak Period: Refers to the time during the day when the water heating system experiences its greatest draw of hot water. With the exception of a few applications that require a continuous hot water draw, tests have shown that the peak period of hot water usage will occur once or twice during the day (or hour in the case of a shower room in a school gym.)

Point of Use Water Heater: Point of use water heaters are actually smaller tank units that are located near the hot-water outlet, such as under a sink. They are often used as boosters.

Sensing Burner Technology: Rheem's Guardian SBT monitor the combustion process and works to correct it before shutting it down due to an improper mixture of fuel and oxygen.

Recovery Rate: The amount of time it takes for water stored in a tank water heater to be heated to the desired temperature. Express as gallons per hour, the recovery rate is calculated as follows:
Recovery Rate = (Rated Recovery (gallons per hour) x 100) / Desired Temperature Rise (°F)

Standby Heat Loss: Heat energy that escapes stored water in a tank-type water heater, requiring that water to be heated even when there is no demand. Manufacturers reduce standby heat loss in tank water heaters by insulating the tank.

Temperature Rise: The difference between the inlet (outdoor) water temperature and the water temperature that the water heater delivers.


(1) Source: GAMA
(2) Source: U.S. Department of Energy

• Find Rheem water heating installers near you
• Find Rheem tankless water heating installers near you