| Parameter | Value | Note |
|---|
Subcooling: confirming liquid refrigerant quality at the metering device
Subcooling is the temperature drop of liquid refrigerant below its saturation (condensing) point at a given pressure. A subcooled liquid line means only pure liquid is reaching the metering device — no flash gas. Flash gas before the metering device causes erratic valve operation, poor capacity, and noise. Subcooling confirms the condenser is doing its job and the charge level is sufficient to fill the liquid line.
In Canadian climates, outdoor temperatures swing from -40°C in Alberta winters to +35°C in Ontario summers. At low ambient, head pressure drops sharply on R-410A and R-32 systems. This compresses the subcooling reading and can mask a low-charge condition. If you're commissioning or checking charge below 60°F outdoor ambient, use a low-ambient kit or interpret subcooling carefully alongside superheat from the superheat calculator.
Subcooling targets by system type
TXV and EEV systems rely on subcooling as the primary charge indicator. Most manufacturers specify 10-15°F at the liquid line service valve near the condenser. Some high-efficiency systems with long line sets target up to 20°F to account for pressure drop between the condenser and metering device. Always check the equipment installation manual — never assume a universal target.
Fixed-orifice systems are charged by superheat, not subcooling. Subcooling on a fixed-orifice system is a secondary check: if subcooling is above 20°F, the system is likely overcharged. If it's below 5°F, the system may be low on charge or have a condenser airflow problem. Use the superheat calculator as the primary method for fixed-orifice charging, then verify subcooling is in a reasonable range.
Reading subcooling with superheat together
The combination of subcooling and superheat tells you far more than either reading alone. High subcooling with low superheat means overcharge. Low subcooling with high superheat means undercharge. High subcooling with high superheat is the restriction pattern — a clogged filter-drier or closed service valve is starving the metering device while liquid stacks up behind the blockage. Normal subcooling with normal superheat confirms correct charge. Enter your actual superheat in the optional field above to get a combined diagnosis.
Where to measure liquid line temperature
Clamp your temperature probe on the liquid line at the service valve on the condenser outlet. This gives you the most accurate reading relative to the high-side pressure you're measuring. Measuring further down the liquid line, especially after a long run in a hot attic or rooftop, will show lower temperature and artificially inflate the apparent subcooling. The P-T chart calculator lets you cross-check saturation conditions at any pressure point in the circuit.
Frequently Asked Questions
On a properly charged R-410A TXV system, subcooling at the condenser outlet should be 10-15°F. Subcooling above 20°F typically indicates overcharge or a restriction downstream of the measurement point. Subcooling below 5°F means the liquid line is starved — either from low charge or a restriction upstream. Always pair subcooling with superheat using the superheat calculator for a complete charge diagnosis before adding or removing refrigerant.
High subcooling with high superheat is the classic sign of a liquid line restriction — usually a clogged filter-drier, a partially closed liquid line service valve, or a kinked liquid line. Refrigerant backs up behind the restriction, creating high subcooling on the condenser side. But the metering device starves for liquid, so superheat climbs on the evaporator side. Adding refrigerant in this condition makes the overcharge worse once the restriction is fixed. Check for a temperature drop across the filter-drier; more than 3°F across the drier confirms replacement is needed.