3.2.1 — Flow Controls
Flow control valves start, stop, or modulate refrigerant (or secondary fluid) flow. In refrigeration, the most common flow control actions are simple on/off (solenoid) and directional control (check valves), but many systems also use modulating valves for capacity control or pressure regulation.
Solenoid Valves (On/Off Control)
Solenoid valves typically provide automatic on/off control based on a thermostat, pressure control, defrost timer, or controller output. When closed, they should stop flow completely; if they leak internally, you can see symptoms such as evaporator flooding during off-cycle or difficulty pumping down.
Solenoid Valve Diagnostics
When diagnosing a solenoid valve, compare three things:
- Coil voltage — confirm rated voltage is present at the coil terminals when the valve should be open.
- Coil temperature — a coil that is cool when energized may have an open winding; a coil that is very hot may be running over-voltage or failing.
- Pressure differential (kPa / psig) — many pilot-operated solenoids require a minimum pressure drop across the seat to open fully. Verify the system can provide the required differential.
Check Valves (One-Way Flow)
Check valves prevent reverse flow and are often used to protect compressors, manage parallel circuits, or maintain pressure relationships during defrost or off-cycle.
Stuck-Open Check Valve
Allows refrigerant migration or reverse flow. Symptoms include evaporator flooding during off-cycle, difficulty achieving pump-down, or unexplained head pressure changes after shutdown.
Stuck-Closed Check Valve
Mimics a restriction — produces a pressure drop and downstream starvation. Often confused with a plugged strainer or metering device. Temperature comparison upstream/downstream helps confirm the fault.
Check valve condition is often confirmed by comparing temperatures upstream and downstream (a stuck-closed valve will have a noticeable temperature drop across it) and by pressure checks where gauge ports are available.