Orion System MHGRV III - Manual (Page 15)

Operator Interface
MHGRV III Controller Technical Guide
15
Initialization
As described on the previous page under the heading, “Power Up and
Operation,” the MHGRV III Controller uses on-board LEDs 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 III 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 the Condenser Hot Gas Valve and the other is
the Reheat Hot Gas Valve. The valves are wired to the MHGRV III
Controller Modulating Hot Gas Valve Output terminals on the control-
ler. These valves work in concert with each other to create 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 are actu-
ally a combination of these two valves working together to achieve the
specifi ed sequence of operation.
Modes of Operation
The MHGRV III 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 “CMP” 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 III 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 III Controller. If
Supply Air Temperature Reset is used, it will initiate when a 0-10 VDC
signal is supplied to the “RESET IN” input. As the voltage increases
from 0 to 10 Volts at the “RESET 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
III Controller. When a 10 Volt input signal is received at the “RESET
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 III Controller behaves as an expansion board
for a VCM controller. The controller begins the dehumidifi cation pro-
cess when the HVAC Unit controller makes a request to the MHGRV
III 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 “CMP”
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 III
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 Tempera-
ture. 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 Temperature 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 III Controller will be ignored.
Additional Features
The following are features which are also designed into the MHGRV
III Controller:
Reheat Coil Flush
To assure positive oil return to the compressor, the Hot Gas Reheat
Coil will be fl ushed of liquid refrigerant by moving the Modulating
Gas Reheat Valve to its maximum position for a short interval. If the
unit is in cooling mode, a fl ush will occur every hour. If the unit is in
dehumidifi cation mode and the valve is below 70% for 20 minutes, a
fl ush will occur.
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 III
Controller to be used in conjunction with the Modulating Hot Gas Reheat
valve. Any time 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 III 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 III Controller.
Sequence of Operation