Hoffman Controls 870-10D Fan Cycle Switch

The Hoffman Controls 870-10D fan cycle electronic head pressure control switches the condenser fan motor on and off to vary the air volume through the condenser consequently regulating head pressure for proper heat rejection in low ambient temperatures. The control is compatible with discrete speed Electrically Commutated Motors (ECM) as well as PSC or Shaded Pole induction motors and will maintain the pressure differential at the expansion valve for proper superheat. It helps preclude evaporator coil freezing and eliminates liquid and oil migration that damages or destroys compressors. The control monitors the head pressure by sensing excessive liquid sub cooling and changes the air volume through the condenser. All control parameters are embedded with no field adjustments required and is specifically applicable for TXV thermal expansion type devices using any refrigerant type. This model’s sensor input allows for the control of a single refrigerant circuit.

The 870-10D Fan Cycle Switch’s operation is dependent on the equipment’s proper refrigerant charge and resulting liquid subcooling of the liquid refrigerant resulting in 4ºF to 6ºF subcooling at ARI conditions at 95ºF ambient. The control’s sensor must be placed at the exit of the condenser, and the sensor should be insulated and weather protected. The control monitors the liquid line temperature (degrees of excessive
sub-cooling) which is directly proportional to the head pressure. Power to the motor is applied at 78°F liquid line temperature (ambient above 60ºF) and allows the motor to operate at full speed. At 60°F liquid line temperature (ambient below 40ºF) the power is removed allowing the motor to cycle “off” to maintain adequate head pressure under low ambient conditions. This control function maintains a minimum pressure differential for proper expansion of the refrigerant that maintains proper system operating conditions, while assuring proper suction pressure (evaporator temperature) over the anticipated ambient operating range.

The purpose of the 870-10D control is to assure adequate pressure in low ambient:
a) for maintaining adequate pressure differential between Hi & Low side pressures for the TXV expansion device.
b) assure an adequate suction pressure to preclude freezing of the DX coil for A/C applications.
c) eliminate oil foaming (oil migration) and/or liquid slugging.

It is extremely IMPORTANT when installing any HCC low ambient control that the installing technician evaluate the systems:
a) Liquid line temperature for the current ambient temperature.
b) Measure and determine the liquid subcooling for the ambient temperature.
c) Adjust units refrigerant charge to obtain liquid line & subcooling values on “Low” ambient vs. condensing temperatures, subcooling, and liquid line temperatures ºF chart