Many hospitals in the Nordic regions use displacement ventilation systems in all inpatient rooms and treatment areas. A typical U.S. system introduces cold air in the ceiling, forces it down through a diffuser and removes it from the ceiling through an exhaust grille. The diffuser mixes the cold air to raise the average temperature and this has the unintentional effect of entraining contaminants and can result in increased germ and contaminant levels. The high air exchange rates in healthcare facilities also contribute to a “cold” feeling in patient rooms.

A displacement ventilation system uses slightly warmer ducted air introduced at or near the floor level and relies on natural warming and convection to raise it to a return system at the ceiling level. The air “pools” across the floor, gains heat from the floor, occupants, or other heat sources inside the room then rises through the breathing zone and is removed. In many U.S. hospitals this system is only used in isolation rooms or other areas with a high potential for disease transmission.

There is also economic benefit for the use of displacement type systems. Studies indicate that the same effectiveness and air quality can be achieved with four air changes per hour, versus 12 or more in most traditional designs. The equipment providing air to the space can be smaller and provide first cost savings in equipment and installation, with continued savings in operation. The air is introduced at a warmer temperature which equates directly to lower energy costs. Studies indicate the first cost savings can be 25% or more.

So why are “traditional” air supply systems still in widespread use in the United States if good alternates are available? Some codes or guidelines do not allow for use of displacement systems in inpatient rooms; there are also minimum air exchange rate requirements that can negate some of the benefits.

Change is coming. The State of California has allowed use of displacement ventilation systems as part of a study at the Kaiser Permanente Acute Care facility in Modesto, California. The projection is that energy savings of 30% will be achieved while maintaining good infection control.