How a TXV Works (60-Second Version)
A Thermostatic Expansion Valve (TXV) is a precision metering device that controls how much liquid refrigerant enters the evaporator coil. It does this by balancing three forces:
- Bulb pressure (opens the valve) — The sensing bulb strapped to the suction line senses vapor temperature. Warmer vapor = more pressure = valve opens wider.
- Evaporator pressure (closes the valve) — The actual pressure inside the evaporator pushes back against the diaphragm.
- Spring pressure (closes the valve) — A superheat spring provides a baseline closing force, adjustable via the valve stem.
When these three forces are balanced, the TXV maintains a target superheat (typically 8–12°F) regardless of load changes. When they're not balanced, things go wrong.
The Three TXV Failure Modes
1. TXV Stuck Closed (Underfeeding)
The valve isn't opening enough — the evaporator is starving for refrigerant.
| Symptom | What You'll See |
|---|---|
| Superheat | Very high (25–50°F+) |
| Subcooling | High (15–25°F) — liquid backs up in condenser |
| Suction pressure | Low — evaporator not getting enough refrigerant |
| Head pressure | Normal to slightly high |
| Evaporator | Mostly warm — only first few inches of coil frosting |
| Compressor amps | Low — compressor working on vapor only |
| Suction line | Warm to the touch (should be cool/sweating) |
If superheat is >25°F and subcooling is >15°F, and the system has adequate charge, the TXV is almost certainly stuck closed or restricted. Don't add refrigerant — you'll make it worse.
Common causes: Wax or moisture contamination in the valve, sensing bulb has lost its charge, power head failure, ice blockage from moisture in the system.
2. TXV Stuck Open (Overfeeding)
The valve is wide open — the evaporator is being flooded with liquid refrigerant.
| Symptom | What You'll See |
|---|---|
| Superheat | Very low (0–3°F) or even negative |
| Subcooling | Low to normal |
| Suction pressure | High — too much refrigerant in evaporator |
| Head pressure | Normal to low |
| Evaporator | Fully frosted — ice everywhere, including suction line |
| Compressor amps | High — compressor working hard on liquid |
| Suction line | Ice cold and sweating heavily, possibly frosted back to compressor |
A TXV stuck open is a compressor killer. Liquid refrigerant flooding back to the compressor causes liquid slugging, which destroys valves and can crack the crankcase. If you see frost on the suction line all the way back to the compressor, shut the system down immediately and investigate.
Common causes: Diaphragm rupture in the power head, foreign debris holding the valve pin open, incorrect bulb installation (not making proper thermal contact).
3. TXV Hunting (Oscillating)
The valve cycles between too open and too closed in a rhythmic pattern, typically every 1–5 minutes.
| Symptom | What You'll See |
|---|---|
| Superheat | Swinging wildly — 5°F to 30°F and back |
| Suction pressure | Surging — rhythmic rises and drops |
| Compressor amps | Fluctuating with suction pressure |
| Supply air temp | Inconsistent cooling — warmer, colder, warmer |
| Suction line | Alternating between sweating and dry |
Common causes: Oversized TXV for the application, sensing bulb poorly clamped (intermittent thermal contact), liquid line restriction causing flash gas before the valve.
Step-by-Step TXV Diagnostic Procedure
- Connect gauges. Read suction and liquid pressures. Calculate superheat and subcooling.
- Check the charge first. Verify subcooling is 8–14°F with a TXV. If subcooling is way off, the charge is wrong — fix that before blaming the TXV.
- Inspect the sensing bulb. It must be strapped tightly to the suction line (10 o'clock or 2 o'clock position), insulated from ambient air, and on a horizontal section of pipe — never on a trap or vertical drop.
- Warm the bulb. Wrap the sensing bulb with your hand for 60 seconds. If the valve responds (suction pressure rises, superheat drops), the valve is mechanically functional. If nothing changes, the power head may have lost its charge.
- Check for restrictions. Feel the liquid line before and after the filter drier. A significant temperature drop across the drier means it's restricted — replace it before condemning the TXV.
- Check the superheat adjustment. If superheat is consistently 5°F higher or lower than target, adjust the TXV stem (clockwise to increase superheat, counter-clockwise to decrease). Only ¼ turn at a time, wait 15 minutes.
The most common TXV misdiagnosis is confusing a dirty evaporator coil with a stuck TXV. A dirty coil reduces airflow, which reduces heat absorption, which lowers suction pressure and raises superheat — just like a stuck-closed TXV. Always check the coil and filter first.
When to Replace vs Repair
- Replace if: The power head has lost its charge (hand-warming test fails), the diaphragm is ruptured, or the valve has been contaminated with acid from a compressor burnout.
- Clean if: Wax or debris is holding the pin. Sometimes a system flush and new filter drier will restore TXV function.
- Adjust if: The superheat is close but not spot-on. TXV stems can drift over time — a simple ¼-turn adjustment may be all that's needed.