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Marvair ComPac AVHDA I&O Manual 03/2019 Rev.3
MODEL
Unit Weight
Shipping Weight
Shipping Dimensions
Height
Width
Depth
LBS
KG
LBS
KG
Inches
MM
Inches
MM
Inches
MM
AVHDA90
1,160
527.3
1,285
584.1
98
2,489
56
1,422
48
1,219
Table 8. Shipping Weights & Dimensions
1.5 General Operation
Hot Gas By-Pass
Normally used in specialty applications (i.e Magnetic Resonance Imaging (MRI) buildings) to prevent
magnetic voltage disturbance caused by cycling. This technology is applied in this product to extend
the operation envelope for the compressor to 20º F (-6.6ºC). Combined with a condenser low ambient
Fan Cycle feature, compressor operation can be extended to 0º F (-17.8ºC). During Hot Gas operation
mode, system performance will be reduced. If product operation is in mild outdoor ambient conditions,
the installed shut-off ball valve may be closed, thus disabling the Hot Gas By-Pass feature.
Refrigerant Cycle (Cooling Mode)
The air conditioners use R-410A refrigerant in a conventional vapor-compression refrigeration cycle to
transfer heat from air in an enclosed space to the outside. A motorized impeller assembly blows indoor
air across the evaporator. Cold liquid refrigerant passing through the evaporator is boiled into gas by
heat removed from the air. The warmed refrigerant gas enters the compressor where its temperature
and pressure are increased. The hot refrigerant gas condenses to liquid as heat is transferred to outdoor
air drawn across the condenser by the condenser fan. Liquid refrigerant is metered with a thermal
expansion valve (TXV) into the evaporator to repeat the cycle.
AVHDA (Dual Compressor) Units:
The AVHDA is factory wired for maximum cooling utilizing both
compressors. If 2 stage compressor operation is desired, the factory installed jumper between terminals
1 and 2 of the low voltage terminal strip must be removed. The 1st stage cooling input is terminal 1 of
the low voltage terminal strip and the 2nd stage cooling input is terminal 2 of the low voltage terminal
strip. The thermostat must be programmed for 2 stage cooling operation when 2 stage compressor
operation is desired.
Heating Mode
A wall-mounted thermostat controls the heating cycle of models which incorporate resistance heating
elements. On a call for heat, the thermostat closes the heat relay to energize the indoor fan and the
resistance elements.
1.6 Electronic Control Board Mode of Operation
Normal
24 VAC power must be continuously applied to “R” and “C”. Upon a call for cooling “Y” and with the
high pressure switch (HPS) closed, the compressor will be energized. (Note: See the delay on make
feature.) The compressor will remain energized during the 3 minute timed low pressure by-pass cycle. If
the low pressure switch (LPS) is open after the 3 minute by-pass cycle, the compressor will de-energize.
Lock-out
If either of the fault conditions (LPS or HPS) occurs twice during the same call for cooling, the control
board will enter into and indicate the lockout mode. In the lockout mode, the compressor is turned off.
If there is a call for indoor air flow “G”, the blower remains energized, the alarm output is energized and
the status LED will blink to indicate which fault has occurred. When the lockout condition is cleared, the
unit will reset if the demand for the thermostat is removed or when the power is reset. With the control
board, the user can now have either normally closed or normally open remote alarm dry contacts. The
air conditioners are factory wired to be normally open.
