Restricted Liquid Line or Filter-Drier: How to Find It and Tell It from Low Charge
A liquid line restriction starves the evaporator. On your gauges it looks a lot like an undercharge, and adding refrigerant to a restricted system just piles more liquid in the condenser. The difference shows up in subcooling.
Restriction vs. low charge
| Reading | Low charge | Liquid line restriction |
|---|---|---|
| Suction pressure | Low | Low |
| Superheat | High | High |
| Subcooling | Low | Normal or high |
| Head pressure | Low to normal | Normal, or high on a bad restriction |
| Temperature drop at one spot on the liquid line | No | Yes |
Use the superheat and subcooling calculator. High superheat with normal or high subcooling means liquid is reaching the condenser outlet but not getting through to the evaporator.
Find the restriction
With the system running:
- Measure the liquid line temperature on both sides of the filter-drier. It should be almost the same going in and coming out. Ferguson's guidance is that a drop of more than 3°F across the drier strongly suggests a restriction.
- Look and feel along the line from the condenser to the indoor coil: service valve, filter-drier, any solenoid valve, kinks, crushed tubing in the wall or attic. Goodman and Daikin service manuals note that a restriction shows a definite temperature drop at that point; in severe cases frost forms there and runs down the line in the direction of flow.
- Check the metering device inlet. A plugged piston, inlet screen or TXV can be the restriction: see TXV troubleshooting.
- On heat pumps, a bi-flow drier and check valves can also restrict. Check in the mode where the problem shows up.
If you have access ports on both sides of the drier, measure the pressure drop directly.
When to replace the filter-drier
Sporlan's contaminant control guidance for Catch-All filter-driers: replace it when the pressure drop across it exceeds 5 psi, or when the drier is cooler than the inlet tubing, which means subcooling has been lost and liquid is flashing in the drier.
Also replace the liquid line filter-drier whenever the system is opened, after a compressor burnout (often along with a temporary suction line drier during clean-up), and when non-condensables or moisture are found.
Common causes of restrictions
- Braze scale from brazing without nitrogen. See brazing with a nitrogen purge.
- Moisture freezing at the metering device, or acid and sludge after a burnout.
- Kinked or crushed line sets.
- A partly closed service valve after installation or service.
- An undersized or failed solenoid valve.
Replacing it
- Recover the charge.
- Install the drier with the flow arrow pointing toward the metering device (bi-flow driers on heat pumps).
- Braze with a nitrogen purge, and keep the flame off the drier body.
- Leak test, pull a deep vacuum: how to pull a deep vacuum.
- Charge to the manufacturer's subcooling target and recheck superheat and the temperature drop across the new drier.
FAQ
What's a normal temperature drop across a filter-drier?
Close to none. A drop of more than about 3°F points to a restriction.
Can a clogged filter-drier look like low refrigerant?
Yes. Both cause low suction pressure and high superheat. The difference is subcooling: low with an undercharge, normal or high with a restriction.
When should a filter-drier be replaced?
Sporlan says when the pressure drop exceeds 5 psi or the drier is colder than the inlet line. Replace it any time the system is opened, and after a burnout.
Why is my liquid line frosting at the drier?
The refrigerant is dropping in pressure at the restriction, so it flashes and gets cold. That's a severe restriction.
Sources
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