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.