Modulating outdoor/return air dampers
ASHRAE Cycle 1 conformance
Tracer ZN521 applications with an outside air damper support ASHRAE Cycle 1 conformance. ASHRAE Cycle 1 operation admits 100% outdoor air at all times except during a
ASHRAE Cycle 2 conformance
Tracer ZN521 controller applications with modulating outside air damper support ASHRAE Cycle 2 conformance. ASHRAE Cycle 2 operation allows the modulating outdoor air damper to completely close when the zone temperature drops 3°F (1.7°C) or more below the active zone temper- ature setpoint. If the zone temperature rises to within 2°F (1.2°C) of the active setpoint, the damper opens to the occupied or occupied standby minimum damper positions. If the zone temperature is between 2°F and 3°F (1.2°C and 1.7°C) below the active zone temperature setpoint, the damper modulates between the minimum position and closed.
If the discharge air temperature is between the discharge air tempera- ture low limit and the discharge air temperature low setpoint, the damper modulates between the minimum position and closed. If this situ- ation is concurrent with ASHRAE Cycle 2 operation, the lesser of the two setpoints will determine the actual damper position.
Economizing (free cooling)
Cooling with outdoor air, when the temperature is low enough to make this possible, is referred to as economizing or free cooling. Tracer ZN521 controller applications with modulating outside air damper support econ- omizing. The modulating outdoor air damper provides the first source of cooling for the Tracer ZN521. The controller initiates economizing if the outdoor air temperature is below the economizer enable point (config- urable using the Rover service tool). If economizing is initiated, the con- troller modulates the outdoor air damper (between the active minimum damper position and 100%) to control the amount of outdoor air cooling capacity. When the outdoor air temperature rises 5°F (2.8°C) above the economizer enable point, the controller disables economizing and moves the outdoor air damper back to its predetermined minimum position based on the current occupancy mode or communicated minimum outdoor air damper position. If an outdoor air temperature value is not present, economizing is disabled.
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