In addition to monitoring and displaying boiler status, the CPW board controls the timing of various operating functions including: boiler pump turn-off delay, blower fan pre-purge, and blower fan post-purge. The location of each user- adjustable timing is shown in Figure L-1.
Figure L-1
CPW Board
•Pump Turn-off Delay
The CPW board has a built-in pump driver with pump turn-off delay, which allows the operator to set how long the pump will run after the boiler shuts off. The delay is factory-set for 7 minutes but can be field-adjusted from 3 minutes to 10 minutes. The pump relay can directly power a pump of up to 12.0 amps draw or 1.0 Hp capacity. Pumps with larger amp draws should be energized through a pump contactor that is controlled by the CPW board.
By properly setting the pump turn-off delay, the boiler pump will continue running for a limited period of time in order to extract any residual heat from the combustion chamber. The pump will then shut off to conserve energy until the next call for heat. The pump-off time delay should be set to the minimum time required to extract all residual heat from the heat exchanger. If temperature as monitored at the boiler outlet increases more than 5°F after the pump turns off following a firing cycle, the pump-off time delay should be extended by thirty seconds. Increase or decrease time delay as appropriate until outlet temperature does not spike more than 5°F after pump shut-off following a prolonged firing cycle (> 5min.)
The boiler requires forced water circulation when the burner is operating and the pump must be interlocked with the boiler. See Section D for minimum water maximum flow rates and pump selection.
Pump will come on when power is first applied to the boiler.
•Blower Fan Pre-purge
The CPW board has a built-in pre-purge delay which operates the blower fan for a user-adjustable time period prior to pilot trial for ignition. The pre-purge period ensures that the combustion chamber is clear of all excess combustible gases prior to commencing the pilot ignition cycle. The pre-purge delay is factory set for 45 seconds and it can be adjusted in the field from 45 seconds to 120 seconds. To optimize boiler response time on call-for-heat, set the pre-purge delay to the minimum allowed by local, state or national code.
•Blower Fan Post-purge
The CPW board has a built-in post-purge delay to exhaust residual combustion products after the main gas valve closes. The post-purge timing is factory set at 3 minutes and is not field-adjustable .
