Chapter 8
Sizing
To enable a heating system to operate at maximum
efficiency it is imperative to properly size
the boiler or furnace.
For hot water and warm air systems this requires
that a “heat loss” calculation be performed
on the building, but a steam boiler should NOT
be sized based on the heat loss of the building.
Steam boilers must be sized based on the
amount of steam required to fill all of the connected
radiation and piping with steam. Properly
sizing a steam boiler requires that every heat
emitting device be measured to determine its
BTU capacity in Square Feet (Sq. Ft.) of Equivalent
Direct Radiation (EDR).
Once the total required Sq. Ft. of steam is determined,
size the replacement boiler by selecting
a unit with a Net I-B-R steam rating equal to or
greater than the BTH/hr. capacity of the connected
radiation.
If the boiler is undersized, the radiators furthest
away will not heat properly. If the boiler is
oversized it will short cycle, the steam flow in
the main piping will cause too much pressure
drop, the water in the returns will rise and water
hammer can occur. In either case, undersized or
oversized, overall system efficiency is compromised
and fuel is wasted.
Figure 1
38 National Oilheat Research Alliance
To properly size a replacement steam boiler
perform the following for each heat emitter:
1. Identify the style of the heat emitter.
2. Measure the height and width of the heat
emitter.
3. Count the number of tubes in each section
if applicable.
4. Count the number of sections if applicable.
5. Determine the square feet of radiation in
each section.
6. Multiply the square feet of radiation in
each section by the number of sections.
7. Total the square feet of radiation for all the
radiators in the building.
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.
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.
For example:
Step 1. Identify the radiation - Go from room
to room and identify the type of each heat emitting
device. There are several types of heat
emitters commonly found in the field:
Column radiators, tubular
radiators, radiant convectors,
cast iron baseboard, thin tube
radiators and wall radiators.
Step 2 to step 6. Determine
the Sq. Ft. of radiation of each
heat emitter.
Use the manufacturer’s
information if it is available, if
not, follow these procedures.
Measure the height and