Commonly, the term "Make Up Air" can be rephrased as "Allowing clean outside air into the building to replace the evacuated, or 'pulled out', smoky air." The smoky air is evacuated from the building so people have clean air to breathe as they escape the fire.
When smoke is being "evacuated" or removed from a building, air is going to be sucked into the building. Either the air can be sucked in slowly, through small cracks and openings, or air can flow in freely, through a designated intake route.
If the replacement air comes in slowly, a low-level partial vacuum is created in the building; if the air comes in freely, there isn't a vacuum.
Buildings aren't designed to have a partial vacuum. Things like crushed ductwork, ballooned out ductwork, crumpled outside doors, etc. happen with a partial vacuum. When there's a partial vacuum, the efficiency of the smoke evacuation fan is also reduced. There's even a possibility that emergency exit doors won't open.
Smoke Evacuation Fan
The smoke evacuation fan can be turned on manually through the fire alarm panel by firefighters, or automatically by the fire alarm panel. An addressable control relay for the fire alarm panel is used to turn on the fan.
Before turning on the fan, though, a sequence of events has to occur.
The sequence is:
- Activate the Makeup Air
- Confirm that the Makeup Air is Working
- Turn on the Exhaust Fan
Makeup Air Turn-on
The first event to occur, when the smoke has to be evacuated, is the makeup air fan is commanded on, or the makeup damper or door is commanded open. An addressable control relay from the fire alarm panel can do this.
Fan Running Detector or End of Motion Switch
The "Fan Running Detector" or "End of Motion Switch" is a physical confidence switch that senses that the makeup air fan is really operating or the makeup air damper or doors are fully open. It's a confidence thing that says "The makeup air is available."
The makeup air fan, damper, or door is activated or opened by the fire alarm control panel.
The confidence switch is directly wired using a Class B input circuit into an addressable input module, and programmed as a supervisory input or monitor input to the fire alarm system. For exact wiring, read the installation sheet that comes with the addressable input module.
Once the switch says "It's Running or Open", then the fire alarm panel can turn on the smoke evacuation fan.
Sequence of Events
When a firefighter turns on the smoke evacuation fan, or when the fire alarm panel turns on the smoke evacuation fan, a sequence of events occurs.
- The makeup air damper or door is commanded open
- The damper or door is confirmed to be open
- The smoke evacuation fan is turned on
Sometimes, a time delay has been used instead of an end of motion sensing switch. This, however, is a "Hope It Works" solution. Using the timer delay is hoping that the damper or door opens before the fan starts sucking the air out of the building.