Air Make-Ups

Is Air Make-Up / Air Replacement Needed?

The terms “air make-up” and “air replacement” are interchangeable. In order to ensure proper air balance, air make-up or air replacement systems are designed to deliver fresh, filtered and heated air into a building or booth. In addition, an air make-up unit can lower heating and cooling costs. When air make-up is added, the building exhaust system works more efficiently.

What Air Make-Up Is

Air make-up is the air required to maintain safe and effective building operation by replacing exhausted air. When an exhaust fan is installed in a building, exhausted air must be replaced from outside. This is done either through the cracks and openings in a building or with an air make-up unit. The function of an air make-up, or air replacement, unit is to introduce outside air into the building. This air is usually filtered, cooled or heated.

Heating Air by Accident vs. Design

Installing an exhaust system without an air make-up unit is a good example of heating ventilation air by accident rather than by design. Air always flows from a higher-pressure area to a lower-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. 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 contaminants from the space.

When more air is exhausted from a building than is supplied by the mechanical systems, the building is under a “negative” condition and air will leak into the building through cracks, windows and doors. A negative condition results in the following:

  • Flues and stacks will experience a backdraft and cause dangerous contaminants to remain in the occupied space. In the case of flues, the products of combustion may condense and corrode the equipment.
  • The exhaust system sees a greater static pressure which reduces the capacity of each fan, resulting in an inadequate removal of contaminants and wasted horsepower.
  • Drafts and cross currents will increase, causing an uncomfortable or unhealthy work environment.

Installing an air make-up unit sized to the building will improve exhaust system efficiency and provide greater control over the interior temperature. With the correct balance of air, it is easier to control air pressures to alleviate problems in opening or closing doors. Balance also prevents contaminants or odors from traveling to different areas of the building. The air make-up unit reduces fuel bills by eliminating drafts. 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 the friction the exhaust fan must overcome to exhaust air. The more cracks and openings in the building, and the larger they are, the easier it is for air to move into the building. As the static pressure rises, the exhaust air decreases.

When to install an Air Make-Up Unit

The following checklist is helpful in determining if a building needs an air make-up unit:

  • Gravity systems, such as vent stacks from a gas-fired furnace or water heater that normally draw air out of the building, are pulling outside air in.
  • Exhaust systems are not operating efficiently, resulting in a build-up of contaminated air within the facility.
  • The inside perimeter of the building is cold because the outside air is being pulled into the building.
  • Exterior doors are hard to open or close because of the pressure exerted by outside air entering the building through them.
  • It is difficult to maintain an even temperature throughout the interior space.


The air make-up system should be sized according to the spray booth exhaust volume plus 10%. If the air make-up duct will be physically connected to the spray booth then the 10% extra capacity can be disregarded. However, some means of volume adjustment must be allowed so that a proper input/exhaust volume balance can be obtained. This can be in the form of an adjustable drive on the air make-up and/or exhaust fan or volume dampers in the system. If the installation is new, then the manufacturer will know the needs of both the exhaust fan and air make-up system. If the booth is older, the exhaust volume can be determined from the manufacturer’s literature, computing from known booth velocity or from fan curves. Air make-up is most easily sized during initial booth purchase and installation.

To determine your air replacement needs:

Multiply the exhaust fan rated capacity (CFM) by 20
(based on three changes per hour: 60 min. / 3 = 20)

10′ wide x 8′ high spray booth
Rated at 125 FPM
10,000 CFM
20 X 10,000 = 200,000 cubic feet of air

If the shop area (width x length x height) is less than this amount, an air replacement system should be installed. In the example above, the booth described would require an air replacement system in any building smaller than 100′ x 100′ x 20′.

Styles of Air Make-Up Units

There are four basic air make-up styles available. They are defined by their intake and discharge mechanisms and include:

Horizontal intake/downblast discharge

An air replacement unit for inside or outside installation. The unit, when weatherproofed, may go on the building roof, has a horizontal intake with a downblast discharge and is curb mounted.

Horizontal intake/horizontal discharge

An air replacement unit generally used indoors. The horizontal intake allows the unit to be mounted through the side wall of a building. The unit has a horizontal discharge.
Vertical intake/horizontal discharge

An air replacement unit for use indoors. The vertical intake allows for mounting through the roof of the building. It has a horizontal discharge.
Floor-mounted vertical unit

An upblast furnace. Horizontal intake and floor mounted vertical units are available in either inside or outside models.