Greenheck DOAS v8.1 Reference Manual page 6

- The outside temperature increases by 5ºF.
- The outside humidity decreases by 20%RH, if
humidity sensor is installed.
- The unit has been locked out for more than 2
hours when a humidity sensor is not installed and
not locked out on low ambient condition.
• Defrost: Periodically, the ASHP need to initiate a
defrost cycle to remove accumulated frost from the
outside coil when operating in heating mode. The
saturated suction temperature, the outside ambient
temperature and/or the outside humidity determine
when a defrost initiates and terminates.
Initiation: One of the following must be true for a
defrost cycle to initiate:
- The saturated suction temperature is less than
-15ºF; or
- The saturated suction temperature is less than
ambient conditions (temp/dewpoint) minus an
offset (35ºF/25ºF).
Termination: The defrost cycle is terminated when
one of the following occur:
- The saturated discharge temperatures of all
refrigerant circuits are greater than the cancel
defrost setpoint (80ºF); or
- The max defrost time (5 min) has been exceeded.
Damper Control: During heat pump defrost, the
control of dampers will be determined based on the
configuration of the unit.
- 100 % Outside Air: When a recirculating damper
is NOT available, the outside air damper will
remain 100% open.
- Recirculating Air: When a recirculating damper
is installed in the unit, by default, the outside
air damper will close, and the recirculating
air damper will open to 100%. This option is
adjustable by changing the HP Defrost Damper
Position to "Control OAD" from "Full Recirc" in
the Advanced Setpoints menu.
º When "Full Recirc" is selected the following
will also occur if the unit is equipped:
» Exhaust Fan will turn off.
» Energy Recovery device will stop or
close.
• Outside Coil Fan Control: Head pressure control of
the outside fans will maintain head pressure control
by utilizing transducers on each refrigerant circuit.
The outside fan options available on the ASHP are
lead modulating or all modulating fans and utilize
refrigerant transducers to stage fans on and off in
cooling/dehumidification and heating modes.
- Cooling/Dehumidification: Reference the Active
Head Pressure Control section of the IOM for
operation in cooling and dehumidification modes
of operation.
6 Microprocessor Controller for DOAS
Sequence of Operation
Heating: In heating mode, the pressure reading
-
from the transducer is converted to a saturated
suction temperature for each circuit. The
temperature, or minimum temperature when two
circuits are present, is compared to a setpoint.
When the saturated temperature is below
setpoint, the modulating fan speed will increase
to maintain head pressure. When above setpoint,
the modulating fan speed will decrease. Non-
modulating fans, if installed, will stage on and
off based on setpoint minus/plus setpoint. This
function is similar to the cooling/dehumidification
active head pressure control for lead modulating
fans.
- Defrost: When defrost is initiated, the outside
fans turn off allowing the heat to build and defrost
the outside coil. When defrost is terminated, the
outside fans turn on to bring the pressure down
before switching back to heating mode
• Secondary Heat: A secondary heating device may
be installed in the unit. This device may be electric
heat, gas furnace, or a hot water coil. The following
sequences are available for secondary heat:
- Backup: Secondary heat only operates when
heat pump heating is not available.
- Supplemental: Secondary heat will operate
simultaneously with heat pump heating when the
compressors are not producing enough heat to
stay within 2ºF of setpoint.
Economizer
If the application requires cooling, and the OA
conditions are suitable for free cooling, the controller
will enter economizer mode. If the unit is economizing
and the discharge temperature setpoint is not being
met, the controller will bring on mechanical cooling.
If equipped with a modulating OA and recirculated
air damper, the dampers will modulate between the
min OA and max positions to maintain the supply
temperature setpoint. If equipped with energy recovery,
reference the Energy Recovery sequence.
• Temperature: The economizer will be locked out
when:
- The outside air is less than the economizer
minimum lockout (40°F) and greater than the
economizer maximum lockout (65°F).
- There is a call for heating.
• Temperature/Enthalpy: The economizer will be
locked out when:
- The outside air is less than the economizer
minimum lockout (40°F) and greater than the
economizer maximum lockout (65°F).
- The outside air is greater than the economizer
high enthalpy lockout (23 btu/lb).
- There is a call for heating.