Specifications

B) 2-Pipe Unit (steam/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
The unit enters cooling mode when the optional changeover t'stat contacts close to indicate that chilled water
is available. Since there is no changeover t'stat on heating only units, such units will not enter mechanical
cooling mode. Heating only units will use outdoor air, when applicable, for cooling; please see the Outdoor Air
Damper Operation section for operational details.
When the space temperature exceeds the cooling setpoint, a proportional-integral control loop shall position
the control valve to maintain the space temperature.
If the discharge air temperature falls below 55°F, the control valve shall be closed. The valve shall remain
closed until the discharge air temperature rises above 57°F.
Heating Operation
The unit enters heating mode when the optional changeover t'stat contacts open to indicate that hot water is
available. Heating only units shall not have a changeover t'stat; such units operate only in heating mode.
When the space temperature falls below the heating setpoint, a proportional-integral control loop shall position
the control valve to maintain the space temperature.
The control valve shall be driven open if the low limit thermostat trips (closes) OR the discharge air
temperature falls below 55°F. The valve shall remain open until the low limit thermostat is opened AND the
discharge air temperature exceeds 57°F.
In heating mode, the control valve shall be closed when the discharge air temperature exceeds 130°F. The
valve shall remain closed until the discharge air temperature falls below 110°F.
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, while chilled water is available (changeover t'stat closed). The damper will not open until the space
temperature falls to less than 2°F above the cooling setpoint. If chilled water is not available, the damper
continues to operate under normal control.
The outdoor air damper shall close to minimum position when the outdoor air temperature exceeds 68°F, while
chilled water is available. If chilled water is unavailable, the damper continues to operate under normal control.
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 drive the control valve open.
In cooling mode, this shall drive the control valve closed.
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 10