An air conditioner with the wrong refrigerant charge will cool poorly, run up the power bill, and in the worst case destroy its own compressor — but undercharge and overcharge fail in nearly opposite ways. Knowing which one you are looking at is the difference between fixing the system and making it worse.
The good news is that you do not have to guess. Two measurements — superheat and subcooling — move in opposite directions depending on whether the system has too little or too much refrigerant. Read them correctly and the diagnosis is usually unambiguous. The catch is that low airflow and dirty coils can fake both conditions, so there is one step you have to clear before you ever connect a charging hose.
The Two Numbers That Tell the Story
Superheat is the suction line temperature minus the saturation temperature that corresponds to the suction (low-side) pressure. It tells you how much the refrigerant vapor has warmed past the point where it finished boiling in the evaporator. A high superheat means the evaporator is starved — not enough liquid refrigerant is reaching it.
Subcooling is the saturation temperature at the liquid-line (high-side) pressure minus the actual liquid line temperature. It tells you how much liquid is stacked up in the condenser. A high subcooling means there is a lot of refrigerant backed up on the high side.
That relationship is the core of the diagnosis. When refrigerant is low, the evaporator starves (superheat climbs) and the condenser has little liquid to hold back (subcooling drops). When refrigerant is high, liquid floods the condenser (subcooling climbs) and feeds the evaporator more aggressively (superheat drops). The two numbers swing in opposite directions, and that pattern is what you read.
Signs of an Undercharged System
An undercharge almost always traces back to a refrigerant leak — these systems are sealed and do not “use up” refrigerant in normal operation. The fingerprint is a starved evaporator and a high side that never builds pressure:
- High superheat — the headline symptom of a starved evaporator
- Low subcooling — little liquid backed up in the condenser
- Low suction pressure and low head (discharge) pressure
- Warm or insufficient supply air and reduced capacity
- Long run times as the system struggles to hit setpoint
- Possible evaporator icing if suction pressure drops far enough
- Bubbles in the sight glass, where one is fitted
If the charge keeps dropping after you correct it, you have a leak to find and repair. Topping off a leaking system is a temporary fix that vents refrigerant into the atmosphere over time.
Signs of an Overcharged System
Overcharge usually comes from a tech adding refrigerant to “fix” a symptom that was actually an airflow problem, or from charging without measuring. Too much refrigerant floods the high side and crowds the evaporator:
- High head (discharge) pressure — the compressor works against a packed condenser
- Low superheat — liquid is feeding the evaporator too aggressively
- High subcooling — liquid is stacked up in the condenser
- Elevated compressor amp draw and high discharge temperature
- Poor cooling, despite the system seeming “full”
- Liquid floodback / slugging risk — the most dangerous outcome
That last point is why overcharge is not just inefficient. When superheat falls to near zero, liquid refrigerant can return to the compressor and slug it, washing out the oil and mechanically damaging valves and bearings. An overcharged system can quietly kill a compressor.
Undercharge vs overcharge symptoms
Which Reading to Trust Depends on the Metering Device
Here is where techs go wrong: superheat and subcooling are not equally reliable on every system. Which one you charge to depends on the metering device.
On a TXV or EEV (thermostatic or electronic expansion valve), charge to subcooling. The valve actively modulates to hold suction superheat roughly constant across a range of charge levels, so superheat barely moves even when the system is two pounds overcharged. Subcooling, on the other hand, reflects the total refrigerant inventory. Target subcooling is typically around 10–12°F, but always follow the value stamped on the manufacturer’s data plate.
On a fixed orifice or piston system, charge to superheat. There is no valve to self-regulate, so the charge directly controls how much liquid feeds the evaporator. Compare your measured superheat against a target pulled from the manufacturer’s charging chart based on indoor wet-bulb and outdoor dry-bulb temperatures. Subcooling on these systems swings too much with conditions to be the primary indicator.
Mixing these up is one of the most common charging errors in the field — chasing superheat on a TXV system will have you adding and removing refrigerant chasing a number that the valve is deliberately holding steady.
Which reading to trust
Verify Airflow Before You Touch the Charge
This is the rule that separates good techs from parts-changers: confirm airflow before adjusting refrigerant. Airflow faults mimic charge faults almost perfectly.
Low airflow across the evaporator — a dirty filter, a fouled evaporator coil, a weak or miswired blower — starves the coil and drives suction pressure down and superheat up. That looks exactly like an undercharge. A tech who adds refrigerant to “fix” it ends up with an overcharged system the moment the airflow is later restored.
A dirty or blocked condenser coil works the other way. It can’t reject heat, so head pressure climbs and the high side reads like an overcharge, even when the charge is correct.
So before you connect a charging hose, confirm three things: a clean filter, clean evaporator and condenser coils, and correct blower airflow at roughly 400 CFM per ton. Only after airflow checks out do superheat and subcooling become trustworthy charge indicators. Skipping this step is how systems end up mischarged and compressors end up dead.
Before you touch the charge
How to Set the Charge Correctly
When you are commissioning a virgin or freshly evacuated system, the most accurate method is to weigh in the charge specified on the data plate, adjusted for line-set length. Superheat and subcooling are then used to fine-tune or to verify the result.
On an existing system, you use superheat and subcooling to diagnose and adjust rather than starting from a known weight. Take your readings only after the system has stabilized under load for 10–15 minutes — numbers taken during pull-down are meaningless. Adjust in small increments, let the system settle, and re-measure before adding or recovering more.
And whenever you remove refrigerant, recover it properly into a recovery machine and cylinder. Venting refrigerant to the atmosphere is illegal under EPA Section 608, and it is also just bad practice — you are throwing away the exact substance you are trying to measure.
Use the Free Calculator
You do not need to do the saturation-temperature math by hand. These tools convert your gauge pressures and line temperatures into superheat and subcooling, and help you land on the right charge:
Superheat Calculator — get your exact answer in seconds.
Subcooling Calculator — check a TXV system’s charge.
Refrigerant Charge Calculator — pull it all together to set the charge.
Enter your refrigerant type, gauge pressures, and line temperatures, and the calculators handle the PT-chart conversions for you.
FAQ
How do I know if my AC is overcharged or undercharged?
Read superheat and subcooling after the system has run for 10–15 minutes. High superheat with low subcooling and low pressures points to undercharge. Low superheat with high subcooling and high head pressure points to overcharge. The two readings move in opposite directions, which is what makes the diagnosis clear — but verify airflow first, because a dirty coil or filter can fake either pattern.
Should I charge to superheat or subcooling?
It depends on the metering device. Charge a TXV or EEV system to subcooling (typically around 10–12°F, per the data plate), because the valve holds superheat roughly constant regardless of charge. Charge a fixed-orifice or piston system to superheat, compared against a manufacturer chart based on indoor wet-bulb and outdoor dry-bulb temperatures.
Why do I have to check airflow before adding refrigerant?
Because low airflow mimics charge problems. A dirty filter or fouled evaporator coil drives superheat up and pressures down, looking just like an undercharge — and a dirty condenser raises head pressure like an overcharge. If you adjust refrigerant before fixing airflow, you mischarge the system. Confirm clean coils, a clean filter, and roughly 400 CFM per ton first.
Is overcharging or undercharging worse for the compressor?
Overcharge is the more immediate threat. When the charge is too high, superheat can fall to near zero and liquid refrigerant returns to the compressor, slugging it and washing out the oil — that causes mechanical failure. Undercharge mostly hurts efficiency and capacity, though severe undercharge can also overheat a compressor over time. Neither is acceptable, but slugging can fail a compressor quickly.
Can I just add a little refrigerant to be safe?
No. Adding refrigerant “to be safe” is how systems end up overcharged, with reduced cooling and a real risk of compressor damage. A sealed system that is low on charge has a leak — the fix is to find and repair the leak, recover or measure the charge, and set it precisely. And any refrigerant you remove must be recovered, never vented, under EPA Section 608.