Boiler Facts...
George R. Carey, Jr.
Vice President, Fluid Industrial Associates
gcarey@fiainc.com twitter: @Ask_GCarey
Proper Steam Boiler
Replacement Etiquette
Recently, a contractor asked me what the correct
way to size a replacement steam boiler was; he
said he had heard conflicting ideas from different
people within the industry. The proper way to size
a replacement steam boiler is to go around and add up
all of the radiation in the house. Back in the day, the
expression used to indicate a radiator’s heating capacity
was referred to as a “square foot of steam,” that
came from the original flat steel radiators used back in
the mid 1800s. They would measure how many actual
square feet of surface area the panel radiator had to
determine its heating capacity.
One square foot of surface area from the radiator
would put out 240 British Thermal Units (BTUs) when
the air temperature around the radiator was 70°F
and the steam temperature inside the radiator was
215°F. If a radiator had 20 square feet of surface area,
it could emit 4,800 BTUs an hour. Also important to
note, 1psig steam has a temperature rating of 216°F,
so every radiator that has been installed in a steam
system can offset the heat loss of that room with less
than 1psig steam pressure at the radiator. If you try to
solve a heating problem by raising the steam pressure,
you’ll overheat the rooms and consume more fuel in
the process. Almost from
the beginning of installing
residential steam heating
systems, homeowners
complained about the size
and look of those flat, steelplated
radiators and so the
industry responded. Radiator
manufacturers started
making tubular style radiators
that took up less space
and aesthetically were
more pleasing to the homeowners.
They were able to
provide increased output in
a smaller footprint by adding surface area to the tubes.
This created a new term in the radiator industry: EDR
which stands for Equivalent Direct Radiation. Because
these radiators were not shaped like a panel radiator,
it was challenging at the beginning to establish a
square foot rating but eventually they figured it out
and the term a Square Foot of EDR was born.
Back to sizing the replacement boiler–there are
sizing capacity charts provided by various boiler
manufacturers in the industry that will list a radiator’s
heating capacity in Square Foot of EDR based
upon the height of the radiator, the number of columns
per section and the
number of sections.
Dan Holohan
offers an excellent
book that contains
virtually every
piece of radiation
ever manufactured
titled EDR…Every
Darn Radiator. If
you are in the business
of replacing
steam boilers, you
should have this
book. When you
are asked to size a
replacement steam
boiler, the last
thing you should
be concerned with
is the heat loss of
the house. I know
it sounds crazy,
but it’s true. If you
were replacing a hot water boiler, you would calculate
the current heat loss and then select the correct boiler.
In a hot water system, you are using a liquid to transport
the heat from the boiler out to the radiation and
this liquid is going to stay a liquid…cooler on its way
back but still a liquid. However, with steam it is different.
Steam is a gas that carries the heat from the boiler
out to the radiators where it is needed. If you don’t
produce enough gas (steam), the heat won’t reach all the
radiators and you end up with cold rooms and unhappy
customers. You have to produce enough steam to fill
the piping and all the radiators. This is because steam
wants to turn back into water as quickly as it can. When
steam enters cold pipes and radiators, the cold metal
robs the steam of its latent heat. With no more latent
heat the steam turns back to a liquid (condensate). The
idea is to have a boiler produce enough steam to overcome
the system’s ability to condense it.
Steam is made up of two types of BTUs. The first
type is called sensible heat. This is the amount of heat
required to bring water to its boiling point. It can be
“sensed” by a thermometer. At every pressure, there
is a corresponding requirement of sensible heat to boil
water (the higher the pressure, the greater the amount
required of sensible heat).The second type of heat is
known as latent heat (the heat of evaporation). This is
the amount of energy required to take the boiling water
16 ICM/September/October 2017