Microprocessor Controller for DOAS
6
Sequence of Operation
Sequence of Operation
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 set point (80ºF); or
- The max defrost time (5 min) has been exceeded.
•
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.
-
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 set point.
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 set point 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 set point. If equipped with an energy
wheel, Reference Energy Recovery Wheel Sequence.
•
Temperature:
The economizer will be locked out
when:
- The outside air is greater than the economizer
high lockout (65°F).
- The unit is operating in dehumidification mode.
- There is a call for heating.
•
Temperature/Enthalpy:
The economizer will be
locked out when:
- The outside air is greater than the economizer
high lockout (65°F dry-bulb).
- The outside air is greater than the economizer
high enthalpy lockout (23 btu/lb).
- The unit is operating in dehumidification mode.
- There is a call for heating.
Dehumidification
The cooling is controlled to maintain the cold coil
set point. Dehumidification is enabled when the OA
temperature is greater than the cold coil set point plus
an offset (adj. 10ºF). Dehumidification is disabled when
the OA temperature falls below the enable point by a
hysteresis (2ºF). If equipped with BMS communications,
the user can also directly set the cold coil leaving air set
point.
•
Optional Room Relative Humidity Sensor or
Thermostat:
The controller will adjust the cold coil
leaving air temperature set point between the min
(50°F) and max (55°F) set point to satisfy the desired
space relative humidity set point.
Reheat
While the unit is dehumidifying, the supply air
temperature is maintained by controlling the reheat
device to the supply air set point.
•
Hot Gas Reheat (valve):
The microprocessor
controller modulates to maintain set point.
•
Reheat Plus:
The microprocessor controller can
be configured to use the primary heat source as
secondary reheat.
Supply Fan VFD Sequence
The factory installed VFD is wired to the controller.
Supply fan speed needs to be set during test
and balance of the unit. If equipped with BMS
