Boiler Sizing Replacement Guide

9
To properly size a replacement steam boiler,
determine the total BTU capacity of the
connected radiation in the building. This
method is necessary so that the new boiler will
produce adequate steam to
fill the entire system. To make
the calculations, determine
the number of square feet
of direct radiation in each
radiator connected to the
existing boiler. Follow these
steps:
1. Identify the style of the
radiator. Figure 6 shows the
relative sizes of three dierent radiator styles.
All are four tube, eight section radiators but
the rating of each is dierent.
2. Measure the height and width of the radiator.
3. Count the number of tubes in each section.
4. Count the number of sections.
5. Determine the square feet of radiation in each
section. See Table F (page 10).
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.
STEP 2: CALCULATE TOTAL HEAT LOSS
The Total Heat Loss for the example is calculated
on the form shown in Figure 5 based on the
following steps:
1. The construction of the house matches
Construction Design Number 17 in Table A
(page 4).
2. Each of the two levels is rounded to 800 sq ft
(the basement is not heated).
3. From Table B (page 5) Construction No. 17 at
800 sq ft has a heat loss of 33,430 BTU/Hr.
4. Table C (page 6) shows a 1.15 correction factor
for 80°F Design Temperature Dierence.
Therefore, the heat loss for each area is
38,450 BTU/Hr. (33,430 x 1.15 rounded).
5. Since it is a two story house, the first floor
does not have a ceiling loss.
Table D (page 6) indicates a Ceiling
Correction Factor for Construction Number 17
of .87. Therefore, the total adjusted heat loss
for the first floor is 33,450 BTU/Hr. (38,450 x
.87 rounded).
6. The total heat loss for the two levels is 71,900
BTU/Hr.
STEP 3: SELECT THE BOILER
The Total Heat Loss of 71,900 BTU/Hr. calculated
in Step 2 is the minimum AHRI NET Rating of the
new boiler. The owner wants a high eciency
gas boiler. Therefore, from Table E (page 7) the
Ultra 105, WM97+ 110, ECO 110, GV90+ 4, CGi-5,
CGa-4, or EG 40 could be selected. The Model
Number of the boiler would be recorded on the
Materials List. (Figure 3, page 7). The list would
then be used to calculate material cost; add
labor, overhead and profit; and determine the
selling price.
LEVEL
CONSTRUCTION
NUMBER SQ. FT.
HEAT LOSS
(AT 70°F
DESIGN
TEMP. DIFF.)
CORRECTION
FACTOR AT
°F TOTAL
CEILING
CORRECTION
FACTOR
ADJUSTED
HEAT LOSS
FIRST LEVEL
x = x =
SECOND LEVEL
x =
MID-LEVEL
x =
BASEMENT
x =
SLAB
x =
TOTAL HEAT LOSS
FIGURE 5: HEAT LOSS CALCULATION FORM
17 33,430 38,450 33,4501.15 .87800
17 33,430 38,450 38,450
71,900
1.15800
SECTION 2
PROCEDURE FOR SIZING STEAM BOILERS
FIGURE 6
thin tube
type
tube
type
column
type
80