Tighter regulations for energy conservation has driven metal building manufacturers to design more efficient roof and wall panel systems that affect ventilation. These new products minimize the amount of air and moisture infiltration and reduce thermal transmission. The end result is a metal building with a more productive working environment for employees and lower energy costs for owners when used as an unconditioned manufacturing facility or warehouse. However, what is a benefit for keeping heat in and moisture out in winter months can be a detriment in summer months when they can be trapped inside. Thus, it is even more critical to incorporate an adequate metal building ventilation system into the design of a building when using these new building envelope systems.
Natural Ventilation Systems
The first choice for ventilating a metal building is to use a simple natural ventilation system. This type of ventilation system uses wind and thermal convection to move ambient air in at the base of the building and hot air out through the roof. The openings around the perimeter of the metal building can be fixed louvers or doors and the openings in the roof can be hooded roof vents or clamshell shaped ridge vents. All of these fixtures should include dampers to restrict cold air infiltration in the winter.
When a natural ventilation system is designed and functioning properly, employees will feel a noticeable cooling effect of ambient air area moving across the building floor. Also, heat generated from the sun and moisture from night time cooling will be vented out and away from people and equipment.
The benefits of a natural ventilation system are:
- No operating costs;
- Lower capital and maintenance costs;
- Minimal rain water incursion; and
- Less load on roof and wall support structures.
The big drawback to a natural ventilation systems is its significant reliance on prevailing winds in order to work. In an area of low prevailing winds, a natural ventilation system will not be effective. Other reasons that a natural ventilation system might not be effective are:
- Prevailing winds are blocked by something adjacent to the metal building such as another building, trees or a hill;
- Low roof height minimizes thermal convection;
- The outside air contains a significant amount of contaminants requiring filtration;
- Activities inside the metal building generate excessive heat, smoke, or particulates that require a more consistent air exchange rate.
Alternative Systems
The alternative to a natural ventilation system is a powered ventilation system. There are three basic types of powered ventilation systems:
- Powered supply and natural exhaust;
- Natural supply and powered exhaust; and
- Powered supply and powered exhaust.
For a metal building, a powered supply and natural exhaust system is the first option to consider. The configuration is similar to a natural ventilation system with hooded roof vents or clamshell shaped ridge vents located on the roof, but instead of louvers around the perimeter, wall fans are mounted at 8 to 10 feet above the building floor. Good design practice will place the fans down low or direct the air from the supply fans down to the floor level and to areas where there is the most activity.
The advantages of a powered supply and natural exhaust system are:
- Temperature at the floor level is maintained close to the outside ambient temperature;
- Consistent sensible cooling effect for employees working at the floor level;
- Positive pressure in the building to keep out contaminants such as dust; and
- The addition of filters and heaters to the supply fans can provide clean air and heat in the winter.
There are two major considerations in designing a powered supply and natural exhaust system. The first is to properly balance the open area of the natural exhaust vents to the volume of supply air coming into the metal building. Too little open area can create excessive pressure in the metal building causing doors to stick or fly open risking personal injury to employees. The second is to select weather hoods for the supply fans that have sufficient open area to prevent rain water from being sucked into the metal building when the fans are on.
A natural supply and powered exhaust system, on the other hand, uses fixed louvers and doors around the metal building perimeter for air supply with upblast or hooded roof fans to exhaust the air. Situations when this type of powered ventilation system is more favorable for a metal building are:
- Building roof height is low;
- High heat sources require the placement of exhaust fans immediately above; and
- A negative building pressure is required to keep fumes or contaminants from entering adjacent office spaces.
A powered supply and powered exhaust ventilation system combines the advantages of the other two types of powered ventilation systems and can be designed to provide either positive or negative building pressure. This type of system works best when there is a high heat load in the metal building and the required air exchange rate per hour is high. A good example is a gas pipeline compressor station building.
A Word of Caution
Don’t allow the placement of a large ceiling fan hanging from the metal building roof to be confused with a properly designed ventilation system. In addition to missing out on all of the advantages previously discussed, the ceiling fan will draw the heat trapped at the roof of the building and blow it down on employees and equipment doing the opposite of the desired effect.
Let us help you get the most out of your metal building with a ventilation system designed to maximize productivity and comfort. We at Eldridge have the engineering know-how and product selection expertise to design and equip a ventilation system for a new metal building or to troubleshoot an existing one. To reach a ventilation system expert, email us at info@eldridgeusa.com or call 713-780-7200 or +1-844-780-7200.