Specifications

D) 4-pipe unit (hot water or steam and chilled water), Valve 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 cooling valve to maintain the space temperature. When the space temperature falls below the cooling
setpoint, the cooling valve shall be closed.
If the discharge air temperature falls below 55°F OR the low limit thermostat closes, the cooling valve shall be
driven closed. This valve 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 heating valve to maintain the space temperature. When the space temperature exceeds the heating
setpoint, the heating valve shall be closed.
If the discharge air temperature exceeds 130°F, the heating valve shall be driven closed, regardless of other
conditions. This valve shall remain closed until the discharge air temperature falls below 110°F.
If the discharge air temperature falls below 55°F OR the low limit thermostat closes, the heating valve shall be
driven open. This valve shall remain open until the discharge air temperature rises above 57°F AND the low
limit thermostat opens.
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 to less than 2°F above 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 both the outdoor air damper and the cooling valve closed. In heating mode, this shall
drive the heating valve open.
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 12