PRIMARY/SECONDARY
BOILER PIPING
The installer must ensure that the boiler has adequate flow without excessive temperature rise. The temperature rise on a multiple zone system should be made when only the zone with the longest length and/or highest head loss is open. Low system flow can result in overheating of the boiler water which can cause short burner on cycles, system noise and in extreme cases, a knocking flash to steam. These conditions indicate the need to open the bypass adjustment valve, installation of a larger circulator pump or installation of the boiler with a primary/secondary piping system. System noise may also indicate an oversized boiler.
CAUTION
At no time should the system pressure be less than
12 PSIG.
FIG. 43 Primary/Secondary Piping of a Single Boiler
Heating boilers with inputs of 315,000 - 500,000 Btu/hr or larger installed on multiple zone systems are recom- mended to be installed with a primary/secondary piping system as shown in Figure 43. A primary/secondary piping system may also be installed on smaller input boilers to ensure proper boiler flow rates. Primary/sec- ondary piping is also ideal for systems using a mixture of propylene glycol and water. A primary/secondary piping system uses a dedicated pump to supply flow to the boiler only. This pump is sized based on desired boiler flow rate, boiler head loss and head loss in the secondary system piping only. The secondary pump, installed in the boiler piping, ensures a constant water flow rate to the boiler for proper operation. Boiler installation with a primary/secondary piping system can prevent noise problems caused by low system water flows. A primary/secondary piping system is recommended on any boiler where low water flow conditions may present an operational problem.
A primary/secondary piping system is ideal for systems filled with a propylene glycol and water mixture. A glycol and water mixture is more dense than water only and may result in a lower boiler flow rate. A lower boiler flow may cause noise on operation or short cycling of the burners. A larger secondary pump may be used on a primary/secondary system to provide an increased boiler flow rate. A glycol and water system may require from 10 to 20% more flow to compensate for the increased density of the glycol and its effect on the heat transfer process in the boiler. The exact increase in flow that may be required is based on the type and percentage of glycol added to the boiler system. Consult the manufacturer of the glycol for additional recommendations when using a glycol and water fill for your boiler system.
WATER
PRV
EXPANSION TANK
TO SYSTEM
LWCO (OPTIONAL)
SYSTEM
PUMP
| BYPASS | SECONDARY | |
12" | BOILER PUMP | ||
|
AIR SEPARATOR
FROM
SYSTEM
FIG. 44 Boiler with Low Temperature Bypass Piping - Models 315,000 - 500,000 Btu/hr
A boiler operated with an inlet temperature of less than 140°F (60°C) must have a bypass to prevent problems with condensation. A bypass as shown in Figure 44 must be piped into the system at the time of installation.
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