Measuring Superheat and Subcooling

A pressure reading alone does not tell you much. The same suction pressure can come from a healthy system or a sick one. Superheat and subcooling turn pressure and temperature into numbers that show what the refrigerant is really doing inside the coils. Almost every refrigeration diagnosis and charge check depends on them.

What You Will Be Able to Do

• Define superheat and subcooling in plain words.
• Measure each one in the right place.
• Calculate both from a pressure and a line temperature.
• Say what a high or low reading suggests.
• Avoid the errors that give false readings.

Saturation Is the Starting Point

Inside a coil where refrigerant is boiling or condensing, liquid and vapor exist together. In that condition the refrigerant is at its saturation temperature, and the pressure tells you exactly what that temperature is. You find it on a pressure-temperature (P-T) chart, or a digital gauge shows it for you once you select the correct refrigerant.

Superheat and subcooling both measure how far the refrigerant has moved away from saturation.

• Superheat is how much warmer a vapor is than its saturation temperature at that pressure.
• Subcooling is how much cooler a liquid is than its saturation temperature at that pressure.

Why Each One Matters

Superheat shows that all the liquid finished boiling before it left the evaporator. If superheat is too low, liquid may reach the compressor and damage it. If superheat is too high, the evaporator is starving for refrigerant and not doing full work.

Subcooling shows that the refrigerant leaving the condenser is fully liquid. Solid liquid is what the metering device needs. If subcooling is too low, vapor bubbles can reach the metering device and cut capacity.

Before You Measure

1. Check airflow first. A dirty filter or coil changes both readings and can fool you.
2. Confirm which refrigerant is in the system by reading the data plate, then select it on your gauges or use the matching P-T chart.
3. Let the system run at least 15 minutes so pressures and temperatures settle.
4. Clean the pipe where the temperature clamp will sit.
5. Clamp the probe tight to bare copper and cover it with insulation so outdoor air does not affect it.

Warning: Wear safety glasses and gloves when you connect to a service port. Liquid refrigerant can cause frostbite, and only trained, EPA-certified technicians should connect gauges to a system.

How to Measure Superheat

Superheat is measured on the suction line, as close to the evaporator outlet as practical. On a home split system that is often done at the outdoor unit's suction service valve, about 6 inches away, because the indoor coil may be hard to reach.

Formula: superheat = suction line temperature minus saturated suction temperature.

Worked example, R-410A:

1. Suction pressure reads 118 psig.
2. The P-T chart gives a saturation temperature of about 40°F.
3. The suction line clamp reads 52°F.
4. Superheat = 52 minus 40 = 12°F.

How to Measure Subcooling

Subcooling is measured on the liquid line near the condenser outlet, usually at the liquid service valve.

Formula: subcooling = saturated condensing temperature minus liquid line temperature.

Worked example, R-410A:

1. Liquid pressure reads about 340 psig.
2. The P-T chart gives a saturation temperature of about 105°F.
3. The liquid line clamp reads 95°F.
4. Subcooling = 105 minus 95 = 10°F.

Notice the subtraction flips. For superheat, the line is warmer than saturation. For subcooling, the line is cooler. If you get a negative number, recheck your readings.

Typical Targets

Always follow the data plate or the manufacturer's chart first. These ranges are common rules of thumb on a system running at normal load.

Below about 5°F of superheat, liquid may reach the compressor. Above about 20°F, the evaporator is starved. Subcooling below about 5°F or above about 20°F also deserves investigation.

What the Numbers Suggest

Use these patterns with pressures, temperature split and amp draw. No single number diagnoses a system.

Which Number Tells You About Charge

On a fixed orifice or capillary tube system, superheat tells you about charge, because nothing is adjusting the flow. On a TXV or EEV system, the valve holds superheat steady on purpose, so normal superheat tells you little about charge. Use subcooling there instead.

Blends and Temperature Glide

Some refrigerant blends do not boil at a single temperature. Between the start and end of boiling, the temperature slides a few degrees. This is called glide. On these refrigerants, a P-T chart lists two columns:

• Dew point: use this for superheat.
• Bubble point: use this for subcooling.

R-407C and R-448A have noticeable glide. R-410A has almost none, and R-454B has a small one. Digital gauges handle this when you select the right refrigerant.

Common Mistakes

Check Your Understanding

A system on R-410A shows 118 psig at the suction port and the suction line is 60°F. What is the superheat, and what might it suggest? Saturation is about 40°F, so superheat is 20°F. That is high, and it suggests a starved evaporator, such as low charge or a restriction.

A TXV system shows normal superheat. Can you say the charge is correct? No. The TXV holds superheat steady across a range of charge. Check subcooling.

You calculate subcooling and get minus 4°F. What should you do? Recheck. A negative number usually means a bad probe contact, the wrong refrigerant selected or a misread pressure.

A blend's chart shows both dew and bubble columns. Which column do you use for superheat? The dew point column.

Key Takeaways

• Superheat is vapor temperature above saturation, and subcooling is liquid temperature below saturation.
• Measure superheat on the suction line and subcooling on the liquid line.
• Check airflow, wait 15 minutes and select the correct refrigerant before you trust the readings.
• Fixed devices are judged by superheat, and TXV or EEV systems by subcooling.
• The data plate or manufacturer chart beats any rule of thumb.