RADIATORS
BY BILL STOCKS
OUT WITH THE OLD
When an old style cast-iron radiator is removed from a space and a new baseboard
heater is installed, it should be self-evident that careful attention must be paid to the
proper sizing and piping of the new unit. That is not always the case as the following
“If the two are on the same
loop you can expect problems
with over- and under-heating
in the rooms on that loop.”
DIFFERENCES IN OPERATION
SPECS FOR COMPARISON
When you are replacing an old
cast-iron radiator pay attention
to its height, depth and length.
Count the number of sections,
they are usually 2 ½” to three
inches wide. Look at the end of
the rad and count the number of
vertical tubes. If you have a digital camera, take a few pictures.
Armed with this information
you are ready to look through
FIGURE 1 HEAT EMISSION RATES – CAST-IRON RADIATORS
Average Water Temperature ºF
Heat Emission Rate (Btuh per sq.Ft.)
170 150
180 170
190 190
200 210
215 240
42
HPAC | APRIL 2007
Older cast-iron radiators and modern
copper/aluminum baseboard heaters
have different operation characteristics. Because of its mass the cast-iron
heats up slowly and stores heat that
it gives off even after a call for heat
is finished. Modern baseboards heat
up and cool down quickly. If the two
are on the same loop you can expect
problems with over- and under-heating in the rooms on that loop.
Imagine two rooms on a loop, one
with a cast-iron radiator (Room A)
and the other with modern copper
tube baseboard (Room B). Consider
the scenario if the thermostat is in
Room A. When the thermostat calls
for heat it takes a long time to heat
up the mass of the cast-iron radiator
and give off enough heat to the room
to satisfy the thermostat.
The stored heat will keep Room A
warm for a long time and the therCONTINUED ON PAGE 44
MODERN HYDRONICS
Photo Jupiterimages
scenario demonstrates. While investigating a customer complaint
about a new radiator that “didn’t
work”, we discovered that a threefoot-long cast-iron radiator that
was two feet high and six inches
deep had been replaced by a three
foot long, eight-inch high, 2 ½”wide piece of copper/aluminum
baseboard.
As it turned out the old castiron radiator had about 32 square
feet of heating surface. That
equates to 4,800 Btus at 150
Btus per square foot of heating
surface. Three feet of baseboard
rated at 170F provides 1,500
Btus (3 x 500 Btus per foot). This
left a major shortfall in heat for
the room.
some old radiator catalogues, if you
have them. You can also check current
publications that list old radiators
and show dimensions and heating
surface. As an alternative, baseboard
manufacturers can help in determining the square feet of equivalent direct radiation (EDR). If you do not
know the make, find something that
is similar in size and design to help
determine its Btus.
Older cast-iron radiators are rated
in square feet of equivalent direct radiation (EDR). Convert square feet
of EDR to Btus by determining how
many square feet of heating surface
a radiator has. Figure 1 shows heat
emission in Btus per square foot of
heating surface. The standard is to
use 150 Btus for hot water radiators
and 240 Btus for steam radiators.
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RADIATORS
co nti n u e d fro m PAG E 42
mostat will not have to call for heat
during this time. Meanwhile there
is a problem in Room B. While the
thermostat in Room A was calling
for heat for a long time, Room B
was overheating because the baseboard heats up quickly and does not
store heat. With the thermostat off
for a long time in Room A, Room B
cooled down and became cold while
waiting for the next call for heat.
Consider what would occur if the
thermostat is in Room B (with the
baseboard). On a call for heat the
baseboard heats the room very quickly and the thermostat becomes satisfied. Meanwhile, Room A has just
started to heat up when the call for
heat ends and the room is still cool.
One way to help avoid this problem is to pipe the radiators on separate loops with individual thermo-
stats and balancing valves. Care must
be taken not to short circuit the existing radiators.
WATER TEMPERATURE
When you are planning the replacement remember to select the length
of copper baseboard at the right water temperature. If baseboard is in
a system with 190F average water
temperature (AWT) it will provide
approximately 640 Btus per foot. At
140F it will drop to approximately
320 Btus per foot. From this example you can see that twice as much
baseboard may be necessary when
used in a lower temperature cast-iron
system.
As an alternate to copper tube baseboard, steel tube baseboard may be
used. It operates in a similar fashion
to the cast-iron radiator. The thick
steel tube has some mass to it and it
will heat up slowly and store some
heat until the next call for heat. There
are also new cast-iron baseboards on
the market. Do not forget to calculate the Btus – these baseboards will
be longer than the taller, thicker, oldstyle cast-iron radiators.
Bill Stocks, is sales manager, Quebec and Atlantic Canada, with Slant/
Fin Ltd/Ltee. He can be reached at
[email protected].
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HPAC | APRIL 2007
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