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Make-up air unit for boiler with variable speed FD fan

Make-up air unit for boiler with variable speed FD fan


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?


RE: Make-up air unit for boiler with variable speed FD fan

There should be exceptions to the fixed rate of makeup air, if you: A) duct make-up air to the boiler FDBs directly from outdoors (I believe there is a min. duct diameter requirement), or B) use passive room inlet gravity dampers. The open louver area would need to meet code for the BTU rating of your devices. With B, you might still need pretty substantial room heaters or heat trace of piping near the louvers.

RE: Make-up air unit for boiler with variable speed FD fan

couldn't you build a direct vent system, like it is used for smaller systems with the vent pipe going straight from outside to the boiler air intake with no room air mixed in?

you should consult the boiler manufacture on their requirements for that. Most give you some duct sizes for certain lengths.  

RE: Make-up air unit for boiler with variable speed FD fan

what about running the combustion air supply fan on room pressure?

knowledge is power

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