Air conditioning systems rely on a variety of different parts to effectively cool your home, and one of the most important parts of any cooling or refrigeration system is the thermostatic expansion valve. This valve works to control the rate, pressure, and temperature of the refrigerant flowing into the evaporator coil inside your air handler. Here is everything you need to know to understand what a thermostatic expansion valve does, how it works, and why it is so important.

The Role of the Thermostatic Expansion Valve in Cooling Systems

To understand exactly what purpose the thermostatic expansion valve (TXV) serves, it is first necessary to go into more detail on how air conditioning systems work to cool a building. Air conditioners and heat pumps work by removing latent heat from the air inside the building and releasing this heat into the air outside. The process works by using a special chemical compound known as a refrigerant that is extremely efficient at absorbing and transferring heat.

Heat energy always wants to naturally flow from a higher temperature area to any nearby areas that are at a lower temperature, and all cooling systems use this principle to transfer heat from the inside to the outdoors. To remove heat from the air inside, the AC unit first compresses the refrigerant. Compressing it instantly reduces the pressure and turns the refrigerant from a warm gas into a cold liquid. The compressor motor then pumps the refrigerant into an evaporator coil located in the indoor air handler.

Whenever the system is currently cooling, the blower fan constantly draws hot air from the building in through the return grates. This air travels through the return duct and is then forced over the evaporator coil. The air is much warmer than the refrigerant inside the coil and thus most of the heat energy in the air is absorbed by the refrigerant. As the refrigerant removes heat from the air, it eventually transforms back into a hot gas. The gas then moves back out to the condenser coil in the AC unit where the fan helps to disperse all of the heat back into the air.

The thermostatic expansion valve is located inside the air handler where the liquid refrigerant flows into the evaporator coil, and it also has a special sensor located at the coil’s outlet where the gas exits back out towards the outdoor unit. The valve serves an essential purpose by regulating the amount of refrigerant that flows into the coil at any one time and thus the speed at which the system cools.

For the system to work properly, the refrigerant must absorb enough heat that all of it turns back into a gas before it exits the evaporator coil. Too much refrigerant flowing into the coil at one time would result in some of the refrigerant remaining in liquid form when it leaves the coil and flows back out into the compressor. Liquid refrigerant entering the compressor is a serious issue that can destroy the compressor motor and will usually force you to replace your entire AC unit. When working correctly, the TXV ensures that this can’t happen.

How a Thermostatic Expansion Valve Works

The thermostatic expansion valve works by using the sensor at the evaporator coil outlet to constantly measure the temperature of the refrigerant flowing out of it. As the refrigerant absorbs heat, it eventually begins to boil and turn into a gas. The sensor works by measuring what is known as the superheat of the refrigerant, which is an expression of how much higher the temperature of the refrigerant is compared to its boiling point.

If the superheat measurement is too low, it indicates that the system isn’t absorbing enough heat to turn all of the refrigerant into a gas. This is caused by the pressure of the refrigerant being too low when it enters the evaporator coil, which results in too much liquid flowing into it. In this situation, the TXV will partially close to slow down the flow rate and increase the pressure of the refrigerant as it enters the coil. Increasing the pressure also raises the temperature and causes some of the refrigerant to turn into a gas as it enters the coil. The more the valve closes, the higher the pressure will be, resulting in less liquid entering the coil.

An easy way to think about how the valve works is to consider it like the spray nozzle on a can of air freshener. The stuff inside the can is compressed into a liquid. As it exits the nozzle, the pressure instantly decreases and much of it transforms back into a gas.

The TXV works in the opposite way if the sensor measures that the superheat of the refrigerant is too high. When the superheat is too high, it indicates that the pressure is too high and thus the valve will open further to reduce the pressure and allow more liquid into the coil. This issue usually occurs when the indoor air temperature is much hotter and the refrigerant is absorbing far more heat in a short time. Opening up the valve allows more cold liquid refrigerant into the coil so that the system can absorb much more heat.

The amount of heat energy in the air as it is blown over the evaporator coil dictates the rate at which all of the refrigerant boils and turns into a gas. Whenever the system is running, the TXV constantly measures superheat and adjusts the refrigerant flow and pressure to ensure that the system is removing as much heat from the air as possible and that it removes enough heat so that all of the refrigerant boils and turns into gas.

What Happens If the Thermostatic Expansion Valve Fails?

As with any other component, a thermostatic expansion valve can eventually break or wear out due to age, and this will prevent the valve from opening or closing. When this happens, it can create different issues depending on which position the valve is stuck in and how much latent heat there is inside the building.

If the valve is stuck mostly open, it will result in the pressure being too low and thus allow liquid refrigerant to flow back out to the compressor. If the valve is stuck in a closed position, the pressure will be too high and much more of the refrigerant will be turned into a gas as it enters the coil. This will drastically reduce the amount of heat the system can absorb and thus result in it cooling much slower and needing to run longer and use more energy. Unfortunately, replacing a TXV can be incredibly time-consuming and expensive, but it is necessary. If you don’t have the valve replaced, your AC either won’t work correctly or could suffer major damage.

If you suspect that your thermostatic expansion valve needs to be replaced or you need any other repairs, you can count on Cook’s Air Conditioning and Heating Specialists to help. We repair and maintain all brands and models of air conditioners and can help with AC installation and replacement. Our team also works on residential and commercial heating systems throughout the Palm Coast area, and we offer a range of indoor air quality services. To schedule an AC inspection or any other service, contact us today.

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