An article from Lennox Industries Inc. publications

There are many parts that make up a quality Lennox cooling product. One of these parts is an expansion valve. Many Lennox cooling products come with an expansion valve, the importance of which is often overlooked.

An expansion valve is very uniquely designed device in an air conditioning or heat pump system that controls the super heat to the compressor over a wide range of conditions. So why should you use an expansion valve on an air conditioning or heat pump system versus a capillary tube or other fixed orifice device? We’re glad you asked! The following covers many advantages of using an expansion valve:


The SEER of a single speed air conditioning system is determined from a single steady state operating condition (at 82 degrees outdoors) and a cyclic test. The rated cooling capacity is determined at the same indoor conditions, but at 95 degrees outdoors. Fixed orifices are typically sized to provide maximum performance at 95 degrees, although some manufacturers are now sizing them at a condition somewhere between 82 and 95 degrees in order to get better seer ratings. The net result is, there is typically not much difference in SEER ratings between expansion valve systems and fixed orifice systems; but expansion valve systems are more efficient in reality because they ensure that the evaporator coil always operates at peak efficiency at all conditions.


Sizing a fixed expansion device at a given outdoor and indoor condition will provide the proper superheat at that condition. AS the outdoor temperature drops, the superheat will increase if the indoor condition is the same. AS the outdoor temperature increases, the superheat will decrease. The superheat also changes as the indoor load changes. Higher indoor loads will also cause the superheat to increase while lower loads will cause the superheat to decrease. An expansion valve can adjust to changes in the system so operation is better over a range of situations.


In today’s energy conscientious society, most homeowners adjust their thermostat settings (manually or with electronic programmable thermostats) depending on whether the space is being used or not. Daytime setup or nighttime setbacks are typical conditions that are prevalent in many homes today. In addition to this, most parts of the country experience a wide range of temperature and humidity conditions. This makes it impractical to expect a fixed orifice device to properly feed the evaporator coil in many applications the way an expansion valve can.


With a fixed orifice device, the superheat is very sensitive to just a few ounces of refrigerant. A system can easily be overcharged or undercharged. With an expansion valve system, most systems can be undercharged or overcharged by up to 10% and still operate very close to rated performance because the expansion valve will still be able to control the superheat coming out of the coil. This always ensures that the coil is operating at peak performance.


Because there are so many choices, there is high likelihood the air conditioning/evaporator unit installed in a home is not a tested combination. This makes it even more difficult to charge a fixed orifice system. To make matters worse, many coil manufacturers will factory install a fixed orifice or capillary tube in the evaporator coil. Since the proper size of the orifice device is much more dependent on the condensing unit that then evaporator coil, this requires these types of coils to be matched to a specific condensing unit. Within reason, expansion valves make it much easier to overcome mix-matched systems since they will control the superheat no matter what coil is installed.


If a fixed orifice allows liquid refrigerant to get back to the compressor, the compressor bearing can be damaged. Liquid refrigerant in the compressor also lowers the temperature of the compressor to the point where liquid refrigerant will migrate to the compressor during the off cycle (refrigerant naturally migrates to the coldest spot). This is a sever operating condition for the compressor on startup that can also cause the oil to be pumped out of the compressor and lead to bearing damage. Higher superheat will cause the compressor motor to operate at higher temperatures that can cause the motor windings to deteriorate prematurely.