Learning Center / Maintenance
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Superheat and Subcool — What They Are, and Why Every Real HVAC Tech Measures Them
Ask any HVAC tech what "superheat" is. If they can't answer in one sentence, they shouldn't be charging your system. This one measurement separates real refrigeration work from the parts-changers.
What is superheat, in one sentence?
Superheat is how many degrees warmer the refrigerant gas is at the outdoor unit than its saturation temperature. It tells you whether all the liquid refrigerant in the evaporator has boiled — the whole point of your AC.
When (and why) do we measure it?
Every time we service or install a system with a fixed metering device (TXV-less systems). On a standard TXV system, we measure subcool at the liquid line instead — which tells us if the condenser is doing its job of turning gas back into liquid.
The targets:
- Superheat: 10–15°F on most systems in typical Triangle conditions.
- Subcool: 8–12°F, per the manufacturer's charging chart.
How does a technician fail this?
By skipping it entirely. "Gauging in" and comparing pressures to a chart on the side of the unit is a guess. The correct method uses digital manifold + probes on the suction line and liquid line, cross-referenced against the outdoor temp and indoor wet-bulb. It takes 15 minutes. It's the difference between a system running at 92% of its rated capacity and 60%.
Frequently asked questions
How do I know if my AC is properly charged?
Ask the contractor for their commissioning sheet: measured superheat OR subcool (depending on metering device), delta-T across the evaporator, static pressure, capacitor microfarads. If they can't produce it, the system isn't properly commissioned.
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