Square Foot of Heating Surface AKA Equivalent Direct Radiation (EDR)
the amount of heating surface which will give off 240 Btu per hour when filled with steam
at 215°F and surrounded by air at 70°F. (Equivalent square foot of heating surface may have
no direct relation to the actual surface area.)
Static Head - the amount of water needed in returns to build up necessary pressure to force
water back into the boiler in a gravity feed system.
Steam Boiler - a closed vessel for conversion of water into steam. It should be equipped
with automatic controls to maintain water level and sufficient safety devices for proper
burning of fuel.
Steam Heating System - a heating system in which the heating units give up their heat
to the room by the way of a radiator, convector, steam baseboard, ect;. and the condensate
returns back to the boiler.
Steam Pop Safety Valve (Relief Valve) - a device to prevent over pressurization in
a boiler. The relief valve in a low pressure steam system should be set for 15 psi. High
Pressure Boilers should be set to the maximum working pressure of the boiler.
Steam Trap - a device that allows the passage of condensate and air but prevents the
passage of steam.
Strainer - a device used to protect valve pins, seats, and other components from dirt and
foreign matter.
Supply Main - primary piping that carries steam from boiler to heating units, and, in a
one-pipe system, condensate from heating units to return main.
Supply Riser - vertical piping that carries steam under pressure from the boiler up to the
supply main(s). A riser then carries steam up from supply main to heating unit.
Supply Valve :
One-Pipe System - located at the bottom of heating unit’s inlet connection. Supply Valves
control the admission of steam and flow out of condensate. In a one-pipe system, the supply
valve should be fully open or closed tightly, since a partially open valve obstructs the free
flow of steam in and condensate out.
Two-Pipe System - Modulating-type supply valves admit steam to the heating unit through
its top inlet connection. Selection depends upon system configuration and system pressures.
Thermostatic Steam Trap - located at the bottom outlet connection of a heat emitter,
thermostatic traps open to drain condensate and air into the return but prevent flow-out of
live steam into the return piping.
56 National Oilheat Research Alliance