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Method 1: Primary/Secondary Piping - Boiler in Secondary Loop
This method can be used in heat-only applications as shown in Figure 8.2 or with an indirect water heater as
shown in Figure 8.3. This method relies on primary/secondary pumping to ensure that the required flow is always
maintained through the boiler. In this system, the flow rate through the boiler is completely independent of the flow
rate through the heating system. Use the following guidelines to ensure that boiler will have the required flow shown
in Table 8.1 regardless of the flow in the heating system.
1) Primary Loop Piping - Size the primary circulator and piping to obtain the design flow rate through the heating
system as you would on any other heating system. All piping between the expansion tank and secondary connection
tees must be at least as large as that shown in Table 8.5, column a. In order to keep the flow rates in the primary
and secondary loops independent of each other, provide at least 8 diameters of straight pipe upstream of the first
secondary tee and 4 diameters downstream of the second secondary tee. Keep the distance between the expansion
tank and the first secondary tee as short as practical.
2) Secondary Loop ("Boiler Loop") Piping – All piping must be the size shown for the boiler in Table 8.5, column a.
To size the circulator:
a) C ount all fittings in the planned secondary loop (the secondary loop consists of the shaded piping in Figure
8.4a). In doing so, do not count the secondary connection tees, unions, or the fittings supplied with the boiler
(these have already been accounted for).
b) U sing Table 8.7, find the equivalent lengths of all fittings in the secondary loop. Total these equivalent lengths
and add them to the total length of planned straight pipe in the secondary loop. The result is the total equivalent
length of the secondary loop.
c) U sing Table 8.5, find the boiler size being installed and select a boiler secondary circulator that shows a
"maximum equivalent length" (column d) in excess of the total equivalent length calculated in Step b.
3) Indirect Water Heater Loop Piping (If Indirect Water Heater is Used) – All piping must be the size shown in Table
8.6, column (a). If the indirect water heater connections are smaller than the pipe size called for in column (a),
reduce the pipe size at the indirect water heater connections. To size the circulator:
a) Count all fittings in the planned Indirect Water Heater Loop (the indirect water heater loop consists of the
shaded piping in Figure 8.4b). In doing so, you will be counting some piping and fittings which are common to
the heating system secondary ("boiler loop") piping and which were counted in Step 2a above. Do not count the
elbows or fittings supplied with the boiler.
b) Using Table 8.7, find the equivalent lengths of all fittings in the indirect water heater loop. Total these equivalent
lengths and add them to the total length of planned straight pipe in the indirect water heater loop.
The result is the total equivalent length of the indirect water heater loop.
c) U sing Table 8.6, find the boiler size being installed and select an indirect water heater loop circulator that shows
a "maximum equivalent length" (column f) in excess of the total equivalent length calculated in Step b.
Example – Assume that a CHG150 is to be installed in a heating system along with a AL40A indirect water heater. A
total of 15 ft of straight pipe will be installed between the boiler and the primary loop. A total of 20 ft of straight pipe
will be installed between the boiler and the indirect water heater. Fittings are arranged as shown in Figure 8.3. The
AL40A requires a flow rate of 8 GPM and has a head loss of 3.0 ft.
Total fittings in Secondary loop ("boiler loop"):
7 90 Elbows
1 Swing Check
Note: Unions, Secondary Connection Tees, and factory supplied fittings are ignored.
Calculate total equivalent length from Table 8.7:
15ft Straight Pipe + 7 Elbows x 3.75 + 2 Runs of Tees x 2.5 + 1 Swing Check x 10 + 2 valves x 0.8 = 57.85
Equivalent Feet Straight Pipe. From Table 8.5, we see that a Taco 007 will pump 8 GPM through a CHG150
with 148 equivalent feet of pipe, so Taco 007 is OK.
Total fittings in Indirect Water Heater Loop:
6 90 Elbows
2 Turns in Tees
Calculate total equivalent length from Table 8.7:
20 Straight Pipe + 6 Elbows x 3.75 + 2 Turns in Tees x 8 + 1 Swing Check x 10 + 2 valves x 0.8 = 70.1
Equivalent Feet Straight Pipe. From Table 8.6, we see that smallest circulator which will pump at 8 GPM
through a CHG150 with 70.1 equivalent feet and an indirect water heater pressure drop of 3 ft is a Taco 0010.
24
2 Runs of Tees
2 Isolation Valves
1 Swing Check
2 Isolation Valves
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