Components

Expansion valve

The metering device that drops refrigerant pressure before the evaporator and controls how much liquid enters it. Thermostatic expansion valves respond mechanically; electronic expansion valves are driven by the control board and report their own faults.

Also called TXV, EEV, metering device

Direct answer

What does an expansion valve do?

An expansion valve is the metering device that drops refrigerant pressure before the evaporator and regulates how much liquid enters it, which is what sets superheat.

Expansion valve: how it works

The valve holds a pressure difference between the high side and the low side, so liquid arriving under pressure flashes to a cold mixture as it passes through. A thermostatic valve senses suction line temperature through a bulb and opens or closes mechanically; an electronic valve is a stepper motor the board drives, which lets it respond faster and log what it did.

Where you meet the term

  • In a superheat reading, because the valve is what that number is measuring the behaviour of.
  • In an EEV fault code, where the board could not confirm the valve reached the step position it commanded.
  • In a capacity complaint on a system with a correct charge, where metering rather than charge is the remaining explanation.
Technician work

How to check it

  1. Measure superheat at the evaporator outlet. Superheat is the direct read on what the valve is doing. Persistently high or low readings with a correct charge point at metering. Technician
  2. Verify the sensing bulb contact and insulation on a TXV. A bulb that has slipped or lost its insulation reads the wrong temperature and drives the valve to the wrong position, which looks like a valve fault. Technician
Read this twice

Expansion valve: what it is not

  • A metering fault is not the same as a charge fault. Both change superheat, which is why subcooling is read alongside it before anything is adjusted.
  • Not every system has a valve. Fixed orifice and capillary systems meter by restriction alone and cannot adjust to conditions.

Expansion valve questions

What is the difference between a TXV and an EEV?

A TXV reacts mechanically to a temperature bulb on the suction line. An EEV is a stepper motor the control board positions, so it responds faster, holds tighter control, and can report its own position as a fault code.

Can an expansion valve cause a frozen coil?

Yes. A valve that underfeeds the evaporator drops the boiling temperature inside it, and if that falls below freezing the coil ices in the same way it does when airflow is restricted.

Is an expansion valve owner serviceable?

No. It sits inside the sealed refrigerant circuit, so any work on it requires recovery and recharge by a certified technician.

Where this definition comes from

Written from standards body publication, government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • ASHRAE TerminologyASHRAE · standards body publicationIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.
  • Section 608 Technician CertificationUnited States Environmental Protection Agency · government guidanceWho may open a refrigerant circuit in the United States, and under which certification type.