A cast-iron radiator is a heat-emitting unit that transmits a portion of its heat by radiation and the remainder by convection. An exposed radiator (or freestanding radiator) transmits approximately half of its heat by radiation, the exact amount depending on the size and number of the sections.

The balance of the emission is by conduction to the air in contact with the heating surface, and the resulting circulation of the air warms by convection.

To size a column-type or tube-type cast-iron radiator, first measure its height in inches and then count the number of sections and the number of tubes or columns in each section (see Figure 2-3). The sections are the divisions or separations of a cast-iron radiator as seen when standing directly in front of it.

When you look at the radiator from its narrow end, you can see that each section consists of one or more vertical columns or pipes.

These vertical columns or pipes (they are called columns in the traditional cast-iron radiators) are 21⁄2 inches wide. In newer radiators, they are called tubes and are only 11⁄2 inches wide.

Find the square-foot EDR (equivalent direct radiation) of one section of the radiator. Multiply that figure by the number of sections in the radiator module to arrive at the square-foot EDR rating of that radiator.

Multiply the square-foot EDR rating by 240 Btu per hour to obtain the heating capacity of that radiator in a steam heating system or by 170 Btu per hour for its heating capacity in a hot-water heating system.

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