Specifications

A) 2-Pipe unit (steam/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
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.
When the space temperature exceeds the cooling setpoint, a proportional-integral control loop shall position
the face & bypass damper to maintain the space temperature.
The face and bypass damper shall be driven closed (full bypass) if the discharge air temperature falls below
55°F. The damper shall be held 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.
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.
The face and bypass damper shall be driven open (full face) if the low limit thermostat trips (closes) OR the
discharge air temperature falls below 55°F. The damper shall remain open until the low limit thermostat is
opened AND the discharge air temperature exceeds 57°F.
In heating mode, the face & bypass damper shall be closed when the discharge air temperature exceeds
130°F. This damper shall remain closed until the discharge air temperature falls below 110°F.
End-of-Cycle Valve Operation (Optional)
A normally open, 2-position valve with return on power failure is optionally used as an end-of-cycle (EOC)
valve. This valve prevents radiant effects from overcooling/overheating the space when the face & bypass
damper is in the full bypass position. The EOC valve shall open when the face & bypass damper is
commanded to open. If the outdoor air temperature falls below 40°F, the EOC valve shall be forced open,
regardless of the face & bypass damper position.
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 force the face & bypass
damper into full face position and the optional EOC valve shall open. In cooling mode, this shall force the face
& bypass damper into full bypass position and the optional 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 9