Practical Guide
16 min 2026-11-02 Short answer: superheat is how many degrees the suction vapor is above its saturation (boiling) temperature, and subcooling is how many degrees the liquid line is below its saturation (condensing) temperature. Techs use superheat to judge how well the evaporator is fed, and subcooling to judge how much liquid is backed up in the condenser. On a fixed orifice system you usually charge by superheat; on a TXV or EEV system you usually charge by subcooling. Neither number means much until airflow, filters and operating conditions have been checked first. You can run both numbers for free with the superheat and subcooling calculator.
This guide is written for working technicians and for apprentices who are past the textbook and onto real roofs and real attics. It covers what the two numbers actually measure, how to take the readings so they mean something, how techs in the field use them to make decisions, the patterns that point to common problems, and the mistakes that lead to overcharged systems and callbacks. It is long on purpose. Keep it open on your phone the next time you have gauges on a unit.
A pressure reading by itself tells you very little. A suction pressure of 118 psig on an R-410A system could be fine, or it could be a starved coil on a mild day, or an overcharged system with low airflow. Pressure only becomes useful once you turn it into a saturation temperature with a PT chart and compare it with an actual line temperature. That comparison is superheat on the suction side and subcooling on the liquid side.
Those two numbers are the closest thing you have to looking inside the coils. They tell you where the refrigerant is changing state, how much of each coil is doing useful work, and whether the metering device is doing its job. That is why almost every manufacturer charging procedure is built on one or the other.
They also protect the compressor. Superheat that is too low can mean liquid is reaching the compressor, which can wash out oil and damage valves. Superheat that is too high can mean the compressor is running hot with poor cooling from returning vapor. Subcooling tells you whether the metering device is getting a solid column of liquid, which it needs to feed the evaporator properly.
Inside an operating evaporator and condenser, most of the refrigerant is a mix of liquid and vapor. While it is changing state, its temperature is locked to its pressure. That locked temperature is the saturation temperature, and the PT chart is simply a table of that relationship for each refrigerant. Read the pressure, look up the saturation temperature, and you know the temperature of the boiling or condensing refrigerant inside the coil.
Once all the liquid in the evaporator has boiled off, any more heat picked up raises the vapor temperature above saturation. The number of degrees it rises is superheat:
Superheat = suction line temperature − evaporator saturation temperature
Example: R-410A suction pressure 118 psig, which is about 40°F saturation. Suction line temperature near the service valve is 52°F. Superheat is 52 − 40 = 12°F.
Low superheat means the refrigerant finished boiling very late, close to the end of the coil or not at all. High superheat means it finished boiling early, so the last part of the coil is just warming vapor instead of absorbing heat by boiling.
In the condenser, vapor gives up heat and condenses to liquid. Once it is fully liquid, further cooling drops its temperature below saturation. The number of degrees it drops is subcooling:
Subcooling = condensing saturation temperature − liquid line temperature
Example: R-410A liquid line pressure 365 psig, which is about 110°F saturation. Liquid line temperature is 100°F. Subcooling is 110 − 100 = 10°F.
Subcooling tells you how much liquid is stacked up in the bottom of the condenser. More refrigerant in the system usually means more liquid backed up and more subcooling. Less refrigerant usually means less.
Many refrigerants are blends. Some, such as R-410A, behave almost like a single refrigerant. Others, such as R-407C or R-454B, have temperature glide: the saturation temperature is not the same at the start and the end of boiling. For those, use the dew point temperature to calculate superheat and the bubble point temperature to calculate subcooling. Many digital gauges handle this for you if the right refrigerant is selected. Brixon's calculator uses dew point for superheat and bubble point for subcooling for the same reason. Always confirm the refrigerant on the nameplate before you trust any calculated number.
Most bad charging decisions come from bad readings, not bad math. Before you look at a single number, make sure the measurement is worth looking at.
Run the system at least 10 to 15 minutes before reading, longer on a cold start or after you changed anything. If you add or recover refrigerant, wait again before judging the result. Superheat in particular moves slowly, and chasing it in real time is how systems get overcharged.
Pressure drop in a long suction line, a restricted filter drier or a heat pump reversing valve can make the pressure at your gauge different from the pressure in the coil. On most residential split systems the service valves are close enough. On long line sets, rooftop units with unusual port locations or systems with suction accumulators, keep in mind that a small pressure difference can mean a few degrees of error.
Charging data is only valid in certain conditions. Many manufacturers want outdoor temperature above a minimum, often around 65°F to 70°F for cooling charge checks, and indoor return air in a normal comfort range. Outside those conditions, the right charging method may be weighing in the charge from the nameplate and line set length. The additional refrigerant charge calculator helps with the line set part.
This is the most important part of the guide. Superheat and subcooling respond strongly to airflow, and low airflow looks a lot like an undercharge or overcharge depending on which coil is affected.
Low airflow across the evaporator means less heat to boil the refrigerant. Suction pressure drops, superheat drops, and the coil can frost. A tech who sees low suction pressure and adds refrigerant without checking the filter can end up with an overcharged system that still does not cool well once the filter is changed.
Low airflow across the condenser means less heat rejected. Head pressure rises, and subcooling can look high or low depending on the system. A dirty condenser coil can hide the real state of the charge.
So the free, easy checks come first:
Only when airflow is right should superheat and subcooling be used to judge the charge. Many experienced techs say most charge complaints they find are actually airflow problems. You do not need to accept that as a rule, but it costs almost nothing to rule airflow out first.
A fixed orifice (piston or capillary tube) cannot adjust its flow. The amount of refrigerant reaching the evaporator depends on the pressures and the charge. That makes superheat a good indicator of charge on these systems, but only once you know what superheat should be for today's conditions.
Target superheat depends on the indoor wet bulb (how much heat and moisture the indoor air brings) and the outdoor dry bulb (how hard the condenser has to work). A common field estimate is:
Target superheat (°F) = (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2
Example: indoor wet bulb 63°F, outdoor dry bulb 85°F. Target = (189 − 80 − 85) ÷ 2 = 12°F. If your actual superheat is 12°F, give or take a few degrees, the charge is probably close. If it is 25°F, the coil is underfed. If it is 3°F, the coil is overfed.
This formula is an approximation of the charging charts manufacturers publish. When the manufacturer chart for the unit is available, it wins. The superheat and subcooling calculator uses this formula and flags a target below 5°F, because at those conditions charging by superheat is not reliable.
Measure return air wet bulb as close to the indoor unit as you can, with a psychrometer or a digital probe that reports wet bulb. Do not guess it from the thermostat. If your probe only gives temperature and relative humidity, the dew point and wet bulb calculator converts them.
If you adjust charge, go slowly. Add or recover in small amounts, wait several minutes, and read again. Superheat lags behind changes. Many techs have overshot a charge because they kept adding while the number was still settling.
A thermostatic expansion valve (TXV) or electronic expansion valve (EEV) adjusts refrigerant flow to hold superheat roughly steady at the evaporator outlet. That means superheat stays fairly constant even when the charge is off by a fair amount, so it is not a good charge indicator on these systems. Subcooling is.
Most manufacturers print a target subcooling on the outdoor unit nameplate or in the installation manual, often in the range of 8°F to 12°F, sometimes with a tolerance and sometimes with a chart that varies with outdoor temperature. Use that number for the unit you are working on. There is no universal subcooling target.
Even when you charge by subcooling, read superheat too. A TXV usually holds superheat somewhere around 8°F to 12°F at the evaporator outlet, though the manufacturer setting varies. A superheat far from that range with correct subcooling tells you the metering device, not the charge, deserves attention.
Experienced techs rarely look at superheat or subcooling alone. They read both, plus suction and head pressure, plus the air temperatures, and look for a pattern. Here is a simple table many techs keep in mind. Treat it as a starting point, not a verdict, and always confirm with other checks.
| Superheat | Subcooling | Suction pressure | Head pressure | Common suspects |
|---|---|---|---|---|
| High | Low | Low | Low | Low charge, possible leak |
| Low | High | High | High | Overcharge |
| High | High | Low | Normal or high | Restriction in liquid line, drier or metering device |
| Low | Low or normal | Low | Low or normal | Low indoor airflow, dirty evaporator, frozen coil |
| Normal or low | Low | High | Low | Weak compressor (inefficient pumping) |
| Normal | High | High | High | Dirty condenser, condenser fan problem, non-condensables |
Every one of these patterns has exceptions. A heat pump in cooling mode with a leaking reversing valve can look like a weak compressor. A system with a liquid line running through a hot attic can lose subcooling before it reaches the indoor unit. Use the table to choose the next check, not the final answer.
A residential R-410A split with a piston. Complaint: house not cooling, last tech added refrigerant two weeks ago. Readings: suction 98 psig (about 32°F saturation), suction line 38°F, superheat 6°F. Indoor wet bulb 64°F, outdoor 88°F, target superheat about 12°F. Liquid line subcooling 16°F, head pressure high for the conditions.
Low superheat and high subcooling point toward overcharge, but the low suction pressure needs explaining. The filter was packed and the evaporator was starting to frost. After a new filter and 20 minutes of runtime, suction came up, superheat rose to 4°F and subcooling stayed high. The system was now clearly overcharged from the previous top up. Recovering refrigerant in small steps brought superheat to about 11°F. The real cause was the filter; the added refrigerant was a second problem created by skipping the airflow check.
A light commercial rooftop unit with a TXV. Complaint: poor cooling on hot afternoons. Readings: superheat 24°F, subcooling 19°F, suction pressure low, head pressure normal. Charge looked high by subcooling, but the evaporator looked starved by superheat.
High superheat with high subcooling suggests a restriction. The liquid line drier felt noticeably cooler on the outlet than the inlet, and a temperature probe showed a 7°F drop across it. Replacing the drier, with proper recovery and evacuation, brought superheat to about 10°F and subcooling to the nameplate target without adding refrigerant. The vacuum decay log is useful for recording the evacuation afterward.
A residential system where superheat, subcooling and pressures were all close to target, but the customer said the house felt clammy. The refrigerant side was fine. The air side was not: the blower was set to a high speed tap that moved too much air for the humidity load, so the coil was not cold enough long enough to dehumidify well. Superheat and subcooling cannot see that. The air temperature split and the sensible and latent heat calculator told the story.
This is the main lesson: correct refrigerant numbers rule out some problems, but they do not prove the system is working the way the customer needs.
Superheat and subcooling are tools in a sequence, not a diagnosis by themselves. A sequence many techs follow on a cooling complaint looks like this:
Step 9 matters more than most people think. Readings recorded at the end of a good service call give the next tech a baseline. If superheat was 11°F last summer and is 22°F today under similar conditions, that change is useful information.
It is worth being honest about the limits:
The Brixon superheat and subcooling calculator runs in your phone browser with no account and no app to install. It lets you:
Saturation temperatures come from a reference PT table with interpolation. Values for some newer refrigerants are approximate, so confirm against the manufacturer's chart when a decision depends on one or two degrees. The results are estimates based on your inputs. Always verify against the manufacturer's data and the codes that apply where you work.
Related free tools you may use on the same call:
You can find all of them on the free field tools page.
Sometimes you take good readings, check airflow, and the pattern still does not match anything obvious. That is normal. Real systems have more than one problem at a time, and some faults only show up under certain loads.
That is where Brixon's diagnostic assistant can help. Describe the equipment, your readings and what you already checked, and it suggests the easy checks first and the next tests to narrow things down, with sources when available. It does not see the unit and it does not replace your judgment on site; it helps you think through the next step. Your first question is free with no account: ask Brixon about your readings.
Running a team and want everyone to share what they learn on these calls? See Brixon for teams.
Superheat is how many degrees the suction vapor is above its saturation temperature. Subcooling is how many degrees the liquid line is below its saturation temperature. Superheat describes the evaporator side, subcooling the condenser side.
Fixed orifice and capillary tube systems are usually charged by superheat. TXV and EEV systems are usually charged by subcooling. The manufacturer's installation manual sets the method and target for the unit.
It depends. On a fixed orifice system the target changes with indoor wet bulb and outdoor dry bulb; a common estimate is (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2. On a TXV system, superheat is often held somewhere around 8 to 12°F, but the manufacturer's setting applies.
Use the target printed on the outdoor unit nameplate or in the installation manual. Many residential systems list targets around 8 to 12°F, but there is no universal value.
Low indoor or outdoor airflow changes pressures, superheat and subcooling in ways that look like a charge problem. Adding or removing refrigerant before fixing airflow can create a second problem.
It often points to a restriction, such as a plugged filter drier, a stuck metering device or a kinked liquid line. Confirm with a temperature drop test across the drier and other checks before replacing parts.
For blends with temperature glide, use the dew point temperature for superheat and the bubble point temperature for subcooling. For near azeotropic refrigerants like R-410A the difference is very small.
No. They indicate whether the charge is likely close for the conditions. Only weighing the charge tells you the exact amount, and a low charge means there is a leak to find.
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