Specifications

C) 4-pipe unit (hot water or steam and chilled water), Damper Control
Fan Operation
The supply fan shall run continuously in the occupied and tenant (unoccupied) override modes. The fan shall
cycle with calls for cooling or heating in the unoccupied mode.
Cooling Operation
When the space temperature exceeds the cooling setpoint a proportional-integral control loop shall position
the face & bypass damper to maintain the space temperature. When the space temperature falls below the
cooling setpoint, the face & bypass damper shall be closed (bypass).
If the discharge air temperature falls below 55°F OR the low limit thermostat closes, the face and bypass
damper shall be driven closed (full bypass). This damper shall remain closed until the discharge air
temperature rises above 57°F AND the low limit thermostat opens.
Heating Operation
When the space temperature falls below the heating setpoint, a proportional-integral control loop shall position
the face & bypass damper to maintain the space temperature. When the space temperature exceeds the
heating setpoint, the face & bypass damper shall be closed (bypass).
If the discharge air temperature falls below 55°F OR the low limit thermostat closes, the face and bypass
damper shall be driven open (full face). This damper shall remain open until the discharge air temperature
rises above 57°F AND the low limit thermostat opens.
If the discharge air temperature exceeds 130°F, the face & bypass damper shall be driven closed. This
damper shall remain closed until the discharge air temperature falls below 110°F.
End-of-Cycle Valve Operation
2-position valves with return on power failure are used as end-of-cycle (EOC) valves. These valves prevent
radiant effects from overcooling/overheating the space when the face & bypass damper is in the full bypass
position.
If the space temperature is at or above the cooling setpoint, the cooling EOC valve shall open. This valve shall
close when the when the space temperature falls below the cooling setpoint. If the low limit thermostat trips
OR the discharge air temperature falls below 55°F, the cooling EOC valve shall drive closed.
If the discharge air temperature exceeds 130°F, the heating EOC valve shall close and remain closed until the
discharge air temperature drops below 110°F, regardless of other conditions. If the outdoor air temperature
falls below 40°F OR the low limit thermostat closes, the heating EOC valve shall be opened and shall remain
open until the outdoor air temperature exceeds 44°F AND the low limit thermostat opens. If the space
temperature falls below the heating setpoint, the heating EOC valve shall open.
Outdoor Air Damper Control
The outdoor air damper shall be closed in the unoccupied mode, regardless of the space temperature.
The outdoor air damper shall be closed in low limit conditions (See Safeties).
The outdoor air damper shall be closed if the space temperature falls more than 3°F below the heating
setpoint. The damper will not open until the space temperature rises within 2°F of the heating setpoint.
The outdoor air damper shall be closed if the space temperature rises more than 3°F above the cooling
setpoint. The damper will not open until the space temperature falls within 2°F of the cooling setpoint.
The outdoor air damper shall close to minimum position when the outdoor air temperature exceeds 68°F
The outdoor air damper shall be controlled with a proportional control loop based upon the deviation from
setpoint. The outdoor air damper shall begin to modulate open from minimum position when the space
temperature exceeds the (space setpoint - 0.5°F). The outdoor air damper shall be fully open when the space
temperature rises 1.5°F above the setpoint. Except as noted in the paragraphs above, the outdoor air damper
shall not close beyond minimum position during occupied operation.
Safeties - Low Limit Conditions
If the low limit thermostat trips OR the discharge air temperature falls below 55°F, the unit enters low limit
mode. This shall drive the outdoor air damper closed. In heating mode, this shall force the face & bypass
damper into full-face position and the optional heating EOC valve shall open. In cooling mode, this shall force
the face & bypass damper into full bypass position and the optional cooling EOC valve shall close.
Normal unit operation shall resume when the discharge air temperature exceeds 57°F and the low limit
thermostat opens.
7/1/03 Part Number 011-200001-003 Page 11