Low velocity headers are used to separate hydraulically the boilers from the rest of the system. In addition to helping maintain a minimum flow through the boiler, they create a low velocity region for system dirt to be deposited and separation of air from the system water. Used in conjunction with a system filter, they are invaluable when conceding a new boiler to an existing system.
Low velocity headers should always be vertical and sized for a maximum water velocity of 0.5 m/s. Low velocity headers are designed to ensure a water velocity of 0.2 m/s or less and for ΔT 10 or 20, so will be suitable for most systems. Fig 2 proposes dimensions for the design of a low velocity header, and Table 2 shows dimensions for a ΔT 10 or 20 system.
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