Cast Iron Radiator Heating Capacity Guide
Use this handy guide to determine how much heat existing "old
style" radiators give off or how much heat a boiler must
supply to them.
Page 1 - Introduction
types of radiators are now used to heat the rooms of homes and
buildings which have central steam or hot water boiler systems
but for the first half of this century cast iron radiators were
the overwhelming favorite form of radiation. Although there are
now very few manufacturers, cast iron radiation is still one of
the most comfortable, quiet and durable ways to heat a room. Cast
iron radiators heat up slowly and cool down slowly to give a very
gentle, even, radiant heating. Since the 50's less expensive copper
finned convectors and baseboard have come to predominate but the
heating is harsher more of an "all on," "all off"
effect. Along with these and other, even newer forms of heating,
Burnham Cast Iron Radiators and Cast Iron Baseboard are still
available at Colonial.
The Heat Output of any radiator is determined by:
A radiator's relative radiating surface area is measured in terms
of Square feet of "Equivalent Direct Radiation" or "EDR".
- Temperature of the surrounding air which is usually assumed
to be 70 degrees
- Temperature of the radiator's surface which is directly related
to the temperature of the water or steam inside the radiator.
Naturally, the hotter the radiator the more heat it will radiate.
- Surface area of the radiator. Naturally, the larger the radiator
the more heat it can radiate.
There are two types of hydronic heating systems which utilize
cast iron radiators:
As mentioned above (#2) and as the following chart shows a radiator's
heat output is directly related to its temperature:
- Hot water heating systems circulate water heated by
a boiler up through the radiators. The water can be mechanically
pushed by a circulating pump or the system may have been designed
to take advantage of the fact that hot water rises because it
is less dense than "cold"water. The latter are called
gravity hot water hrating systems. Most of these systems were
designed to deliver no more than 180 degree water to the cast
- Steam heating sytems for homes make & use steam
at very low pressures, usually under 1 pound per square inch.
Steam rises naturally and quickly from the boiler up to the
radiators. When a radiator is filled with this low pressure
steam it can reach a temperature of 215 degrees farenheit.
In most cases a Square foot EDR of radiation will emit 240 BTUs
per hour on steam heating systems or 170 BTUs per hour on hot
water heatings systems. This "rule of thumb" proves
reliable for cast iron radiators as applied in existing homes
in the United States. The above graph shows that these outputs
will be higher or lower if the steam or water within them is hotter
or cooler. It will also vary if the radiators are exposed to blowers
or fans and if they are covered or obstructed. Any
Okay, that's how much heat each square foot of Equivalent Direct
Radiation can be expected to emit in normal home heating design
The TOTAL of the Square feet EDR ratings of all the radiators in
a house multiplied by either:
is how much heat the boiler in the house must deliver to make all
of those radiators hot.
- 170 BTUs per hour if its a hot water based system or
- 240 BTUs per hour if its a steam system
Usually that's more than enough heat to keep the house warm - even
on the coldest day - and even enough to dry everyone's wet mittens
at the same time!
In fact, when they were built most houses with cast iron radiators
weren't very well insulated and often didn't have storm windows.
Since then many have had energy saving improvements so the total
radiator heating capacity is now much greater than the minimum
needed according to the building's actual heat loss. It is, however,
very comforting to really "cook" all those radiators
on bitter cold days.
Replacement steam boilers must have enough power to fill every
radiator with steam but replacement hot water boilers for the
improved home can be selected according to the actual heat loss
instead of radiator heating capacity if the residents can accept
the lower water and radiator temperature which will result.
This all sounds pretty simple, huh? Well it is, the only trick
is determining the EDR square footage for each radiator.