Learning About Furnace Upgrades and Repairs

As the weather heats up, it's time to turn up the air conditioning and cool down. However, for owners of window units or other small cooling systems, it is helpful to understand what can go wrong with their units. Below is more information on a component you may never have heard about: the capillary tube. Information is given on its function as well as a guide to inspection.

What is a capillary tube and what does it do?

A capillary tube merely consists of a thin metal tube within in a cooling system, but its simplicity belies its importance. Whether it is an air conditioner or refrigerator, the tube provides a bottleneck for the flow of refrigerant from the high-pressure side of a system toward the low-pressure side. As refrigerant decreases in pressure, it boils and evaporates; refrigerant evaporation occurs at extremely low temperatures, and this process cools the surrounding evaporator coil. The evaporator coil, in turn, is the source of cooling for air or refrigerated space.

Larger air conditioning systems often use a thermostatic expansion valve instead of a capillary tube but, capillary tubes are prized in smaller systems for their simplicity and low cost. The capillary tube is critical to the functioning of the cooling system because it regulates the amount of refrigerant passing into the low-pressure side of the system. Either too much or too little flow will result in inadequate cooling, and the tube is measured to a precise length and diameter to provide optimal cooling power.

How to check your capillary tube

If you are having difficulty with your small air conditioning system or window unit, then a good place to check for trouble is the capillary tube. While replacement of the tube requires advanced skills suited for a professional technician, finding problems with the tube is often simply a matter of visual inspection. Below is how you can check your capillary tube for obvious signs of malfunctioning:

1. Remove the power source of your air conditioner – always unplug an air conditioner or turn off the power at the circuit breaker panel before performing any work. Allow the unit to sit idle for at least 1 hour after removing the power so any capacitors can slowly release their charge; a charged capacitor is capable of giving you a painful, or even fatal, electric shock.

2. Open the unit – for many window units, the cover is attached via screws in the rear and the cover can be pulled up and over the unit. However, you may need to consult your owner's manual for specific information about opening the unit.

3. Locate the evaporator coil – the evaporator coil in most cooling systems consists of a long pyramid-shaped structure that contains cooling fins on its surface. Tubing can be seen entering and exiting the coil housing. On one end of the evaporator coil, you will see thin metal tubing coiled together, and this will be the capillary tube.

4. Inspect the tube carefully -  below are several potential problems that you can identify, so be sure to learn the list below or write it down before performing your air conditioner inspection:

• Crushed or crimped tubing – this can block the flow of refrigerant altogether, or it can trap passing debris which could cause internal damage.
• Frosted or icy tubing – capillary tubes should be cool, but dry, to the touch. Frosted tubes are evidence that the  refrigerant amount is insufficient for the system's needs.
• Shaky or loose tubing – capillary tubes should be firmly mounted in position and not permitted to rattle or vibrate. Any type of repetitive motion can cause the loose tubing to rub against another object and eventually introduce holes into the material.

What to do if you find a deficiency in your capillary tube

If you locate a problem with the capillary tubing, you will want to contact a licensed heating and air conditioning technician for help. They can provide you with additional info and a more definitive diagnosis of the trouble and perform repairs or replace the tubing altogether.