Aircon Capillary Tube

A capillary tube is a narrow copper pipe that controls how much refrigerant flows into the evaporator coil on some aircon systems. It works like a straw that limits flow — the tube's diameter and length determine the correct rate of refrigerant delivery. Not all systems use capillary tubes; some use an expansion valve instead, which adjusts flow electronically.

The capillary tube sits in the sealed refrigerant circuit and has no moving parts, which means it rarely fails on its own. Its job is to maintain the right balance of refrigerant between the high-pressure and low-pressure sides of the system. When that balance is disrupted by a blockage, the entire cooling cycle is affected because the indoor coil receives too little or too much refrigerant.

Capillary tubes get clogged with moisture, debris, or frost that builds up inside the refrigerant circuit over time. A partial blockage restricts flow and you notice cooling becomes weak or unstable, with temperature swinging up and down. In more severe cases, ice forms on the indoor pipes because the coil is starved of refrigerant and the surface drops below freezing.

These symptoms look nearly identical to a refrigerant leak or an airflow blockage, which is why capillary tube problems are often misdiagnosed at first glance. A system with low gas and a system with a clogged tube both produce weak cooling and possible icing. Pressure testing under running conditions is the only reliable way to separate the two causes.

Technicians check the most common causes first — refrigerant leaks and airflow blockages — because these account for the majority of weak-cooling complaints. If those check out fine, they test the refrigerant flow and pressure behavior in the metering section while the system is running. Comparing how the system behaves under load versus when stopped reveals whether the tube is restricting flow.