Equivalence of direct radiation (EDR) is a standardized comparison method for estimating the output ability of space-heating

radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

s and convectors. Measured in square feet, the reference standard for EDR is the mattress radiatorhttp://www.heatinghelp.com/newsletter.cfm?Id=157 mattress radiator invented by Stephen J. Gold in the mid 19th century. One square foot of EDR is able to liberate 240

BTU The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI un ...

per hour when surrounded by air and filled with

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizat ...

of approximately temperature and 1

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet * Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviation ...

of pressure. EDR was originally a measure of the actual surface area of radiators. As radiator (and later convector) design became more complicated and compact, the relationship of actual surface area to EDR became arbitrary. Laboratory methods based on the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

of steam allowed for very accurate measurements. While now somewhat archaic, EDR is still computed and used for sizing steam

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...

s and

radiators Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

, and for modifying and troubleshooting older heating systems using steam or hot water.

References

* *{{cite book , last=Emerick , first=Robert Henderson , title=Heating Design and Practice , location=New York , publisher=McGraw-Hill , year=1951 , ref=Emerick, 1951 Heating Heat transfer Electromagnetic radiation