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Cooling · RepairAC Expansion Valve (TXV) Replacement
AC expansion valve (TXV) replacement is the fix when the thermostatic expansion valve sticks and starves the indoor coil, but the valve is condemned only after three readings agree: low suction pressure, high superheat, and normal or high subcooling. Low suction with low subcooling is a refrigerant leak, not a valve. The job opens the sealed circuit, so it is priced in writing after a $90–$150 diagnostic.
The thermostatic expansion valve meters liquid refrigerant into the evaporator coil, opening and closing to hold a steady superheat at the coil's outlet. When it sticks nearly shut, too little refrigerant enters. Suction pressure falls, the first few tubes of the coil frost while the rest stays warm, and the house cools slowly or not at all. On the gauges this looks like a system low on charge, and that is how it gets misdiagnosed: refrigerant is added, the pressures barely move, and the system ends up overcharged with the original fault still in place.
Subcooling separates the two faults. A system that has leaked has little liquid stacked in the condenser, so subcooling is low. A system with a restricted valve has its full charge backed up in the condenser, so subcooling is normal or high while superheat is high. The technician also checks for a temperature drop across the filter-drier, because a plugged drier upstream produces the same starved coil.
The sensing bulb is checked before the valve is blamed. It is a small copper bulb strapped to the suction line at the coil outlet, and it tells the valve how far to open. It has to be clamped tight on clean pipe, at the position the manufacturer shows and never on the underside where oil runs, and wrapped in insulation so it reads the pipe and not the plenum air. A loose, bare or misplaced bulb makes a good valve hunt or flood; a bulb whose capillary tube has rubbed through has lost its charge, and the valve stays shut.
Replacement is a refrigerant job from start to finish. The charge is recovered, the old valve is cut or unthreaded from the coil, and the new one, matched to the refrigerant and tonnage, is brazed in with its body wrapped in a wet rag or heat-blocking paste so the torch does not cook the internals. Nitrogen flows through the pipe during brazing, a new filter-drier goes in, and the circuit is pressure-tested and evacuated before the charge is weighed back. Some coils take a valve with threaded fittings, which removes the brazing but not the recovery or the vacuum.
Signs that point to the expansion valve
- Frost on the coil inlet or the small tubes feeding it, with the rest of the coil warm
- Weak cooling that did not improve after another company added refrigerant
- Suction pressure low and superheat high on the gauges, with subcooling normal or high
- The suction line and compressor sweating or frosting heavily, the sign of a valve stuck open and flooding
- Pressures that swing up and down every minute or two as the valve hunts
How it works
With the system running: suction and head pressure, superheat, subcooling, and the temperature on both sides of the filter-drier. If you have a previous invoice showing refrigerant added, have it ready; the amount tells us how far the charge may be over.
The bulb's mounting and insulation are corrected, and the valve is tested by warming and cooling the bulb while watching suction pressure respond. A valve that responds is left alone. One that does not move is quoted for replacement in writing.
Recovery, valve and drier replaced, pressure test, vacuum, charge weighed in. The system then runs long enough for the new valve to settle, and superheat and subcooling are recorded against the manufacturer's targets.
Common questions
What does a TXV replacement cost?
The valve is a modest part; the labour around it is the cost. Recovery, access to the coil inside the plenum, brazing, a new filter-drier, the pressure test, evacuation and recharge all belong to the job, and coil access varies widely between installations. You pay the $90–$150 diagnostic first, folded into the repair if you proceed, and get the full figure in writing. See pricing.
Is the expansion valve covered by warranty?
Often the part is. Many manufacturers cover parts for ten years on a registered unit, and the valve is a part. Labour, refrigerant and the filter-drier are normally not included unless you bought a separate labour plan. Bring the installation invoice or the model and serial numbers and we check the coverage before quoting. Understanding HVAC warranty coverage explains the usual terms.
Can a stuck TXV free itself?
Sometimes a valve sticks intermittently, cooling well one day and starving the coil the next, and that pattern is itself a clue. It does not get better with time. Tapping the valve or adding refrigerant are not repairs. If the readings come and go, the technician may need to see the system during a bad spell, so note the time and conditions when cooling drops.
Does every air conditioner have a TXV?
No. Many builder-grade and older systems use a fixed orifice, a small brass piston with a drilled hole, which has no moving parts to stick and is charged by superheat. Higher-efficiency systems use a TXV, and some inverter units use an electronic expansion valve driven by the control board. The indoor coil's label or the installation manual says which one you have.
Should I replace the valve or the whole coil?
Replace the valve when the coil is clean, tight and under about ten years old. If the coil is also leaking or badly corroded, the labour to open the circuit is better spent on an evaporator coil replacement, which comes with a new valve. On an R-22 system or one past twelve years, ask for the repair beside a replacement price. Book the diagnostic through AC repair or the contact page.