THERMOSTATS – ON or AUTO?

David R. Olson, PE, LEED AP
Most thermostats utilized in commercial applications include one or more operating functions. They all can control to a heating or cooling setpoint, or both. Some thermostats include manual changeover from heating to cooling and visa-versa, and some have a setting which will permit the thermostat to determine if heating or cooling operation is necessary. Most thermostats sold today include automatic set-back features which promote energy savings during unoccupied time periods. Many commercial thermostats have an “on-auto” fan switch. In my experience, I have seen numerous examples of this control feature being mis-understood. I will attempt to clarify this confusion in this article.

When in the “fan-on” mode, the thermostat will keep the supply fan in an operational mode. The supply fan will continue to circulate supply airflow throughout the occupied space at all times that this setting is chosen. If the thermostat senses that the surrounding temperature is lower than setpoint, it will energize the heating operation. The heater will continue to operate until the thermostat is satisfied, all the time with the supply fan operational. Then the thermostat determines that the space has adequate temperature, it will shut off the heating operation, but will keep the supply fan working. The thermostat operation for cooling operation is similar.
When the thermostat is set to the “fan-auto” position, the fan will stay off until there is a call for heating or cooling. When the thermostat senses a need for heating or cooling it will energize the supply fan and will signal the heating system or cooling system, as appropriate, for operation. When the thermostat determines that the space has reached its desired temperature, it will de-energize the heating or cooling operation and signal the supply fan to discontinue operation.

Some will argue that the “fan-auto” setting is appropriate for commercial spaces. I disagree. The International Mechanical Code requires occupied spaces to be ventilated at all times. Ventilation may be accomplished either naturally or mechanically. Natural ventilation can occur when operable windows are in place, with open or closed position is at the discretion of the occupants. By code, operable door and window area must exceed 4% of the adjacent floor area, with a maximum of one occupied space between the operable windows and an interior area without windows or doors to the outdoors.

I would suggest that most commercial buildings do not possess adequate operable windows to allow ventilation to occur in this manner. I recognize that there is a trend in modern construction to provide operable windows in commercial buildings, there still remains the potential for mis-utilizing operable windows to the detriment of building temperature controls and the resultant energy utilization. Obviously, natural ventilation cannot be tempered upward or downward from the exterior temperature condition prior to being introduced to the occupied space via operable windows. Therefore, realistically, the building is only likely to be properly ventilated during periods of moderate outdoor temperatures, perhaps 55°F to 75°F. How many people will open a window for ventilation if the outdoor temperature is excessively cold or excessively hot? Not very many, and for not very long.

When buildings do not have sufficient doors and windows to satisfy the IMC ventilation requirements, then they must be mechanically ventilated. This most commonly occurs via an adjustable outdoor air intake on a packaged rooftop air handler, or via a dampered connection between a return air duct and a source of ducted outdoor air. With either of these alternatives, ventilation can only occur when the supply fan is operating. When the fan is off, such as with the fan control switch in the “fan-auto” position and the conditioned space has a satisfied space temperature, no ventilation is occurring. This violates the building code, and can result in a stuffy space. However, if the fan is operated in the “fan-on” mode, then the air distribution system will introduce outdoor ventilation airflow even when there is no call for heating or cooling at the thermostat. The indoor air quality will remain more satisfactory due to the introduction of suitable quantities of fresh outdoor airflow.

Outdoor airflow is introduced into the return air side of the system. This is either performed by a manual or operable damper on an outside air duct or by a similar manual or automatic damper on an outdoor air source into a packaged rooftop mounted HVAC unit. By code, the building requires ventilation regardless of outdoor temperature. Therefore, due to its introduction at the return air position, the heating or air conditioning component can function automatically upon detection of the mixed air temperature, or space temperature with less sophisticated control systems. Mechanical ventilation airflow is filtered prior to being delivered to the occupied space. This enables the air handler to remove dust, pollen and similar contaminants from the ventilation airflow prior to introducing this outdoor airflow to the building.

Some residential thermostats also have a fan operation “on-auto’ switch. I suggest that it is normally appropriate to have the thermostat set in the “auto” fan position in this case. Most homes have suitable windows to provide ventilation airflow. Houses are also more prone to infiltration airflow, which serves to ventilate the home to some extent. Additionally, only rarely have I seen residential buildings with ducted outside airflow. Windows, doors and leakage must usually provide all make-up air for kitchen hoods, clothes dryers and toilet exhaust fans. This is satisfactory and normally the indoor air quality will remain acceptable.
In summary, I recommend that thermostats remain in the “fan-on” position during occupied time periods. If air handlers include automatic dampers within the outdoor air source, these should close during unoccupied time periods. Ventilation is critical for maintenance of acceptable indoor air quality. The governing building codes require ventilation at all times that a building is occupied.