Cast iron radiators are used in hydronic (hot water) and steam heating systems. They transmit heat by both convection and radiation heat transmission. Air contacting the heated iron surfaces becomes heated through convection, then circulating through the room. Warmed air radiates out into the room air as well. Most modern radiators use hot water

Radiator modules are constructed by joining together a series of hollow vertical sections, each section having a waterway opening at the top and bottom. Metal round fittings and gaskets are attached to the openings to form passages, through which hot water or steam flows. The number of section used varies depending on the size of the space to be heated and configuration of the room, window placement, etc.

Sizing a radiator involves knowing how much BTU per hour the room will require. From there, you can calculate how many sections of radiator you need to install as follows.

The manufacturer’s spec sheet should state the square foot EDR (equivalent direct radiation) per single section, which will vary with it’s height. Multiply the EDR figure by the number of sections in the assembled radiator you think you will need; this will give the total sq. ft. EDR for that radiator. Take the resulting number and multiply that by 170 BTU per hour to arrive at the heating capability in a hot water system, or by 240 BTU per hour in a steam system.

Heating Efficiency

In order to operate a radiator heating system most efficiently, there are some guidelines to observe.

-Radiator covers or enclosures must have sufficient openings equal to 40 percent of their total surface.

-The radiator unit must be level for best results. Use a carpenter’s level to check position, and apply shims or wedges under it if required.

-Because metallic paint can inhibit transmission of heat, a 10 to 15 percent boost in heating capability can be had simply by stripping metallic paint from the top, sides and bottom of a painted radiator.

-Make sure the radiator valve is not clogged. Heat produced by a radiator can be reduced if the valve is clogged and dirty. Bleed valves yearly.

-Do not block radiators with curtains or furniture. There should be nothing to impede the flow of heat.

-Extra heat can be reflected into a room by placing aluminum foil or a piece of sheet metal against the wall directly behind a radiator.

Heat Output of Radiators

Cast iron radiators output of heat will depend on several factors. One aspect is the temperature of the surrounding air in the room being heated, known as ambient temperature. For sizing purposes, ambient temperature can be assumed to be 70 degrees F.

The second factor is the temperature of the surface of the radiator, which in turn depends on the temperature of the steam or water going through the hot water or steam in the radiator.

Another aspect determining heat output is the size of the radiator’s surface area. A typical cast iron radiator has been shown to give off around 1.6 BTU per square foot of heating area per hour per degree difference in temperature between the ambient air and the water or steam within the radiator.

For example, when it is supplied with steam of 2 ½ lbs of pressure, with a surrounding air temperature of 70 degrees F, a cast iron radiator will give off 240 BTU per hour of heat. A rule of thumb is that a cast iron radiator has a square foot EDR of 170 BTU per hour. EDR stands for equivalent direct radiation, and is expressed in square feet; it is a measurement of radiators’ relative radiating surface.

Photo by Clearly Ambiguous, Creative Commons Attribution License