11
MHGRV-X Field Technical Guide
OPERATION MODES
Additional Features
5. Relay Operation
• Fan Relay will be activated in this mode.
• Compressor Relay will be activated in this mode.
• Valve Relay will be activated in this mode after
the initial 30 seconds.
• Auxiliary Relay
○
It will be activated if the Reheat Valve stays
at 100% for two minutes and the Supply Air
Temperature is 5°F below the Supply Air
Setpoint.
○
It will deactivate if the Reheat Valve stays at 0%
for two minutes and the Supply Air Temperature
is 5°F above the Supply Air Setpoint.
Communications Operation
In this mode, the MHGRV-X behaves as an expansion board for an
AAON unit controller. The controller begins the dehumidification
process when the AAON unit controller makes a request to the
MHGRV-X for dehumidification. 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 MHGRV-X will open the Hot Gas Reheat Coil by
activating the “VALVE” output which opens the Reheat Solenoid
Valve. At this time, the MHGRV-X 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 unit controller ends 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 unit controller. It
will send a request to move towards the Supply Air Temperature
Reset Setpoint based on its setpoints and configuration.
The controller will conclude the Dehumidification process
when the HVAC unit controller sends a request to terminate
Dehumidification or a Cooling or Heating Override request is
made by the HVAC unit controller. Any setpoints or signals at
the inputs to the MHGRV-X will be ignored.
Auxiliary Mode Communicating
1. Same as the Stand-Alone Auxiliary Mode, but the
Reheat Enable and Auxiliary Valve position will be
sent using E-BUS commands.
Additional Features
Reheat Coil Flush
To assure positive oil return to the compressor, the Hot Gas
Reheat Coil will be flushed of liquid refrigerant by moving the
Modulating Gas Reheat Valve to its maximum position for a
short interval.
Cooling Flush
If the unit is in cooling mode, a flush will occur when the unit’s
flush cooling interval timer has elapsed. The time is accumulated
whenever it is in cooling mode and resets after each flush cycle.
The flush cooling interval timer is a setpoint that is configurable
using the keypad and display (0 to 120 minutes in 10 minute
increments).
Reheat Mode Flush
If the unit is in dehumidification mode and the valve is below
70% for the flush reheat interval timer value, a flush will occur.
If the valve goes above 70%, the timer is reset. The flush reheat
interval timer is a setpoint that is configurable using the keypad
and display (0 to 120 minutes in 10 minute increments).
Condenser Coil Flush
If Condenser Coil Flush is configured and the unit is in
Dehumidification Mode and the reheat valve is above 70% for
the flush reheat interval timer value, a condenser flush will occur.
This shares the timer with the reheat flush so if the valve is below
70% a Reheat Flush would occur.
Optional Second Stage Reheat
On larger systems, where more hot gas reheat capacity may be
required, a two-position Hot Gas Reheat Valve can be connected
to the MHGRV-X 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 two-position
valve would be energized to supply additional hot gas to the
Hot Gas Reheat Coil. As the reheat demand is satisfied, the
MHGRV-X will de-energize the two-position valve and control
reheat with the Modulating Hot Gas Reheat Valve.
