Updated: Nov 20
In a dry climate, certain considerations must be taken prior to installing or servicing air conditioning systems. A TXV metering device and an accumulator are important factors. In a dry climate, a TXV metering device should be installed instead of a piston metering device. Not only is this important for efficiency, the TXV will help protect the compressor somewhat during low indoor humidity and high outdoor ambient temps. The TXV does this by keeping the superheat steady at around 10-14 degrees instead of allowing the superheat to lower to 0 degrees. A superheat near 0 degrees will allow the compressor to get damaged by liquid refrigerant entering it. The accumulator is an important protection device as well and we will discuss this later in this article.
When charging a system with a fixed orifice, the target superheat needs to be determined. The target superheat is a moving number so it will depend on the Indoor Wet Bulb Temp and the Outdoor Dry Bulb Temp. If you were to take an indoor WB and outdoor DB measurement in a dry climate, and line them up on a target superheat chart, you will likely see that the target superheat is not on the chart, meaning that the target superheat number for the system is too low to print (such as 5°F or lower). In that scenario, you must set a fixed orifice system at an inefficiently high superheat just in order to keep the compressor safe from having liquid refrigerant enter the vapor compressor. Liquid refrigerant entering a vapor compressor will damage it. Remember that if a system has superheat, the refrigerant entering the compressor will be in vapor form. If the system has no superheat, the refrigerant will be saturated (Saturated means that both liquid and vapor refrigerant exist in the same location) so the vapor compressor will get damaged by the saturated refrigerant entering it. Check out our full article on the Total Superheat Charging Method. Also, check out our video on the Total Superheat Method.
A TXV will keep the superheat in the evaporator steady at a preset amount. Usually this amount is 10-14°F but you may read anywhere from 8-20°F when measuring total superheat on an air conditioning system. The TXV will not allow the superheat to lower unless the heat load is so low that the TXV cannot control the superheat anymore such as in a lower indoor airflow scenario. Remember that heat is stored in the air within a building and the job of the indoor blower motor is to move that heat across the evaporator coil in order for the refrigerant within the coil to absorb the heat from the air. The refrigerant then travels to the outdoor unit where it rejects the heat. To learn more about how air conditioning systems work, how to check the charge, and how to troubleshoot, check out our book (on our website or on Amazon). Sample pages are here!
If the blower motor is broken, the TXV will not be able to hold the superheat steady because there is no heat load to work with. The TXV will still allow a small amount of refrigerant through while the system runs because it cannot shut the flow of the refrigerant off completely. However, in circumstances other than low airflow, the TXV can keep the compressor fairly safe from saturated refrigerant entering it as long as it is installed right and correctly working.
Regarding efficiency, a TXV can reduce the heat load in a building faster than a system with a fixed orifice, if there is a high heat load in the building. Over average run conditions, a TXV will allow the system to gain a higher electrical efficiency and capacity due to the efficient removal of heat and humidity. This is because the TXV will make better use of the space inside the evaporator coil by varying the amount of refrigerant allowed into the coil.
For all these reasons, a TXV metering device is a much better choice to install on a system than a fixed orifice. In locations with a dry climate such as in the state of Arizona, the positives for using a TXV grow even more, especially in terms of compressor safety. I can’t tell you how many times (it’s a lot) that I am asked about circumstances in which the target superheat calculated is 4 degrees or 0 degrees and what to do about it when checking the charge of a fixed orifice system. The answer is if you do not have a protection device such as an accumulator and you only have a fixed orifice, you would need to set the total superheat to a safer (higher) yet less efficient number for the sake of the compressor. Otherwise, the only other options are to switch the metering device to a TXV or to add an accumulator. I try to encourage those that are doing installations in these circumstances to include both the TXV and the accumulator in their new system installations.
So what is the accumulator?
The accumulator is a tank that stores saturated refrigerant and protects the compressor from liquid refrigerant entering in. If there is no superheat at the indoor coil due to a stuck open TXV or a fixed orifice during a low indoor WB and/or high outdoor DB scenario, the refrigerant will enter the accumulator tank as a saturated refrigerant and exit the tank as a vapor refrigerant. The accumulator is located directly before the compressor so no other change in the refrigerant state occurs after the refrigerant exits the tank or before the refrigerant enters the compressor.
Systems that have a rotary compressor will automatically have an accumulator tank installed because a version of the accumulator tank is mounted onto the side of this type of compressor. In the case of a heat pump, these will automatically have an accumulator tank installed in them because of the possibility of the outdoor fins freezing during heating mode. An accumulator tank can be field installed on any system if it is suspected of having the possibility of low to no superheat. The accumulator will keep the compressor safe from saturated refrigerant.
I know the addition of a TXV or an accumulator adds cost to a system install but these are crucial components that are needed on a system installed in a dry climate where low indoor WB and high outdoor DB temps are prevalent. The TXV will not only increase the safety for the compressor but for its small cost, it will increase the systems efficiency dramatically. TXV’s are also built much better than the older ones that we may find failing. This is due to the newer design of the stainless steel capillary tubing and stainless steel bulb that connect to the head of the TXV. TXV’s also make it extremely easy to check and set the system for an accurate refrigerant charge by using the subcooling method. To learn about the subcooling charging method click here for a full length article!
If you want to learn more about all the fine details on charging methods and troubleshooting, check out our book which is available on our website and on amazon. The full outline and sample pages are available here. We have a 1,000 question workbook with an answer key that you can use to apply your knowledge as well.
Check out our free quizzes to test your knowledge here!
If you want to learn the full Total Superheat Charging Method, check out this article!
If you want to learn the full Subcooling Charging Method, check out this article!
If you want to learn about Delta T, check out this article!
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Published: 6/11/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/