Why Make-up Air
There are many buildings, especially industrial buildings, that are exhausting fumes, material, noxious gases, etc… from the air, but are not forcing make-up air back into the spaces. This is a common situation, especially in manufacturing operations where fumes or other airborne contaminants are a problem. Of course in most cases there is some make-up air entering the building, generally under doors, through cracks, bad window seals, and various other openings. The problem with this situation is that this is typically insufficient air, and it is untreated. The rule of thumb for building balance is that for every cubic foot of air that is removed through forced exhaust, a cubic foot of forced make-up air should replace it. Forced make-up air, fan-fed, is the best way to balance a space that is being exhausted. Granted, some air will infiltrate the space through migration, but it will most likely not be balanced. The result will be higher fuel bills, as the existing heating system will be strained to temper incoming air; higher electrical bills, as the fans have to work harder to exhaust, while working against the increased static pressure of the resulting negative pressure; and accordingly, decreased performance in these same exhaust fans as a result of working against this higher static pressure.
The best solution for negative space conditions is forced make-up air. In some cases this can be accomplished with a supply fan (fans) that matches the amount of air being exhausted. However, in most cases the best approach is to temper the make-up air as well to provide for employee comfort and/or a consistent process environment. While comfort cooling is possible with make-up air, it is harder to accomplish and typically more expensive. The most common approach is to heat the air in colder months, and supply non-tempered air in warmer months. Make-up air is typically heated with gas, steam, hot water, electrical strip heat, and sometimes heat pump technology. In some cases a mixing box can be used to mix the warmer air that rises in the space with the outside air to temper the air.
Sizing a make-up air unit is often as simple as matching the supply volume to the amount of air being exhausted. It is often advisable to oversize a little, (+/- 10%), to ensure the space is positive. However, there may be other considerations such as heat loss or other negative pressure influences from adjacent spaces that need to be accounted for. Another major consideration is heat load for heated make-up air. While it is a fairly straightforward process, there are several variables to be considered when sizing and selecting make-up air systems. I will address these in further entries per topic. For now, suffice it to say that make-up air is crucial for healthy happy employees, consistent process environment, and in many cases state and federal regulations.
RJ Owen Associates offers make-up air systems from Weather-Rite, LJ Wing, Brasch Manufacturing, and Canarm. We have solid experience in the design, selection, installation, start-up, commissioning, repair, and service of all types of make-up air, heating, and ventilation systems.
[...] should match demand and with these gases demand typically varies. In addition, as noted in a previous post, building ventilation and the associated building pressure should be balanced. Therefore, in most [...]