APPENDIX C - REHEAT EXPANSION MODULE
MHGRV-X Field Technical Guide
24
Initialization
The Reheat Expansion Module uses on-board LEDs to indicate vari-
ous diagnostic conditions during power-up and operation. See the
LED Operation section on
page 25
for more information.
Modulating Hot Gas Valves
Each Reheat Expansion Module utilizes two modulating 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 Reheat Expan-
sion Module’s Modulating Hot Gas Valve Output terminals. 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 opera-
tion that follow referring to the Hot Gas Reheat Valve are actually
a combination of these two valves working together to achieve the
specifi ed 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 8
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 Controller.
Operation in Communicating Mode
In this mode, the MHGRV-X Controller behaves as an expansion
board for an AAON Unit controller. See
page 8
for detailed opera-
tion. Once the Reheat Expansion Module’s Binary Input Compres-
sor input is enabled, the valve positions follow the MHGRV-X
Controller.
Additional Features
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 Modulat-
ing Gas Reheat Valve to its maximum position for a short interval.
Cooling Flush:
If the unit is in cooling mode, a fl ush will occur
when the unit’s fl ush cooling interval timer has elapsed. The time
is accumulated whenever it is in cooling mode and resets after each
fl ush cycle. The fl ush cooling interval timer is a setpoint that is
confi gurable using the keypad and display (0 to 120 minutes in 10
minute increments). If enabled, each Reheat Expansion Module fol-
lows the same sequence as the MHGRV-X, using the same cooling
fl ush interval timer of the MHGRV-X. The expansion boards do not
fl ush at the same time.
Reheat Mode Flush:
If the unit is in dehumidifi cation mode and
the valve is below 70% for the fl ush reheat interval timer value, a
fl ush will occur. If the valve goes above 70%, the timer is reset. The
fl ush reheat interval timer is a setpoint that is confi gurable 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 fl ush reheat interval timer of the
MHGRV-X. The expansion boards do not fl ush at the same time.
Optional Flush Mode Confi gurations:
A confi guration option is
available that will provide a 30 second fl ush cycle whenever the unit
enters the cooling, dehumidifi cation, and heat pump modes. If using
Reheat Expansion Module(s) in conjunction with the MHGRV-X
Controller, a confi guration option is available to force all the valves
to fl ush synchronously instead of independently.
Contact AAON Controls Support for assistance in making these
confi guration changes.
Operation Modes
