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Makeup Air Units

Commercial and industrial buildings exhaust large volumes of air. Food service kitchens employ large hoods over cooking equipment that pull out the unwanted byproducts of cooking such as heat, moisture, smoke and grease. Industries exhaust air to rid the factory floor of odors, fumes, vapors and particulates. All buildings require fresh air to maintain a healthy environment and for the proper operation of fuel-burning equipment.

Air that is removed from the building through exhaust hoods, dust collectors, process exhaust, ventilators and other devices must be replaced with an equal amount of outside air, a process known as makeup air. Without sufficient makeup air, negative pressure problems can result, especially as buildings become better insulated and “tighter.” The symptoms of negative pressure can be costly and serious—from faulty products and excessive costs to poor working conditions and employee absenteeism.

Makeup air units, sometimes called duct heaters, are used to replace the air exhausted from a building. They are widely used in industrial and commercial applications such as manufacturing plants, warehouses, garages, shopping malls, kitchens and office buildings. The units bring facilities into compliance with building codes for proper replacement air.

Makeup air units are available in a number of equipment options. Basic packaged units can be equipped with accessories such as filters, intake hoods, cooling coils, inlet and outlet dampers, and temperature control systems. Installation options include mounting on the rooftop or within truss space, exterior mounted with discharge through walls, or upright units set on a floor. Custom-designed units are also widely available and offer component options to meet any system design need. A newer type of unit, the direct-fired industrial air heater, provides both space heating and makeup air.

How It Works
During warm weather or in warm climates, a makeup air unit may simply replace the air exhausted from the building. On hotter days, evaporative cooling, direct expansion coils or chilled water coils fitted to the unit can reduce the ambient air temperature to the space. During colder periods, the makeup air equipment may be used to warm the outdoor air before it is introduced into the building.

Makeup air units can operate on a variety of fuels such as liquefied petroleum, hot water, steam, electric resistance or natural gas. Although electric units generally have a lower first-time cost, natural gas models are the preferred choice due to their ease of installation, efficiency and economy.

Gas-fired models are either direct-fired or indirect-fired. In the indirect-fired models, the heat is transferred to the incoming air through a heat exchanger. In direct-fired models, which are more efficient, combustion occurs directly in the supply air stream to heat the air. Direct-fired units offer turndown ratios of up to 40 to 1, allowing them to closely track the heating load as outdoor temperatures vary.

Efficiency
80% to 83% for gas models; 100% for electric models.

Sizes
17MBtu/h to 8000MBtu/h 740 cfm to over 100,000 cfm

Important Considerations
An improperly designed makeup air system can negatively impact the effective operation of exhaust equipment. Design considerations include speed of the air introduced by the makeup air equipment, cross drafts from delivery and service doors and drive-through windows, and the location of the makeup air units with respect to the exhaust system. If not properly addressed in the design phase, any of these factors can result in incomplete removal of combustion and other undesirable products.