Boiler Fuel Gas Control

For the case of multiple boiler system, the lead and lag are the terms to indicate the priority sequencing of the boilers. The lead indicates the master or first priority boiler and the lag indicates the slave or second priority boilers. The boilers that are chosen to run always are referred to as the 'lead' boilers. All the other boilers are 'lag' boilers but are prioritized such that a boiler with a high priority always runs before a boiler with a low priority and so on. The most effective boiler is always started first and the least effective one is always stopped first.

In multiple boiler installations, the burner management systems of each boiler may be controlled by a master plant controller (also referred to as a lead-lag controller) to stage boiler operation to minimize on/off operation. A Lead/Lag option turns boilers on or off as the plant demand requires. When the pressure is below the setpoint for a user-configured period of time, the boiler controller starts the lead boiler first. The remaining boilers are brought on line based on the controller output rising above a predetermined value for a user-selected time period.

For more detail, see these links:

http://www.rightangle.bc.ca/download/Boiler%20Controls%20and%20Heating%20Strategies.pdf

http://www.centurycontrols.com/Pdf/CC-600_Hot_Water_How_to_Specify.pdf

http://www.toxalert.com/datasheet/Lead.LagController.pdf

http://www.tekmarcontrols.com/literature/acrobat/p441web.pdf

http://www.boilerspec.com/speci-fire_pdf/123002update/D04_LeadMisc.pdf

- MS