Charging a system with an indoor TXV is different from charging a system with a fixed metering device. TXV’s merely require a full line of liquid in the liquid line (or “subcooling”) to perform. The TXV will then factor in the current system superheat and adjust refrigerant flow accordingly.
We are regularly asked, “What should the superheat read when using an indoor TXV?” Checking the system superheat on a system with a TXV is important, but it is not a way to check or adjust the system charge. TXV’s typically run between 12 and 20 degrees superheat. The system charge must be adjusted based on the system subcooling to verify that the TXV has a charge that will allow proper operation.
A high superheat reading on a TXV system can indicate low charge, but it can also indicate airflow problems, or restrictions. Low system superheat can indicate an overcharged system, restricted airflow, or a loose/poorly insulated sensing bulb. Both low and high superheat can also indicate a failed valve, but checking the system subcooling and airflow will confirm the diagnosis.
The system subcooling is typically listed on either the data tag on the unit, or in the installation manual.
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