PRIMARY/SECONDARY BOILER PIPING
TO SYSTEM
TO EXPANSION TANK
AND MAKEUP WATER SYSTEM
12” MAX.
FROM SYSTEM
FIG. 80 Primary/Secondary Piping of a Single Boiler
A properly sized primary system pump provides adequate flow to carry the heated boiler water to radiation, air over coils, etc. The fittings that connect the boiler to the primary system should be a maximum of 12" (0.30 m) (or 4 pipe diameters) apart to ensure connection at a point of zero pressure drop in the primary system. There should be 10 pipe diameters of straight pipe before and after the boiler secondary loop connections to prevent turbulent flow at the secondary loop connections. Multiple boilers may also be installed with a primary/secondary manifold system. Multiple boilers should be connected to the common manifold in reverse return to assist in balancing flow to multiple boilers.
The installer must ensure that the boiler has adequate flow without excessive temperature rise. Low system flow can result in overheating of the boiler water which can cause short burner cycles, system noise relief valve discharge and in extreme cases, a knocking flash to steam. These conditions indicate the need to increase boiler flow to and from the boiler. This is generally accomplished by increasing the diameter of the piping that connects the boiler to the primary system. A larger diameter pipe reduces head loss and increases flow.
CAUTION
At no time should the system pressure be less than 12 PSIG.
FROM SYSTEM
CAP EACH MANIFOLD
TO SYSTEM
FIG. 81 Primary/Secondary Piping of Multiple Boilers
MINIMUM BOILER WATER
TEMPERATURES
Inlet water temperatures below the specified minimum recommendations can excessively cool the products of combustion resulting in condensation on the primary heat exchanger. The boiler is designed to condense only on the secondary heat exchanger. Condensation on the primary heat exchanger can cause operational problems, bad combustion, sooting, flue gas spillage and reduced service life of the related components.
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