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Posted by Bob D. on October 05, 2009 at 13:49:29:
In Reply to: Re: Mechanically supplied combustion air posted by Gas Inspector on October 02, 2009 at 15:17:06:
Several Codes (see your own Jurisdiction's) require the design airflow of the boiler room supply air fan(s) be sized to meet or exceed the combustion air requirements of all "devices" located within the room (Let's not get into the NFPA 31 and 54 requirements for very small, essentially residential, units). Some guys interpret this as matching the maximum "design" operation, excluding back-up or redundant units, while others add everything up all together. [I still get in arguments about that.] There is usually (again, depending on which Code you are under) an electrical interlock, through auxillary contact(s) in the fan starter and the burner management systems (BMS), that prevents the burners from firing unless the supply air fan is running.
In larger installations, the individual units will also be factory equipped with: a combustion airflow switch (CAF) that is integral to the flame safety system (BMS), as well as residual flue gas oxygen (excess O2) monitoring for control optimization as well as low O2 trip function(s).
I would think hard about using a duct-mounted flapper or pitot tube for a couple reasons - 1. Maintenance required - which probably wouldn't get done; 2. Access to devices may be difficult, and operators would tend to forget where they are; 3. Additional cost/complexity, even if relatively minor; and, 4. "Atypical" control approach.
Then again, it doesn't hurt anything, and you can exceed Code minimum requirements . . . . ."Opening the door" only counts for the little installs.
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