Orion System MHGRV II - Manual (Page 9)

MHGRVII Controller
Technical Guide
9
Initialization
As described on the previous page under the heading “Power Up And
Operation” the MHGRV II Controller uses an on board LED to indicate
various diagnostic conditions during power-up and operation. Please
review this information for a complete description of the controller
initialization sequence.
Modulating Hot Gas Valves
The MHGRV II controller utilizes two modulating Hot Gas Reheat
Valves to control the fl ow of Hot Gas through the Hot Gas Reheat Coil.
One of these valves is refered to as the Condensor Hot Gas Valve and
the other as the Reheat Hot Gas Valve. The valves are wired to the
MHGRV II controller Modulating Hot Gas Valve Output terminals on
the controller. These valves work in concert with each other to effect a
“three way valve” confi guration. As one closes the other opens etc.. All
modes of operation that follow referring to the Hot Gas Reheat Valve
actually is a combination these two valves working together to acheive
the specifi ed sequence of operation.
Modes Of Operation
The MHGRV II controller can be used in two different modes of opera-
tion. These modes behave in a similar manner; the main difference is the
way they receive information to control the dehumidifi cation process.
The following is a description of these modes.
Stand-Alone Operation
As the name implies, in this mode, the controller behaves as an inde-
pendent unit. The controller begins the dehumidifi cation process when
the Dehumidifi cation Input “H1” receives a 24 VAC signal from an
outside source. When the signal is received, the controller will activate
the “FAN” output to energize the HVAC unit fan. At the same time the
controller will initiate Cooling Mode by energizing the “COMP” output
starting the HVAC unit compressor. In addition the controller will open
the Hot Gas Reheat Coil by activating the “VALVE” output. At this time
the MHGRV II controller will start to modulate the Modulating Hot Gas
Reheat valve. The controller will modulate the MHGR valve to maintain
the Supply Air Temperature Setpoint by activating the stepper motor
outputs on the MHGR valve. The Supply Air Setpoint is confi gured
by setting the SETPOINT DIP Switch on the MHGRV II controller. If
Supply Air Temperature Reset is used, it will initiate when a 0-10 VDC
signal is supplied to the “RST IN” input. As the voltage increases from
0 to 10 Volts at the “RST IN” input, the Supply Air Temperature will be
reset towards the Supply Air Reset Temperature Setpoint. This setpoint
is confi gured by setting the RESET DIP Switch on the MHGRV II con-
troller. When 10 Volts input signal is received at the “RST IN” input it
will be controlling at the Supply Air Temperature Reset Setpoint. The
controller will conclude the Dehumidifi cation process when input “H1”
is deactivated, the input “Cool Override” is activated or the input “Heat
Override” is activated.
Operation As An Expansion Board
In this mode, the MHGRV II controller behaves as an expansion board
for a VAV/CAV or MUA II Unit controller. The controller begins the
dehumidifi cation process when the HVAC Unit controller makes a
request to the MHGRV II controller for dehumidifi cation. At that time
the controller will activate the “FAN” output to energize the HVAC
unit fan. At the same time the controller will initiate Cooling Mode by
energizing the “COMP” output starting the HVAC unit compressor. In
addition the controller will open the Hot Gas Reheat Coil by activating
the “VALVE” output which opens the Reheat Solenoid Valve. At this
time the MHGRV II controller will start to modulate the Modulating
Hot Gas Reheat valve. The controller will modulate the MHGR valve to
maintain the Supply Air Temperature Setpoint by activating the stepper
motor outputs on the MHGR valve. The Supply Air Setpoint is set by
programming the HVAC unit controller. If Supply Air Temperature
Reset is used, it will initiate when the HVAC sends a request to reset
the Supply Air Temperature. The Supply Air Temperature will be reset
towards the Supply Air Reset Temperature Setpoint stored in the HVAC
controller. It will send a request to move towards the Supply Air Tem-
perature Reset Setpoint based on its setpoints and confi guration. The
controller will conclude the Dehumidifi cation process when the HVAC
control sends a request to terminate Dehumidifi cation or a Cooling or
Heating Override request is made by the HVAC unit controller. Any
DIP switch settings or signals at the inputs to the MHGRV II controller
will be ignored
.
Additional Features
The following are features which are also designed into the MHGRV
II controller.
Reheat Coil Flush
During Dehumidifi cation Mode, to ensure positive oil return, the Hot
Gas Reheat Coil will be fl ushed of liquid refrigerant periodically by
moving the Modulating Gas Reheat Valve to its maximum position for
a short interval. The AUX Relay will not be energized during the Flush
Cycle. There is no Flush Cycle during the Cooling Mode.
Optional Second stage Reheat
On larger systems, where more hot gas reheat capacity may be required,
a 2 Position Hot Gas Reheat valve can be connected to the MHGRV II
controller to be used in conjunction with the Modulating Hot Gas Reheat
valve. Anytime the reheat demand moves above the Modulating Hot Gas
Reheat valve capacity, this 2 position valve would be energized to supply
additional hot gas to the Hot Gas Reheat coil. As the reheat demand is
satisfi ed the MHGRV II controller will de-energize the 2 position valve
and control reheat with the Modulating Hot Gas Reheat valve.
Reheat Solenoid Valve Control
The Hot Gas Reheat Solenoid valve for the Reheat Coil is activated when
there is a call for Dehumidifi cation. In this mode, the Hot Gas Reheat
Solenoid will be deactivated 2 minutes after the reheat demand ceases.
The Hot Gas Reheat Solenoid valve will be reactivated when a request
for reheat is received by the MHGRV II controller.
Sequence Of Operation