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Metering Devices (TXV, EEV, Piston, Cap Tube)

The metering device drops refrigerant pressure into the evaporator and controls how much refrigerant flows. This sheet covers how TXVs, EEVs, fixed pistons and capillary tubes work and what to match when one is replaced.

Ref HV-0013AHRI 750

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Liquid refrigerant leaves the condenser at high pressure. The metering device restricts flow so pressure and boiling temperature drop as refrigerant enters the evaporator. A thermostatic expansion valve (TXV) adjusts flow to hold evaporator superheat, using a sensing bulb on the suction line. An electronic expansion valve (EEV) uses a stepper motor driven by a controller that reads temperature and pressure sensors. A fixed orifice (piston) is a small brass piston with a precise bore. A capillary tube is a long small-bore tube used in appliances and small systems. Fixed devices cannot adjust; the system charge controls performance.

How to identify

A TXV is a brass valve with a power head, a capillary to a sensing bulb, and sometimes an external equalizer line. Its label shows refrigerant, nominal capacity and model code. An EEV has a cylindrical coil with a wire harness. A piston sits inside a distributor or a flowrater body and has a number stamped on it that tells the bore size. A cap tube is a coiled small tube between the filter drier and evaporator.

Metering device types

Type	How it controls flow	Typical systems	Match on replacement
TXV	Bulb and spring hold set superheat	Residential and commercial AC, heat pumps, refrigeration	Refrigerant, capacity, equalizer type, bleed or hard shutoff, connections
EEV	Controller opens and closes a stepper valve	Inverter systems, VRF, mini-splits	Exact OEM valve and coil; controller compatibility
Piston (fixed orifice)	Fixed bore sized to the system	Many residential split systems	Piston size from the outdoor unit maker for that match
Capillary tube	Length and bore restrict flow	Appliances, small refrigeration	Same bore and length, or a maker-approved replacement

Key points

Bleed TXVs let pressures equalize during the off cycle, which helps compressors with low starting torque. Hard shutoff valves do not, and some compressors need a hard start kit with them.
External equalizer TXVs have an extra line to the suction. Use them where the evaporator has a large pressure drop, such as with a distributor.
Piston sizes are set by the outdoor unit's installation instructions for each indoor coil match. Do not swap pistons by guess.
Changing refrigerant type usually requires a TXV or piston change. Check the conversion guidance.
Mount the TXV bulb in good contact on a horizontal suction line, insulated, at the position in the maker's instructions.

Common mistakes

Replacing a bleed TXV with a hard shutoff valve without checking the compressor.
Leaving the old piston in a new coil match.
Mounting the TXV bulb loose or on the bottom of a large suction line.
Ordering an EEV coil by appearance instead of OEM part number.

Governing standards: AHRI 750 (thermostatic refrigerant expansion valves); equipment maker installation instructions for piston sizes and matches.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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