Primary/Secondary Piping - Boiler In Primary Loop; Heating System Secondary Loop Piping; Heating System Primary Loop (“Boiler Loop”); Direct Connection To Heating System - Crown Boiler BWC Series Installation Instructions Manual

Method 2: Primary/Secondary Piping - Boiler in Primary Loop
This method can be used in heat-only applications as shown in Figure 8.8 or with an indirect water heater
as shown in Figure 8.9. Like Method 1, 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 zones. The boiler is installed in the heating system primary loop as
shown.
This system provides more accurate control of the water temperature entering the heating zones as the number
of zones calling for heat changes. When this piping method is used, the boiler control is set to provide the highest
supply temperature required by any heating zone and some other means, such as 3-way valves, are used to control
the water temperatures required by any zones requiring lower temperature water. This system is only recommended
when the primary loop (shown bolded in Figure 8.10b) is to be constructed as part of the boiler installation and can
be constructed as shown. Also, if the boiler is to accurately control the temperature entering each secondary loop, the
flow rate in the primary loop must be at least as great as the sum of the flows through all secondary loops.
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) Heating System Secondary Loop Piping - The heating system secondary loop piping is shown in Figure 8.10a.
Size each secondary zone circulator and piping to obtain the design flow rate for that zone as you would on any
other heating system. When the heating zone requires a water temperature below that to be controlled by the
boiler, provide a mixing valve or blending station for that zone. 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.
2) Heating System Primary Loop ("Boiler Loop") – The primary loop ("boiler loop") piping is shown bolded in
Figure 8.10b. All piping in this loop must be the size shown for the boiler in Table 8.5, column a. To size the
circulator:
a) Starting at the boiler, trace a path through the heating system primary loop from the boiler supply connection
to the boiler return connection, counting fittings and straight pipe as you go. Where the primary loop breaks
into parallel paths, follow just one of these paths (it doesn't matter which one). Do not count the secondary
connection tees, unions, or the fittings supplied with the boiler (these have already been accounted for).
b) Using Table 8.7, find the equivalent lengths of all fittings counted in (a). Total these equivalent lengths and
add them to the total length of straight pipe measured in step (a). The result is the total equivalent length of the
primary loop.
c) Using Table 8.5, find the boiler size being installed and select a boiler primary 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
bolded piping in Figure 8.10c). In doing so, you will be counting some piping and fittings which are common to
the heating system primary piping (the "boiler loop") 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 for the indirect water heater loop.
c) Using 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.
Method 3: Direct Connection to Heating System (Generally NOT Recommended)
The BWC can be connected directly to the heating system as is done with conventional boilers (Figure 8.11). If
this is done, the flow rate through the boiler will equal the flow rate through the system. The flow rate through the
system must therefore always remain within the limits shown in Table 8.1. For this reason, the pressure drop through
the entire system must be known, added to the boiler pressure drop, and a circulator selected which will provide the
required flow at the total calculated pressure drop.
This method is generally not recommended because it is often very difficult to accurately calculate the pressure
drop through the system. In replacement installations, it may be impossible to get an accurate measurement of the
amount of piping and number of fittings in the system. In addition, if the system is zoned, the system flow may drop
well below the minimum required when only one zone is calling for heat.
The one advantage to this method is its installation simplicity. It may make sense to use this method when the
boiler is to be installed with a new single zone system having a low-pressure drop. Pressure drop curves for the
BWC Series boilers are shown in Figure 8.12. Calculation of the system pressure drop, and selection of the
circulator, must be performed by someone having familiarity with pressure drop calculations, such as an HVAC
engineer.
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