The metering device sits between the high and low sides of a system, controlling how much refrigerant enters the evaporator. The two common types — thermostatic expansion valves and fixed orifices — behave differently, including how you charge them.
Thermostatic expansion valve (TXV/TEV)
A TXV actively modulates refrigerant flow to maintain a target superheat, adapting to changing conditions. This makes it efficient and stable across a wide range of loads. Charge TXV systems by subcooling, since the valve already controls superheat.
Fixed orifice (capillary tube or piston)
A fixed orifice is a simple, fixed restriction with no moving parts. It’s cheap and reliable but less adaptable to varying conditions. Charge fixed-orifice systems by superheat, typically using a charging chart based on indoor and outdoor temperatures.
Why the difference matters
Charging by the wrong method gives misleading results. Always identify the metering device first, then charge to the correct target (subcooling for TXV, superheat for fixed orifice) per the manufacturer’s figures.
FAQ
How do I know which metering device a system has? Check the manufacturer’s documentation or inspect the liquid line near the evaporator — a TXV has a sensing bulb clamped to the suction line.
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