Fresh air is introduced into the return air of most industrial and commercial air-handling systems, for several reasons:
1. to replace exhausted air or air that has been lost because of exfiltration;
2. to provide healthy levels of fresh outside air to occupants;
3. to reduce cooling costs when the temperature and humidity of the fresh air are low enough to provide free cooling, e.g., with an “economizer” cycle.
The outside- and return-air streams are usually mixed before entering the air handler when the air is heated or cooled by the heating and cooling coils.
In the winter, the mixed-air temperature may drop below freezing, so most systems are designed with a preheater to warm the outside air in the heating season. The preheater is modulated, according to a thermostat downstream of the supply fan or dampers. (The heater can be either gas-fired or electric. Although more costly to operate, electric heaters are less expensive to install and are sometimes installed if the device is used only occasionally.)
There may be circumstances in which the thermostat setpoint is not met, e.g., when the airflow is limited. For example, the fan might be off-line, or the outside-air louvers blocked, causing the heater to remain on, but without a steady airstream to transport the heat away. The excess heat may cause a fire if preventive measures are not taken.
A simple solution is to interlock the electric heater (or valve if a gas element is used) with an airflow switch. The heater will not operate if airflow is insufficient to keep the switch open. A similar circumstance applies to the cooling season when the cooling coils can freeze up during low-airflow situations.
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Excerpt from Energy Conservation Projects.