Specifications

Part Number STH12 9/10 145
Head Pressure Control Valve
Multiplex remote systems require head pressure
control valves with special settings. Replace defective
head pressure control valves only with “original”
Multiplex replacement parts.
OPERATION
The R404A head pressure control valve is non
adjustable.
At ambient temperatures of approximately 70°F (21°C)
or above, refrigerant flows through the valve from the
condenser to the receiver inlet. At temperatures below
this (or at higher temperatures if it is raining), the head
pressure control dome’s nitrogen charge closes the
condenser port and opens the bypass port from the
compressor discharge line.
In this modulating mode, the valve maintains minimum
head pressure by building up liquid in the condenser
and bypassing discharge gas directly to the receiver.
DIAGNOSING
1. Determine if the coil is clean. Air passes through
the condenser from the bottom up. Verify the coil
is clean by looking from the bottom up. Do not
look down through the fan.
2. Determine the air temperature entering the
remote condenser.
3. Determine if the head pressure is high or low in
relationship to the outside temperature. (Refer to
the proper “Normal Operating Pressures of
Refrigeration Units” in the Charts section.)
4. Determine the temperature of the liquid line
entering the receiver by feeling it. This line is
normally warm; “body temperature.”
5. Using the information gathered, refer to the chart
below.
NOTE: A head pressure control valve that will not
bypass, will function properly with condenser air
temperatures of approximately 70°F (21°C) or above.
When the temperature drops below 70°F (21°C), the
head pressure control valve fails to bypass and the ice
machine malfunctions. Lower ambient conditions can