Step 5 Choose a Whole-Building Ventilation Control Strategy

Continuous Operation

The amount of airflow required by ASHRAE Standard 62.2 for whole-building ventilation is based on continuous ventilation. The simplest control strategy is to operate the whole-building ventilation system continuously to provide the airflow calculated in Step 3. A labeled manual control must be provided to allow the occupant or owner to override automatic operation. This control may be a switch or time controller that allows the occupant or owner to modify the operating schedule or speed temporarily, or to turn off the fan when the home is not occupied for long periods. To achieve the lowest possible operating costs for ventilation, choose fans that have the lowest possible wattage.

Intermittent Operation

To compensate for less operating time, intermittent operation requires the whole-building ventilation fan to operate at a higher flow than would be required for continuous operation. The required intermittent flow is the inverse of the percentage of on-time in the duty cycle, as long as the duty cycle is no longer than four hours. All intermittently operated whole-building ventilation systems require a programmable timer as a control.

For example: a duty cycle of one hour on/three hours off requires four times the whole-building ventilation airflow calculated in Step 3. The on/off cycle cannot exceed four hours. If the ventilation duty cycle exceeds four hours, more complex calculations are required to determine how large the airflow must be for whole-building ventilation.

Ventilation Systems Integrated with a Central Air Handler (CFIs)

A number of companies manufacture controls that use the central HVAC air handler to bring in outdoor air and distribute it throughout the house for whole-building ventilation. The most common of these controls includes an accumulated-runtime feature that looks back over the past hour (or three hours) and determines whether the air handler has operated for enough time to provide the required amount of outdoor air for ventilation. If it has, the control "decides" not to operate the air handler again until the end of the duty cycle. If the air handler has not operated for the designed percentage of the duty cycle, (typically one-third, for a total of 8 hours out of 24), then it turns on the air handler for the required number of minutes to meet that 33% on-time requirement. The intermittent whole-building ventilation formula says that a system that operates for 1/3 of the time must bring in at least three times the amount of outdoor air that would have been required if the system was operated continuously, assuming a duty cycle of four hours or less.

So if a house needed 60 cfm of continuous ventilation, setting the control for 33% on-time would require that 180 cfm be brought in during that 1/3 on-time. If it were set for 25% on-time, then it would require 240 cfm (4 x 60 cfm = 240 cfm). The duty cycle cannot exceed 4 hours or a ventilation effectiveness factor has to be included, driving up the required ventilation rate even higher.

Because systems using an air handler depend on the small amount of negative pressure in the return air plenum to bring in outdoor air, it is very difficult to prescriptively design the duct. The designer also has to choose an air handler speed that will provide the necessary level of negative pressure to draw in outdoor air. Reducing the air handler speed to avoid comfort problems and to lower fan energy costs reduces the outdoor air intake significantly. (Cutting air handler speed by half cuts outdoor air intake to a quarter.) Also, the airflow has to be measured to verify the required ventilation rate.