Refrigerant Charge Calculator
Compare measured versus target superheat and subcooling on TXV and fixed-orifice systems. Includes line-set adjustment for extra tubing. R-410A, R-22, R-32, R-134a.
HVAC Toolkit runs this math at zero bars
Worked example
TXV R-410A system. Measured superheat 15°F, measured subcooling 8°F, target subcooling 10°F, line set 10 ft longer than factory allowance.
Target superheat & subcooling reference
| Metering | Refrigerant | Target SH (°F) | Target SC (°F) | Primary method | Notes |
|---|---|---|---|---|---|
| TXV | R-410A | 10 | 10 | Subcooling | SH is a secondary check only. |
| TXV | R-22 | 10 | 10 | Subcooling | Compare against the condenser charging chart. |
| TXV | R-32 | 10 | 10 | Subcooling | Higher pressures require accurate instruments. |
| TXV | R-134a | 10 | 10 | Subcooling | Common on medium-temp refrigeration equipment. |
| Fixed | R-410A | 15 | 8 | Superheat | SC is a secondary check only. |
| Fixed | R-22 | 15 | 8 | Superheat | Indoor + outdoor conditions affect target SH. |
| Fixed | R-32 | 15 | 8 | Superheat | Confirm compatible charging procedures. |
| Fixed | R-134a | 15 | 8 | Superheat | Evaporator feeding is the main indicator. |
Common mistakes
- Adjusting charge before verifying airflow. A dirty filter, iced coil, or wrong blower speed distorts superheat and subcooling. Fix airflow first.
- Using superheat on a TXV. The valve modulates evaporator superheat. Subcooling is the primary indicator of charge level on TXV systems.
- Topping off a leaking system. Refrigerant does not get consumed. Repeated low charge means a leak — find it, fix it, then recharge by weight.
- Mixing recovered and virgin refrigerant. Unknown blend composition, oil contamination, potential safety issues. Recover per EPA 608 and charge with clean refrigerant.
FAQ
How do I know if my AC is low on refrigerant?
Low charge often shows up as high superheat on a fixed-orifice system or low subcooling on a TXV system. You may also see reduced cooling capacity, longer run times, low suction pressure, or a starved evaporator. Always verify airflow, coil cleanliness, and metering device operation before concluding the system is undercharged.
What is the difference between superheat and subcooling charging methods?
Superheat tells you how far refrigerant vapor has been heated above saturation after the evaporator — useful on fixed-orifice systems where charge directly affects evaporator feeding. Subcooling tells you how far liquid has been cooled below condensing saturation — useful on TXV systems where the valve regulates evaporator superheat and condenser liquid inventory becomes the better charging indicator.
How much refrigerant should I add per foot of extra line set?
Field estimate for a 3/8 in liquid line: about 0.6 oz/ft R-410A, 0.6 oz/ft R-454B, 0.3 oz/ft R-22, 0.6 oz/ft R-32, 0.4 oz/ft R-134a. Manufacturer line-set tables are more reliable because standard factory-charge length, tubing size, and application details can change the value.
Can I mix refrigerants when topping off charge?
No. Mixing refrigerants creates an unknown blend with different pressure-temperature behavior and oil compatibility. Recover the charge properly and recharge with the exact refrigerant listed on the nameplate.
What causes refrigerant leaks?
Vibration wear, poor brazing, flare leaks, damaged Schrader cores, rubbed copper lines, coil corrosion, and mechanical damage during installation or service. Refrigerant is not consumed in normal operation — find and repair the leak instead of repeatedly topping off.
Sources
The web page won't load where you charge the unit
HVAC Toolkit keeps R-410A/R-454B PT curves on the phone, logs each check, and exports the run for the ticket. No signal, no login. Pay once.
Related
CalcSpec is an estimator for qualified technicians. Results do not replace manufacturer charging charts, AHRI-certified gauges, or the judgment of a licensed HVAC contractor. Always follow EPA Section 608 refrigerant-handling requirements.