TXV Trouble: How a Failing Expansion Valve Starves Your AC

Somewhere near your indoor coil sits a metering device about the size of a small fist, and it makes a decision several times a second that determines whether your air conditioner cools the house or merely runs. In most systems installed in the last fifteen years, that device is a thermostatic expansion valve, usually shortened to TXV. When it drifts, sticks, or clogs, the system produces symptoms that look almost exactly like a refrigerant leak, which is why a failing expansion valve is one of the most misdiagnosed problems in residential air conditioning. More than one Brevard County homeowner has paid for refrigerant twice before anyone looked at the valve.

The Job of a Metering Device

Refrigerant leaves your outdoor unit as a warm high-pressure liquid. Before it can absorb heat from your indoor air, its pressure has to drop sharply so it can boil at a cold temperature inside the evaporator coil. The metering device is the gatekeeper that creates that pressure drop, sitting right at the coil’s inlet.

Older and simpler systems used a fixed orifice, a small brass piston with a precisely sized hole. It works, but it only meters correctly at one set of conditions. A TXV is smarter. A sensing bulb clamped to the coil’s outlet pipe reads how much the refrigerant has warmed by the time it leaves the coil, a value technicians call superheat, and the valve throttles itself open or closed to hold that number steady. On a mild morning it feeds less. On a 95 degree Palm Bay afternoon with the house full of guests, it opens up and feeds more. That constant adjustment is a big part of why modern systems hold their rated efficiency across the wild swings of a Florida cooling season.

How an Expansion Valve Goes Wrong

TXVs fail in a few distinct ways, and the direction of the failure decides the symptoms. A valve that sticks closed, or clogs at its inlet screen, starves the coil. Too little refrigerant enters, the little that does boils off almost instantly, and parts of the coil run far colder than design while the system as a whole moves almost no heat. A valve that sticks open floods the coil, sending liquid refrigerant toward the compressor, which is built to pump vapor and suffers every time liquid arrives instead.

The causes are usually upstream of the valve itself. Debris from a compressor failure, copper shavings from installation, or sludge from moisture in the system collects on the valve’s inlet screen until flow chokes off. Moisture is a particular villain: a system that was not evacuated properly during installation carries water vapor that can freeze into ice crystals right at the valve’s tiny opening, blocking it intermittently. The valve works, then does not, then works again as the ice melts, which produces the maddening complaint of an air conditioner that fails only on certain afternoons. The sensing bulb can also lose its charge or come loose from the suction line, leaving the valve blind and guessing.

Why It Looks Exactly Like Low Refrigerant

Here is the trap. A starved evaporator coil, whether it is starved because refrigerant leaked out of the system or because a valve will not let it through, shows the same surface symptoms: weak cooling, long run times, a coil that ices over, and low pressure on the suction gauge. A technician who reads only that one gauge can easily conclude the system is undercharged, add refrigerant, and leave. For a few days things may even seem better. Then the ice returns, the head pressure creeps up, and the homeowner is back where they started, minus the cost of a pound or two of increasingly expensive refrigerant.

Telling the two apart requires measuring both superheat and subcooling, numbers that describe conditions on each side of the system. A low charge shows low subcooling, because there is genuinely less refrigerant in the circuit. A restricted TXV shows high superheat at the coil with normal or high subcooling, because the refrigerant is all there, stacked up behind a valve that will not pass it. Those twenty extra minutes of measurement are the difference between fixing the system and renting improvement by the pound. It is fair to ask any technician who recommends refrigerant what the subcooling reading was; on maintenance visits we record both numbers precisely so this history exists when something changes. ENERGY STAR’s guidance on keeping a cooling system healthy at energystar.gov makes the same point: measurement beats guesswork.

Symptoms Worth Writing Down

Before a technician arrives, the pattern of the problem is genuinely useful diagnostic data, and homeowners are the only ones positioned to observe it. A TXV problem tends to produce some mix of the following: cooling that fades specifically on the hottest afternoons when the valve should be opening wide, ice that forms on the refrigerant line or coil and begins right at the valve, a hissing or whistling sound near the indoor unit, short bursts of cold air followed by long stretches of mediocre air, and a system that recovers completely after being shut off for a few hours, only to fail again the next day. That last one points hard toward moisture freezing at the valve, since the shutdown gives the ice time to melt.

None of these symptoms is exclusive to the valve, and honest diagnosis sometimes lands elsewhere: a plugged filter drier produces similar restriction, and a failing blower produces similar ice. But a written note that says the AC dies every day around 4 p.m. and heals itself overnight will shorten the diagnostic visit considerably.

What the Fix Involves

A TXV is not a serviceable part in the field. When one has genuinely failed, it gets replaced: refrigerant recovered, the old valve unsweated or unbolted from the coil inlet, a new valve installed with careful heat control so its internal parts survive the brazing, a new filter drier fitted as cheap insurance, and the system evacuated deeply enough to remove the moisture that may have caused the problem in the first place. That evacuation step matters more than the part itself. A rushed vacuum leaves the same water vapor in the lines, and the new valve inherits the old valve’s disease.

Cost-wise, this repair sits in the middle of the range for residential AC work, well below a compressor and well above a capacitor. On a newer system it is clearly worth doing, and the valve may even be covered under a parts warranty. On an older system, the calculation belongs alongside the general health of the cooling system: a fifteen-year-old unit with a failing TXV, tired capacitors, and a rusty coil is asking for a bigger conversation.

Frequently Asked Questions

Can I clean a clogged expansion valve instead of replacing it?

The inlet screen on some valves can be removed and cleaned, and when the restriction is nothing more than debris on that screen, cleaning plus a new filter drier can genuinely solve the problem. But a valve that has been passing contaminated refrigerant usually has wear inside as well, and a bulb that has lost charge cannot be recharged. Most technicians will clean a screen once; if trouble returns, replacement is the durable answer.

How long does a TXV last?

In a clean, dry, properly installed system, the valve routinely outlives the rest of the equipment. Most failures trace back to something else: contamination from a previous component failure, moisture from a sloppy installation, or physical damage to the sensing bulb. That is why the fix always includes a filter drier and a deep evacuation, not just a new part.

Does every AC have an expansion valve?

Every system has a metering device, but not all use a TXV. Many older and some budget systems use a fixed orifice piston, which cannot stick the way a valve can but also cannot adapt to changing conditions. Higher efficiency equipment almost always uses a TXV or an electronic version of it, because the adaptive metering is part of how the efficiency rating is achieved.

Why does my AC only struggle late in the afternoon?

Late afternoon is when the heat load peaks and the valve is asked to pass its maximum flow, so a partial restriction that hides at 10 a.m. runs out of headroom at 4 p.m. Intermittent icing from moisture in the system follows a similar schedule. An air conditioner that keeps banker’s hours is a strong hint that something in the refrigerant circuit is marginal rather than dead, and marginal problems are cheapest to fix before they finish failing.

If your air conditioner has been fed refrigerant more than once without a found leak, the metering valve deserves a hard look before anyone reaches for the gauges again. Inlet Mechanical runs full superheat and subcooling diagnostics on every refrigerant call in Melbourne, Palm Bay, and across the Space Coast. Call us at (321) 723-0858.

Written & Reviewed By

Inlet Mechanical Team

The Inlet Mechanical team brings over 85 years of combined experience in HVAC, plumbing, and mechanical construction across Florida. Our licensed professionals hold Florida Mechanical HVAC License (CMC1250858) and Florida Plumbing License (CFC1433105), along with EPA Section 608 certifications. Based in Brevard County, we serve residential, commercial, and industrial clients with expert knowledge of Florida building codes, climate-specific HVAC solutions, and local plumbing requirements. Every article is reviewed by our licensed technicians to ensure accuracy and practical value for Melbourne-area homeowners and businesses.

Last Updated: September 16, 2026