TXV Troubleshooting: Starving, Flooding and Hunting Expansion Valves
A thermostatic expansion valve (TXV or TEV) does one job: it holds a steady superheat at the evaporator outlet. Measure superheat and you'll know whether it's doing that job. Most "bad TXVs" turn out to be airflow, charge or liquid-line problems.
Measure first
- Airflow and coil: clean filter and coil, correct blower speed, static pressure in range. Low airflow lowers suction pressure and looks like a starving valve. See total external static pressure.
- Subcooling at the condenser outlet (liquid line temperature vs. saturated condensing temperature). TXV systems are charged by subcooling, so this tells you whether the valve has a full column of liquid.
- Superheat at the bulb location: suction line temperature where the bulb is mounted minus saturated suction temperature. Use the superheat and subcooling calculator.
Reading the results
| Superheat | Suction pressure | Subcooling | Likely cause |
|---|---|---|---|
| High | Low | Low | Undercharged or a leak (not the valve) |
| High | Low | Normal or high | Valve starving: restriction, flash gas, bulb charge lost, plugged valve or screen |
| Low | High | Normal | Valve overfeeding: bulb contact or location, leaking seat, oversized valve |
| Low | Low | Normal | Low airflow or low load across the coil |
| Swinging up and down | Swinging | Normal | Hunting: bulb location or contact, oversized valve, low load |
When the valve is starving the coil (high superheat)
Check these before you replace the valve:
- Liquid line restriction or flash gas. Feel or measure the temperature across the filter-drier and any solenoid valve. A noticeable drop means a restriction. Long vertical lifts and undersized lines can make the liquid flash before it reaches the valve.
- Capped, kinked or restricted external equalizer line. The valve can't open properly without it.
- Lost bulb charge. If the power element has lost its charge, the valve closes. Warming the bulb in your hand should make the valve open and the suction pressure rise. No response points to a dead element.
- Plugged valve or inlet screen. Debris from brazing without nitrogen, or moisture freezing at the port, blocks flow. See brazing with a nitrogen purge.
When the valve is overfeeding (low superheat)
- Bulb contact: the bulb must be clamped tight to a clean, straight section of the suction line, with full metal contact.
- Bulb location: on small suction lines the top of the line works; on larger lines (Sporlan uses 7/8 in. and up), mount it at about 4 or 8 o'clock, never on the bottom where oil runs. Keep it away from where liquid can trap, and insulate it so ambient air doesn't affect it.
- Leaking seat or a valve that's too big for the load. An oversized valve also tends to hunt.
Hunting
A hunting valve swings superheat high and low as it overshoots. Sporlan calls this the superheat swing, and the more oversized the valve, the bigger the swing. Bulb placement and contact are the usual fixes; an oversized valve is the next suspect.
Adjusting a TXV
Many residential TXVs are factory set and non-adjustable. On adjustable valves, turning the stem clockwise raises superheat (less refrigerant), counterclockwise lowers it. Make small changes, about a half to one turn, and wait at least 15 minutes for the system to settle before the next adjustment. Never adjust the valve to make up for a charge or airflow problem.
Replacing a TXV
- Recover the charge, protect the valve body from heat (wet rag or heat paste), and braze with a nitrogen purge.
- Mount the new bulb correctly and insulate it.
- Pull a deep vacuum: how to pull a deep vacuum.
- Charge to the manufacturer's subcooling target.
FAQ
What superheat should a TXV hold?
It depends on the system; residential TXVs are commonly set somewhere around 8 to 14°F at the evaporator outlet. Check the equipment maker's data. TXV systems are charged by subcooling, not superheat.
How do I know the TXV is bad and not the charge?
Look at subcooling. Low subcooling with high superheat is low charge. Normal or high subcooling with high superheat and low suction means the valve isn't feeding.
Where does the TXV bulb go?
On a clean, straight section of suction line near the evaporator outlet, clamped tight, on top of small lines or at about 4 or 8 o'clock on larger ones, and insulated.
Can you adjust a TXV to fix low suction pressure?
Only after airflow, charge and the liquid line check out. Adjusting a valve to cover another problem creates a new one.
Sources
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