Greenheck DOAS v8.1 Reference Manual page 5

• Hot Water: Microprocessor controller will modulate a
hot water valve (provided by others) to maintain the
supply temperature setpoint. Coil freeze protection
must be provided by others in the field!
• Electric Heater: Microprocessor controller will
modulate an electric heater to maintain the supply
temperature setpoint.
Cooling
The cooling is controlled to maintain the supply
temperature setpoint. The cooling will be locked out
when the outside air temperature is below the cooling
lockout (55°F).
• Chilled Water: Microprocessor controller will
modulate a chilled water valve (provided by others) to
maintain supply air setpoint. Coil freeze protection
must be provided by others in the field!
• Mechanical Cooling: The microprocessor controller
enables stages of cooling to maintain the supply
air setpoint. A single modulating compressor or a
modulating compressor with additional fixed-stage
compressors will stage on and off based on the
current supply temperature and desired setpoint.
Active Head Pressure Control
Mechanical cooling systems with integrated condenser
sections will maintain head pressure control by
utilizing transducers on each refrigerant circuit. The
pressure reading from the transducer is converted to
a saturated discharge temperature for each circuit.
The temperature, or maximum temperature when two
circuits are present, is compared to a setpoint.
The following sequences are based on the type of
condenser fan modulation installed in the unit.
• No Modulating Fans (All AC): Condenser fans
are staged using digital outputs and the saturated
discharge temperature. The first fan stages on with
the start of the first compressor. Each additional
stage turns on based on the saturated temperature
reaching setpoint plus an offset and turns off when
the temperature falls below setpoint. Built-in delays
between stages assist in staging fans off or on too
quickly.
• Lead Modulating Fan: A unit with this option has
one modulating condenser fan per fan bank. The
modulating condenser fan utilizes an analog output
to vary the speed of the fan. The modulating fan
turns on with the start of the first compressor. When
the saturated temperature is above setpoint, the
modulating fan speed will increase to maintain head
pressure. When below setpoint, the fan speed will
decrease.
Sequence of Operation
Additionally, non-modulating fans are staged using
digital outputs and an offset. Each additional stage
turns on based on the saturated temperature
reaching setpoint plus an offset and turns off when
the temperature falls below setpoint. Built-in delays
between stages assist in staging fans off or on too
quickly.
• All Modulating Fans: A unit with this option
has all modulating condenser fans. One analog
signal modulates all fans in a bank. The first fan
stages on with the start of the first compressor.
The fans modulate to maintain the saturated
discharge temperature setpoint. When the saturated
temperature is above setpoint, the fan speed will
increase to maintain head pressure. When below
setpoint, the fan speed will decrease.
Sliding Head Pressure Control
The head pressure control setpoint changes based
on the outside air temperature and an offset. As the
outside temperature increases so does the control
setpoint for the condenser fans. This feature is active in
cooling and dehumidification modes unless disabled in
the controller. Sliding head pressure control is enabled
by default.
Air Source Heat Pump
When a unit is configured as an ASHP, compressors are
used for cooling and heat pump heating. A reversing
valve is energized when the unit is in heating mode to
reverse the flow of the refrigerant. The ASHP is only
available as a packaged unit with an inverter scroll as
the lead compressor.
• Cooling: Mechanical cooling operates the same as
any other unit with compressors by controlling the
compressors to maintain the supply air temperature
setpoint in cooling mode and to maintain the cooling
coil temperature in dehumidification mode.
• Heat Pump Heating: When heat is required, the
reversing valve is switched, and the compressors
are staged to maintain the supply air temperature
setpoint.
• Heat Pump Heating Lockout: Heat pump heating
may be locked out for any of the following reasons:
- Defrost is initiated 3 times in one hour.
- Supply Air temperature is 5ºF below setpoint
for more than 10 minutes and secondary heat is
available as backup only.
- Outside ambient temperature is below the HP
ambient lockout setpoint(10ºF).
• Resetting HP Heating Lockout: One of the
following conditions must occur to return to HP
heating:
Microprocessor Controller for DOAS 5