Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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AIR MAKE-UP Fig. 19. Roof-mounted horizontal intake blast. An air make-up unit can lower heating and cooling costs. When air make-up is added, the building exhaust system works more ef- ficiently. The information in this section will help to determine when an air make-up sys- tem is needed. Air make-up is the air re- quired to maintain safe and effective building oper- ation by re- placing ex- hausted air. When an exhaust fan is installed in a building, exhausted air must be replaced from out- side. This is done either through the cracks and openings in a building or with an air make-up, or air replacement, unit, which introduces outside air into the building. This air is usually filtered, cooled, or heated. Installing an exhaust system without an air make-up unit is a good example of heating ven- tilation air by accident rather than by design. Air always flows from a higher pressure area to a low- er pressure area. Installing an exhaust fan in a building creates negative pressure within the interior space. Air will flow from the higher pressure outside the building to the lower pressure inside. Because most buildings are closed in, the flow is restricted, but not completely. Cracks around doors and windows and in the masonry and vent stacks allow air to flow into the building. This air creates drafts and cold spots until it can mix sufficiently with space air to reach room temperature. Fig. 20. Inside ceiling mount vertical intake. Fig. 21. Inside ceiling mount horizontal intake. The normal heating system must work longer and at higher temperature to heat the air seeping from the outside. In addition to the increased heating cost, the negative pressure keeps the exhaust fan from doing its job—exhausting conta- minants from the space. Exhaust fans are rated for a certain air delivery measured in cubic feet per minute (cfm). This rating is based on a specific static pressure. Static pressure is 216

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