Thermostatic Expansion Valve Basics

October 20, 2021
Thermostatic Expansion Valve in Jackson, MO

The thermostatic expansion valve, or TXV, regulates the refrigerant flow in your air conditioning system. Here is how this valve works.

Sensing Bulb

Although the sensing bulb is not inside the TXV, it is critical to the valve’s operation. The sensing bulb is next to the output end of the evaporator. When the evaporator temperature increases, the sensing bulb gets warmer. When the sensing bulb temperature increases, so does the pressure of the refrigerant. As a result, the refrigerant level rises toward the capillary tube.

Capillary Tube

The capillary tube is a small-diameter metal pipe that carries refrigerant from the sensing bulb to the TXV. As this line approaches the powerhead or top of the valve, it forms a spiral shape.


Once the refrigerant reaches the valve, it begins to fill the top portion of the powerhead. As it does so, it lowers a diaphragm that is under the powerhead. The diaphragm is a piece of thin sheet metal that drops when the pressure inside the powerhead increases.

As the diaphragm lowers, it opens the valve. This causes additional coolant to travel from the condenser to the evaporator. Since the diaphragm responds to pressure, a higher evaporator output temperature will lead to more significant pressure increases within the sensory bulb and more movement of the diaphragm.

Pressure Spring

The pressure spring is an adjustable spring located on the other side of the valve. It and pressure from the evaporator counteract the pressure from the capillary tube. These combined pressures close the valve.

As the evaporator coils cool, the decreased pressure in the sensing bulb raises the diaphragm, and the temperature increases until your room warms to your desired temperature. Then, the TXV operation cycle repeats.

Get Answers for Your TXV Questions

These valves can be confusing, and we’re here to answer TXV questions from our neighbors in Jackson and the surrounding areas. Our team at Obermanns can handle all of your heating, cooling or indoor air quality needs, including installations, maintenance and repairs. You can rely on us for a complete range of plumbing services as well. Call us today to request more information or schedule an appointment.

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