30 November 1998
Figure 4-20j. DDC control system I/O table and data terminal strip layout for single zone HVAC system
with dual-temperature coil.
b. General sequence of operation.
(1) Supply fan off. When the fan is off, the outside air and relief air dampers are closed and the
return air damper is open. The dual-temperature coil valve is under control.
(2) Supply fan operating. When the fan is on, the control dampers and the dual-temperature coil
valve are operated as required by the system's operational modes. The control dampers are positioned
for full recirculation of air, positioned to introduce minimum outside air, or modulated to maintain space
temperature. The dual-temperature coil valve is under control.
(3) Control of supply fan. Unless the fan is stopped as the result of a safety shutdown, it is on or
off as required by the control system mode of operation.
(4) Safety shutdown of the fan. The control system shuts down the fan if there is a low
temperature condition or if smoke is detected.
(5) Low temperature detection. On a fall in temperature to its setpoint, a low temperature
protection thermostat stops the supply fan. To restart the fan, the thermostat and the control panel must
be manually reset.
(6) Smoke detection. Duct-smoke detectors stop the supply fan whenever either detects the
presence of smoke. To restart the fan, the smoke detectors and control panel must be manually reset.
(7) Filter condition. Filter condition is monitored by a pressure gauge and a differential pressure
switch. When the rise in pressure drop across the filter reaches the switch setpoint, the switch turns on a
(8) Economizer control. When the control system's mode of operation no longer requires the
outside air, return air, and relief air dampers to be in their full recirculation positions, the dampers are
positioned to admit outside air for ventilation. When both these contacts close, the dampers are
modulated as part of the space temperature control.
(9) Space temperature control. The space temperature controls have different sequences
depending on whether hot water or chilled water is being delivered to the unit dual-temperature coil.
(a) Whenever hot water is being supplied to the unit's dual-temperature coil, on a rise in
space temperature, the dual-temperature coil valve is modulated toward closed. On a further rise
through the temperature deadband, the controls make no changes. Then, on a still further rise in space
temperature above the deadband, the outside air and relief air dampers are modulated from the
minimum position toward fully open, and the return air damper is simultaneously modulated toward
(b) Whenever chilled water is being supplied to the unit's dual-temperature coil, on a rise in
space temperature above the deadband, economizer control permitting, the outside air and relief air
dampers are modulated from minimum position toward fully open, and simultaneously the return air
damper is modulated toward fully closed and the dual-temperature coil valve is modulated toward open.