make-up air for boilers with variable speed FD fans

[pipesnpumps]

also cross posted to HVAC/R forum:

What types of approaches have others used to supply make-up air for boilers which have variable speed FD fans?

The boilers will have wide turn-down capability, with O2 trim, which means the forced draft (FD) fan is variable speed.

The code requires that a fixed rate of make-up air be supplied to the space, based on the installed capacity of the boiler.

My thought was to supply a constant amount, with a relief hood. The problem is in the winter I will be heating the make-up air to maintain a temperature in the space, and this will be a huge energy loss for all air that is exhausted, but can't be helped because the code is over simplified and requires that set amount outdoor air regardless of how much air the boilers require

Any ideas or experience with this?

 

[curtis2004]

What code are you talking about?

 

[pipesnpumps]

NFPA 54.. 0.35 cfm/1000 Btu of heat input, where make-up air is supplied from a mechanical air supply system.

 

[curtis2004]

Can you copy here full paragraph where those restrictions are imposed?

 

[cjw81]

Methinks a closer look at the code would be warranted and remember that the code was written for good reason by reasonably reasonable people. Take a look at the NYC high pressure boiler code and you will see that with an installed economiser will save you 10% by heating the makeup air to the boiler. In addition to that, the safety aspect of it by bringing in outside air fo rproper combustion.