A low refrigerant charge in a system using both TEV and capillary tube will cause which pressure change?

• A. High head pressure
• B. Low head pressure
• C. No change in head pressure
• D. Unpredictable pressure

Answer: (B)

When refrigerant charge is low, there is less refrigerant circulating through the system, so the compressor moves less vapor to the condenser. With less vapor arriving, the condenser pressure (the high-side, or head pressure) drops because there’s less heat to reject and less vapor to condense. The thermostatic expansion valve will respond to the reduced evaporator load by restricting flow further, and the fixed restriction of the capillary tube means the overall mass flow to the condenser is reduced. The net effect is a lower head pressure.

That’s why a low charge tends to reduce the high-side pressure. High head pressure would usually come from overcharge, high ambient condenser temperature, or restricted airflow, not from a reduced charge.