26
MHGRV-X Field Technical Guide
Initialization
The Reheat Expansion Module uses on-board LEDs to indicate
various diagnostic conditions during power-up and operation.
See the LED Operation section on
page 27
for more information.
Modulating Hot Gas Valves
Each Reheat Expansion Module utilizes two modulating valves
to control the flow of Hot Gas through the Hot Gas Reheat Coil.
One of these valves is the Condenser Hot Gas Valve and the
other is the Hot Gas Reheat Valve. The valves are wired to the
Reheat Expansion Module’s Modulating Hot Gas Valve Output
terminals. These valves work in concert with each other to create
a “three-way valve” configuration. As one closes, the other opens,
etc. All modes of operation that follow referring to the Hot Gas
Reheat Valve are actually a combination of these two valves
working together to achieve the specified sequence of operation.
Mode of Operation
The Reheat Expansion Module(s) can be used in MHGRV-X Stand-
Alone operation and when the MHGRV-X is communicating to
an AAON unit controller.
Stand-Alone Operation
See
page 10
for MHGRV-X Stand-Alone operation. Once the
Reheat Expansion Module’s Binary Input Compressor input is
enabled, the valve positions follow the MHGRV-X.
Operation in Communicating Mode
In this mode, the MHGRV-X behaves as an expansion board for
an AAON unit controller. See
page 10
for detailed operation.
Once the Reheat Expansion Module’s Binary Input Compressor
input is enabled, the valve positions follow the MHGRV-X.
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). If enabled, each Reheat Expansion Module follows
the same sequence as the MHGRV-X, using the same cooling
flush interval timer of the MHGRV-X. The expansion boards do
not flush at the same time.
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). If enabled,
each Reheat Expansion Module follows the same sequence as
the MHGRV-X, using the same flush reheat interval timer of the
MHGRV-X. The expansion boards do not flush at the same time.
Optional Flush Mode Configurations
A configuration option is available that will provide a 30 second
flush cycle whenever the unit enters the cooling, dehumidification,
and heat pump modes. If using Reheat Expansion Module(s) in
conjunction with the MHGRV-X, a configuration option is
available to force all the valves to flush synchronously instead
of independently.
Contact AAON Controls Support for assistance in making these
configuration changes.
APPENDIX C: REHEAT EXPANSION MODULE
Operation Modes
