CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A rooftop condensing unit operates in cold outdoor conditions (around 40°F), causing low head pressure and inadequate liquid pressure to feed the TXV properly. Which device is commonly used to maintain adequate head pressure under low-ambient conditions?
- AA crankcase heater mounted on the compressor shell
- BA suction line accumulator installed before the compressor
- CA hard start kit consisting of a start capacitor and relay
- DA head pressure control device, such as a condenser fan cycling switch or flooded condenser control valve
Show answer & explanationAnswer & explanation
Correct answer: D. A head pressure control device, such as a condenser fan cycling switch or flooded condenser control valve
Low ambient temperatures reduce condenser heat rejection needs, causing head pressure to drop below the level needed for proper TXV operation. Head pressure control devices — fan cycling switches or flooded condenser (receiver) control valves — restrict condenser capacity to maintain adequate head pressure in cold weather.
Why the other options are wrong
- A. A crankcase heater prevents refrigerant migration/oil dilution, unrelated to head pressure control.
- B. An accumulator protects against liquid floodback, not head pressure regulation.
- C. A hard start kit assists motor starting torque, unrelated to head pressure.
Low-Ambient Head Pressure Control
Devices such as fan cycling switches or flooded condenser control valves maintain adequate head pressure during cold-weather operation by limiting condenser heat rejection.
- Low ambient reduces head pressure, starving the TXV
- Fan cycling switches turn off condenser fan at low pressure
- Flooded condenser valves back up liquid to reduce effective coil surface
Memory trick: Cold outside means the fan needs to slow down.