HVAC Service Clinic: Replacing Exhausted Air

May 11, 2018
Contractors who want to build a healthy space for customers should know the benefits of a make-up air system, and consider it based upon client need.

by Dennis Mueller

Contractors who want to build a healthy space for customers should know the benefits of a make-up air system, and consider it based upon client need.

Warehouses, factories and offices can all be candidates for make-up air systems. Some units can provide both ventilation and make-up air (defined as the use of outdoor air to replace exhaust air and air lost via leaks) lost, for example, via exhaust hoods or bathroom fans. The best make-up air solutions will heat or cool and dehumidify external air to the temperature and moisture level as the air exhausted.

Despite fairly rigid construction standards designed to make buildings as energy-efficient as possible, buildings lose air in a variety of ways. They can lose air via exhaust fans (think kitchen hoods) or vented gas equipment such as furnaces. This leads to negative pressure relative to adjacent spaces, which can draw in outside air via cracks or drafty windows. It makes terminal systems work harder and less efficiently, and presents comfort and humidity issues. That air needs to be replaced.

Make-up air systems can also help adjust pressure in some building sections. Here are some basics:

  • To increase positive pressure, and prevent drafts and lessen the load on HVAC systems, makeup air should be pulled into a space at a rate of 1.1 times the cubic feet per minute exhausted.
  • Neutral air pressure, which is obviously attained through the introduction of air at the same rate it’s exhausted, can also attain goals of improved air quality.
  • Negative air pressure is attained through the introduction of air at a rate slightly less than the exhaust level. This strategy is good in settings such as hotel corridors or schools, where odors need to be confined to, for instance, bathrooms and locker rooms or individual rooms.
  • Make-up air systems can be targeted to pressurize specific areas of a building, such as apartment corridors, where positive pressures can ease cohabitation with neighbors who cook oily fish on a regular basis.
  • Growing concerns about indoor air quality and a modern focus on energy efficiency mean that make-up air systems are an integral part of any new construction or extensive renovation.

The types of make-up air units vary, and are determined by the specific needs of a building or space. Warehouses or car washes, for instance, can be served by a fairly simple, more economical direct-fired make-up air system because humidity levels are of less concern than occupied spaces such as offices.

Other considerations contractors should keep in mind is blower control; occupied and unoccupied modes of operation; and location and protection of the make-up air exchange unit to limit impacts from snow, ice and animal intrusion.

Replacing air is more complicated than simply exhausting air, and these units should only be installed by licensed experts. But the results will leave inhabitants and workers breathing easier.

Dennis Mueller is engineering section manager for commercial products at Modine Manufacturing Company in Racine, Wisconsin. He can be reached at [email protected].