Radiator Heat Output
The heat output of a cast-iron radiator is determined by the following factors:
• Ambient temperature (that is, the temperature of the air surrounding the radiator). The ambient temperature is assumed to be 70ºF for purposes of sizing estimates.
• Temperature of the radiator surface. The surface temperature will depend on the temperature of the steam or hot water circulating through the radiator sections. Steam is always hotter than hot water.
• Surface area of the radiator. Repeated tests have shown that the amount of heat given off by ordinary cast-iron radiators per degree difference in temperature between the steam (or water) in the radiator and the surrounding air is about 1.6 Btu per square foot of heating surface per hour.
A relative radiating surface of a radiator is measured in terms of the square feet of equivalent direct radiation (EDR). A cast-iron radiator will give off heat at the rate of 240 Btu per hour when sup- plied with steam at 21⁄2 lbs of pressure (220°F) with a surrounding air temperature of 70°F. It is determined as follows:
(220 – 70) X 1.6 = 240 Btu
One square foot of steam radiation equals 1.6 square feet of hot-water radiation or 1.4 square inches of warm-air pipe area. Tables 2-1, 2-2, and 2-3 list the heating surfaces for various column and tubular cast-iron radiators.
The cast-iron radiators in hot-water heating systems deliver water at a temperature of no more than 180ºF. As a rule, a square-foot EDR for a hot-water cast-iron radiator will emit 170 Btu per hour.
There is no IBR code covering recessed radiation. As a result, man- ufacturers of recessed heat-emitting units must rate and certify their own product. The Weil-McLain Company is typical of these manu- facturers. Its certified ratings are determined from a series of tests conducted in accordance with the IBR Testing and Rating Code for Baseboard Type Radiation whenever the provisions of the code should be applied. The Weil-McLain ratings include a 15 percent addition for heating effect and barometric pressure correction factor allowed by the IBR code. Other manufacturers use similar rating methods.