❄️ Refrigerant Charge Calculator
Diagnose HVAC charge using Superheat (Fixed Orifice) or Subcooling (TXV)
🔧 Understanding HVAC Charging Methods
1. Superheat Method (Fixed Orifice)
Used for systems with pistons or capillary tubes. Superheat ensures liquid refrigerant doesn't return to the compressor.
- Formula: Actual Superheat = Suction Line Temp - Saturation Suction Temp
- Target Formula: ((3 × Indoor WB) - 80 - Outdoor DB) / 2
- Goal: Actual Superheat should be within ±3°F of Target.
2. Subcooling Method (TXV/EEV)
Used for systems with Expansion Valves. Subcooling ensures a full column of liquid reaches the metering device.
- Formula: Actual Subcooling = Saturation Liquid Temp - Liquid Line Temp
- Target: Specified on the unit's data plate (typically 8°F - 12°F).
- Goal: Actual Subcooling should be within ±3°F of Target.
"Never charge a system without checking airflow first! A dirty filter or blocked coil will give you false readings (low superheat/low suction). If you add refrigerant to a system with bad airflow, you'll overcharge it and potentially kill the compressor. Always clean coils and filters before hooking up your gauges."
⚠️ Common Diagnostics
| Condition | Superheat | Subcooling | Pressures |
|---|---|---|---|
| Undercharged | High | Low | Low / Low |
| Overcharged | Low | High | High / High |
| Low Airflow | Low | Normal/Low | Low / Low |
❓ FAQ
Can I use Superheat on a TXV system?
No. A TXV modulates flow to maintain a constant superheat, so superheat readings won't tell you if the charge is correct (unless it's extremely low/high). You MUST use Subcooling for TXV systems.
What if the outdoor temperature is below 60°F?
Charging by Superheat/Subcooling is inaccurate below 60°F outdoor ambient (unless you have a low ambient kit and block the fan). In cold weather, it's best to weigh in the charge according to the factory specification.
Why is my Target Superheat calculation negative?
If the calculation ((3×WB)-80-DB)/2 yields a negative number or a value below 5°F, conditions are likely too hot inside or too cold outside for a valid test. Ensure Indoor WB is at least 50°F and Outdoor DB is at least 60°F.