What is Makeup Air in a mill building ?

Makeup or compensating air is outside air which is introduced into the building to replace air that is exhausted from the building through ventilation or combustion processes.


Most exhaust systems are installed for employee comfort and safety or for product process requirements. In the past, when energy was inexpensive and plentiful, exhaust fans were frequently added and often oversized. Little thought was given to bringing in outside air to replace the exhausted amount. If adequate air is not introduced into the building, a negative pressure condition will develop. This causes uncomfortable conditions and reduces the effectiveness of exhaust systems. Traditionally, when exhaust effectiveness is reduced, someone often adds another exhaust fan without compensating air which reduces the exhaust effectiveness further, which causes someone to add another exhaust fan , which you get the idea. The end result is an ineffective and uncomfortable system that is extremely wasteful and costly to operate.


Buildings which have inadequate makeup air (or excess exhaust) and are operating with a negative pressure condition have several common characteristics. These include:

A) The building is uncomfortable and has noticeable temperature fluctuations. The perimeter of the building may be cold and drafty due to high infiltration at the same time that other central areas may be overheated.

B) Exterior doors may be difficult and hazardous to open. A door which slams shut can result in broken bones and serious injuries.

C) Cold drafts can be felt rushing into the building around cracks and openings.

D) Exhaust systems do not perform at the rated design capacity. Inadequate or ineffective exhaust may result in a hazy atmosphere.

E) Gravity vents from heating equipment or process may back-vent into the conditioned space, drawing potentially hazardous products of combustion (Flue Gases) into the building.


The ideal, energy efficient building would have compensating air equal to actual exhaust air requirements - no more, no less. A positive pressure can result in excessive energy operating costs, particularly if overhead doors are opened frequently or for long periods. A positive pressure can also drive fumes into office areas and other spaces. If a building has a negative pressure condition, several things can be done to improve the efficiency and comfort of the facility including:

A) Evaluate the existing exhaust systems

1) Consider how much exhaust is actually required to accomplish the ventilation goal. Excessive exhaust is extremely expensive to operate. For example, if natural gas costs $9.10 per Mcf, in Northern Missouri a single 48" ( 20,000 CFM) exhaust fan, could cost as much as $38,000 each year. Do the math - over a fifteen year period, that single exhaust fan could waste over $570,000.

2) Operate exhaust fans and makeup air heating systems only when necessary for proces or safety requirements. The savings to operate makeup air heating equipment for only one shift, five days per week can be significant. For example, operating makeup air heating equipment for only one shift can save approximately $1380 per 1,000 CFM each year in Jackson,Missouri

3) Investigate local ventilation instead of general ventilation. Localized capture of heat or contaminants is usually much more effective and energy efficient than general ventilation

B) Install automatic controls

1) Install temperature control or a timing device for automatic control of exhaust systems. In many cases, the addition of a simple thermostat or time clock can significantly reduce operating costs. If the building doesn't need the exhaust, it doesn't need the makeup air.

C) Install air cleaning or heat reclaiming equipment

1) In many cases, a combination of general air cleaning and localized exhaust can provide a very effective and efficient means of improving indoor air quality while reducing exhaust requirements

D) Install additional makeup air equipment

1) Proper makeup air equipment may be as simple and inexpensive as an outside air intake louver or it could require separate makeup heating equipment to bring in and condition outside air. The best, energy efficient approach for a particular building can only be determined after careful evaluation of all building characteristics, process requirements, and employee considerations.


The following methods may be used to introduce makeup air into a building in a controlled manner:

A) Outside Air Intake Louver - A motorized intake louver interlocked with the exhaust system is an effective and inexpensive method of introducing outside air into a facility. However, the system may result in localized cold and drafty conditions. Building will continue to operate in a slight negative condition.

B) Supply Air Fan - A wall or roof mounted supply air fan is a relatively inexpensive method of introducing a controlled amount of outside air into a facility. Supply fan should be interlocked with exhaust systems and designed for total air balance (Supply=Exhaust). Supply air fans also can result in localized cold and drafty conditions within the conditioned space.

C) Fan Tube / Mixing Type Makeup Air System - Fresh, unconditioned makeup air can be introduced into a building through perforated duct system which promotes uniform mixing of outside air with indoor air. Modulating mixing dampers provide additional flexibility in matching exhaust and temperature requirements.

D) Air-Mixer Ventilation System- Air Mixer ventilation system allows fresh, unconditioned makeup air to be introduced into a warehouse building. The design of the Air Mixer system promotes effective ventilation by uniform mixing of outside air with indoor air. Modulating mixing dampers provide additional flexibility in matching exhaust and temperature requirements.

E) Packaged Heat Recovery Systems - Several types of packaged heat recovery equipment are available which permit incoming outside air to be preheated by outgoing exhaust air. System permits energy efficient heat recovery and tempers incoming outside air.