40 National Oilheat Research Alliance
Step 7. Add up the Sq. Ft. of all the connected
radiation in the building.
For example, using the Sq. Ft. of steam determined
for the various types of heat emitters
determined earlier, assume the building had:
Number Type Sq. Ft.
4 Column radiators @ 36 144
4 tubular radiators @ 42.5 170
2 radiant convectors @ 22.5 45
2 cast iron baseboard sections @ 34 68
1 thin tube radiator @ 14 14
1 wall radiator @ 21 21
Total 462 Sq. Ft.
8. Convert the total square feet of radiation
to BTU/Hr. Each square foot of steam radiation
is based on a heat emission of 240 BTU/
Hr. with standard 70°F air temperature and
215°F steam temperature in the radiator.
462 X 240 = 110,880 BTU/Hr.
9. Size the replacement steam boiler by
selecting a unit with a AHRI NET Steam rating
equal to or greater than the BTU/Hr. capacity
of the radiation.
In this case, the 4 section boiler, fired at 1.25
gph is the proper boiler for this application.
Figure 5
Figure 6
Thin tube (AKA
“midget”) radiators.
The sections
of thin tube radiators
are generally
1.5 to 1.75" wide.
(as opposed to
standard tubular
radiators @ 2.5"
wide)
Assume the radiator is 32" high and has 3
tubes.
• The chart shows that there are 2.33 Sq. Ft.
of radiation per section (Figure 5)
• There are 6 sections
• 2.33 X 6 = 14 Sq. Ft. of radiation for this
radiator.
Wall radiators.
This picture
shows 3 wall
radiators manifolded
together.
• Each is 13.5" X 21" X 3"
• The chart shows that there are 7 Sq. Ft. per
unit. Figure 6
• 3 X 7 = 21 Sq. Ft. Of radiation for this
radiator.
Note: Sizing calculations assume
that steam main piping in
unconditioned areas is insulated.