TXV Hunts: Superheat Swings 4 to 20 F During a Single Call: Decision Tree

Why this matters

A thermostatic expansion valve that hunts swings superheat wide and rhythmically: 5 F, then 18 F, then back, on a 30 to 90 second cycle, while the call holds steady. The compressor sees alternating liquid floodback and starvation, the coil never settles, and capacity wanders. Techs misread it as a low charge and add refrigerant, which masks the swing briefly and sets up a floodback failure later. Hunting is a control-loop instability, not a charge level, and the cure depends on whether the bulb, the equalizer, the valve sizing, or upstream conditions are driving the oscillation. Walking this tree before touching the charge prevents a slugged compressor and a comeback.

Symptom presentation

The clean version: gauge superheat oscillates rhythmically through a wide band (often 4 to 20 F) at steady load, suction pressure breathes with it, and the swing has a regular period. Subcool is roughly normal and stable. The coil may frost-and-clear in waves. The customer reports inconsistent cooling or a hissing that comes and goes at the air handler.

Variants that change the tree:

• Superheat is low and stable (not swinging): overcharge or flooding bulb, not hunting.
• Superheat is high and stable: undercharge or restriction, a different tree.
• Swing only at part load or low outdoor ambient: oversized valve or low head pressure.
• Swing started right after a coil or valve replacement: bulb mounting or equalizer error.

Quick checks before chasing the valve

Confirm steady load first. A short-cycling thermostat, a hunting blower (ECM ramping), or a varying duct damper will make any valve appear to hunt. Lock the system at a stable call with constant airflow before judging the valve.

Inspect the sensing bulb. It must be clamped to clean bare suction line at the 4 or 8 o'clock position on lines over 7/8 inch, never on the bottom (oil) or top (vapor stratification), and fully insulated. A loose, top-mounted, or uninsulated bulb senses air temperature and drives oscillation. Re-clamp and insulate before any other step; this fixes a large share of hunting calls outright.

Verify the external equalizer line is open and connected downstream of the bulb. A pinched or capped equalizer makes an externally equalized valve unstable.

Isolation tree

Branch 1: bulb sensing fault.

• Bulb loose, painted-over corrosion under the clamp, or thermal contact lost: re-seat on clean bare copper with proper clamp pressure and full insulation. Re-test.
• Bulb on the wrong line orientation or downstream of a trap that holds liquid: relocate per the valve manufacturer instructions, typically a horizontal run upstream of the equalizer tap.

Branch 2: valve sizing and load mismatch.

• A valve oversized for the actual load hunts at part load because it cannot modulate fine enough. Confirm valve nominal tonnage against the coil and the real load. An oversized valve on a downsized or low-ambient operation is the classic chronic hunter.
• Low condensing pressure (cool outdoor ambient, oversized condenser, low charge) reduces pressure drop across the valve and starves modulation. Confirm head pressure is within the valve's rated range before condemning it.

Branch 3: refrigerant and circuiting.

• Flash gas at the valve inlet from a partially plugged liquid line filter-drier or insufficient subcool causes erratic feed. Check subcool and the temperature drop across the drier (more than 3 F across a drier signals restriction).
• Distributor or feeder tube uneven feed after a coil change makes superheat read unstable at the common port; this is a measurement artifact, not true hunting.

Branch 4: the valve itself.

• A valve that hunts after the bulb, sizing, equalizer, head pressure, and drier all check out has lost charge in its power element or has a worn pin/seat. Replace with the correct nominal tonnage and equalization type for the coil.

Confirming the diagnosis

After the corrective step, hold a stable call for 20 minutes and watch superheat settle into a band of roughly 2 to 4 F around the target. A correctly mounted, correctly sized valve at adequate head pressure parks superheat near setpoint with only slow drift, not rhythmic swing. Log superheat min/max and period, subcool, head pressure, drier delta-T, and bulb mounting position. Reserve a valve replacement for when every upstream control variable has been eliminated.

References

• ASHRAE Handbook Refrigeration, Chapter on Thermostatic Expansion Valves - bulb mounting, equalizer, and hunting causes.
• Sporlan Bulletin 10-9 and Danfoss TXV technical literature - sensing bulb placement, superheat adjustment, and instability diagnosis.
• AHRI 750 - thermostatic refrigerant expansion valve performance and rating.
• EPA 40 CFR Part 82 Subpart F (Section 608) - refrigerant handling and recovery during valve service.