The filter drier traps moisture, acid and small debris in the refrigerant line so they do not damage the valve or compressor. A clogged drier restricts flow and can cause poor cooling, a temperature drop across the drier and bubbles in the sight glass. It is replaced whenever the system is opened.
A drier is cheap protection for expensive parts. The trouble is that it hides in plain sight on the liquid line, and few operators know it exists until a technician mentions it.
What it does
Refrigerant flows through the drier body, where a desiccant absorbs water and acids and a screen catches particles. Moisture in a system can freeze at the expansion valve, form acid with the oil and damage windings. The drier keeps those risks low. It has a limited capacity, so it must be replaced when it becomes saturated or the system is opened for repair.
Signs it is failing
- Frost or a cold spot on the drier body.
- Noticeable temperature drop across the drier.
- Bubbling in the sight glass with a normal charge.
- Slow cooling and low suction pressure.
- Moisture indicator in the glass shows wet.
- Repeated icing at the expansion valve.
Common causes of failure
| Cause | What happens |
|---|---|
| Moisture entered the system | From a leak or poor evacuation after repair |
| Debris or scale | Old pipework releases particles |
| Compressor burnout | Acid and sludge overwhelm the drier |
| Age | Desiccant becomes saturated over time |
| Oversized or wrong drier | Poor match to the system |
What you can safely check
- Look for frost. Frost on only the drier suggests a restriction.
- Check the sight glass. Note bubbles and the color of any moisture indicator.
- Feel for temperature difference. Only if safe to reach. A big difference between inlet and outlet is a clue.
- Note when it started. Recent repairs may explain moisture.
The liquid line is under pressure, and refrigerant can cause frostbite. Do not loosen fittings. The system must be recovered by a technician before a drier is changed.
What a technician does
- Measures temperature drop across the drier.
- Checks the sight glass and moisture indicator.
- Recovers refrigerant and replaces the drier.
- Pulls a deep vacuum to remove moisture.
- Recharges by weight and checks performance.
Keeping it healthy
- Replace it whenever the sealed system is opened.
- Insist on evacuation and proper recharge by weight after repairs.
- Check the sight glass moisture indicator during service.
- Use the correct size for the system.
Cutting corners on the drier or evacuation is a common source of repeat callouts. Ask whether it was replaced and keep the invoice in your records.
Repair or replace
Driers are replaced, not repaired. Replacement is normal whenever the sealed system is opened. If the old drier was clogged by burnout debris, the technician may recommend an acid test and further cleanup. See the TXV page for how moisture affects the valve.
Related reading: the parts and components guide, the sight glass and recovery, evacuation and recharge.
Questions about this part
How often should a drier be replaced?
Whenever the sealed system is opened, and when it tests wet or restricted.
Can I clean a drier?
No. The desiccant cannot be restored.
What does frost on the drier mean?
A pressure drop from a restriction. It usually needs replacement.