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IM 893-10 • ROOFPAK SINGLEZONE UNITS
Hot-Gas Bypass (HGBP)
CAUTION
Lifting points may not be symmetrical to the center of gravity
of the unit. Ballast or unequal cable lengths maybe required.
Hot-gas bypass (
) provides protection against low
suction and frosting. It also helps keep the unit operating at
light load without excessive cycling.
The system consists of a pressure regulating valve that starts
to modulate open at 100 psig (32°F). The valve is fully open at
90 psig (26°F). The factory settings can be modified in the field
as the valve is adjustable.
Adjustments are made by turning the adjustment screw
clockwise (to increase valve setpoint) or counter clockwise.
The average psi change per turn is 16 psi.
On split systems, it is recommended to mount the HGBP valve
near the condensing unit to minimize refrigerant and oil lagging
in the line upstream. The HGBP line downstream from the
valve will need to be insulated against sweating because itis
exposed to low temperatures.
NOTE:
The regulating valve opening point can be determined
by slowly reducing the system load or reducing the
required discharge air temperature setting while
observing the suction pressure. When the bypass
valve starts to open, the refrigerant line on the
evaporator side of the valve will begin to feel warm as
your hand approaches the copper tube.
HGBP is not recommended for variable speed
inverter compressors .
Figure 76: Hot-Gas Bypass (HGBP) Diagram
External Time Clock
You can use an external time clock as an alternative to (or in
addition to) the MicroTech III controller’s internal scheduling
function. The external timing mechanism is set up to open and
close the circuit between field terminals 101 and 102. When
the circuit is open, power is not supplied to binary input MCB-
BI1. This is the normal condition where the controller follows
the programmable internal schedule. When the circuits closed,
power is fed to BI1. The MicroTech III controller responds
by placing the unit in the occupied mode, over riding any
set internal schedule. For more information, see the “Digital
, “MicroTech III Applied Rooftop Unit
Controller.”
Not Recommended Hot Gas Bypass for
Vaiable Speed Inverter Compressors
