By Brian Murray, CECS, President of Providet Service Associates Inc.
If our job as CECS professionals is to clean kitchen “exhaust” systems of grease to prevent fires, why should we be concerned with make-up air? First of all, let’s define make-up air. Make-up air is used to replace air being drawn out of the kitchen by the exhaust system.
It has two basic purposes:
- It reduces energy costs. Without make-up air, the kitchen exhaust system would pull only the “conditioned” air out of the building, forcing the HVAC system to run continually to keep the building at acceptable comfort levels (kind of like having a fan pulling air into the building while the door is open).
- Without make-up air, the kitchen exhaust system could keep the kitchen under negative pressure. Like a vacuum, the exhaust system can only draw out what is available to draw in. It cannot exhaust to design specifications when the room is under negative pressure.
Although the first purpose stated has importance, it does not concern us in terms of fire protection. The second purpose, however, does. If the make-up air system is not working properly and results in a “negative-pressure kitchen,” then the kitchen exhaust system cannot work in accordance with design. This will result in grease-laden vapors accumulating in the system at faster rates.
Suppose you submitted a proposal to service an account four times per year. You based that recommendation on the requirements spelled out in NFPA 96 and your own observations of what is needed. However, after you have serviced the account a time or two, the fan belt breaks on the make-up air system. This sets up a chain of events that may not be immediately noticed. First, the exhaust system may have less air to draw; therefore, it is not working at optimal capacity. Grease-laden vapors begin to accumulate in the kitchen exhaust system at a faster rate. Instead of the system needing a service in three months, it is needed in two. The building is at risk of fire two months after your last service.
There is another potential problem in terms of fire safety. In the same scenario in which the fan belt breaks on the make-up air system, the kitchen exhaust system is working at less than design specifications for the reasons stated. Now, at peak hours the grease vapors overflow from the hood and are not all captured by the exhaust system. Where does that air go? I have seen it end up going up the make-up air duct. Hot air rises, and the make-up air duct can act like a chimney to draw this hot air. This can end up being a very dangerous problem over time.
Although kitchen exhaust ducts are specified to be 16-gauge black iron, the make-up air duct does not meet that requirement. It is designed only to bring fresh air into the space. It is not designed to remove grease vapors, nor is it equipped with the fire-suppression system that is in place in the exhaust duct. A fire starting or reaching here could spread undeterred.
What can we do? What should we do? I think it is important that part of our service should be to at least inspect make-up air units and duct systems periodically. This first means finding the units-which are not often in the same areas the exhaust fan-making sure the system is working and the fan belts are in good condition. Putting a hand over the air outtake of the make-up air in the kitchen and feeling air coming in is not an adequate way to inspect the system. Because our service does not take place during peak hours, the exhaust system will not be overwhelmed with heat and vapors and will therefore draw air from the make-up air duct even if it is not running, making it seem as if air is being pushed into the space from a properly working system when it may be simply pulled from the exhaust. Part of the inspection should include looking at the make-up air ductwork, making sure there are tight connections and that air is being blown into the specified kitchen area and not other spaces, such as the ceiling plenum.
Of course, this overall inspection should be noted in your report to the customer with any recommendations that may be needed for repair, protecting your customer and yourself from potential fire and liability.