Superheat and Subcooling: Reading What the System Is Telling You

Superheat and subcooling are 2 temperature differences measured at opposite ends of the circuit, and they are read together. Their 4 combinations point at 4 different faults, and only 2 of the 4 are corrected by adding or removing refrigerant. The other 2 are a metering device or a restriction.

The short version

A pressure gauge tells you what a pressure is. It does not tell you why.

Superheat and subcooling turn two pressures and two temperatures into a statement about what the refrigerant is doing. Superheat says what is happening on the low side, where the refrigerant finished boiling. Subcooling says what is happening on the high side, where it finished condensing.

Taken alone, each number has several explanations. Taken together, the pair narrows a no-cool call to one of four causes before a part is ordered.

The target numbers are not universal. They come from the manufacturer for that model, under the conditions you are measuring in, which is why a charging chart beats a remembered figure every time.

What is superheat, and where is it measured?

Superheat is how much warmer the refrigerant vapor is than the temperature at which it boiled, at the pressure it is under. It exists only after the refrigerant has fully turned to vapor.

Taking it is three steps and one lookup.

• Read the suction pressure at the point you are measuring
• Convert that pressure to its saturation temperature for the refrigerant in the system, from a pressure temperature chart or an app
• Measure the actual line temperature at the same point, with a clamp that has good contact
• Subtract the saturation temperature from the measured temperature

Where you stand changes what the number means. At the evaporator outlet it describes the evaporator. At the compressor inlet it describes the evaporator plus everything the suction line picked up on the way, which on a long run through a hot kitchen can be a large part of the reading.

Low superheat means liquid refrigerant is reaching further down the coil than it should, and the extreme case is liquid arriving at the compressor. High superheat means the refrigerant ran out of liquid early and the last part of the coil is doing nothing useful.

What is subcooling, and why is it a liquid line number?

Subcooling is how much colder the liquid refrigerant is than the temperature at which it condensed, at the pressure it is under. It is measured on the liquid line, usually at the condenser outlet or close to the metering device.

Same arithmetic in the other direction. Convert the liquid line pressure to its saturation temperature, measure the actual liquid line temperature, and subtract the measurement from the saturation temperature.

Subcooling answers a different question than superheat. It tells you whether there is a solid column of liquid arriving at the metering device. Without that, the device cannot meter anything, because it was designed to control liquid and not a mixture of liquid and vapor.

Which is why low subcooling and high superheat so often appear together. One is the cause and the other is the consequence.

What are the target numbers?

They come from the equipment, not from memory. The manufacturer sets the target for that model at the conditions it is running in, and it is printed on the data plate, in the service literature or on the chart inside the panel.

Published guidance varies, and that variation is the point rather than a problem with the sources. A reach-in at 70F ambient and a remote condensing unit on a roof at 95F have different correct answers, and the same equipment has a different target in July than in January.

Two rules survive every disagreement about the exact figure.

• A thermostatic expansion valve controls superheat. On a system with one, superheat is the number the valve is holding, so a superheat that will not move while the charge changes says the valve is the subject of the conversation
• A fixed orifice does not control anything. On a capillary tube or piston system, superheat moves with the charge and the load, so the manufacturer's charging chart for the ambient and return conditions is the only correct reference

Record the ambient temperature next to every reading. A superheat figure without the conditions it was taken under cannot be compared to the one from the last visit, which turns two useful measurements into two numbers in a notebook.

What do the 4 combinations mean?

This is the table worth carrying. Read the pair, not either number alone.

Superheat	Subcooling	What it points at	What to do next
High	Low	Undercharge, which in a sealed system means a leak	Find the leak and repair it before adding anything
Low	High	Overcharge, or a condenser that cannot reject its heat	Check the condenser and the fan first, then recover refrigerant if the charge really is high
High	High	A restriction between condenser and evaporator, or a metering device closed too far	Look at the filter drier, the liquid line and the valve. Refrigerant will not fix this
Low	Low	The metering device is passing too much, or the load on the coil is far above design	Check the valve and its sensing bulb, and check what the kitchen just loaded into the box

The third row is the one that costs money. High and high looks like a charge problem to anyone reading one number, and a technician who adds refrigerant watches the subcooling climb while the superheat sits where it was. That behavior is itself the diagnosis, and it is worth recognizing before the cylinder comes off the van.

The fourth row is the one that is often not a fault at all. A cooler that just took 200 pounds of warm product has a load far above design and will read low on both until it catches up.

Which misreadings send a technician after the wrong part?

Four of them account for most wasted trips, and none of them is about the arithmetic.

• Measuring in the wrong place. Compressor inlet superheat on a long suction line reads high because of the line, not the coil. The number is correct and the conclusion is wrong
• Poor sensor contact. A clamp on a painted or dirty line, or on a line without insulation around the probe, reads the room as much as the pipe
• The wrong refrigerant on the chart. A blend has a different pressure temperature relationship, and several blends also have a temperature glide, so the saturation point depends on which end of the glide the chart is giving you
• Reading a system that has not settled. Immediately after a defrost cycle or a door being held open, nothing in the circuit is at steady state and every number is a snapshot of a transient

A fifth one deserves its own sentence. Taking readings on a coil choked with frost or a condenser packed with grease measures the blockage, not the charge, and the readings come good the moment the equipment is clean.

Why do ambient and airflow come before the gauges?

Because both of them move superheat and subcooling without anything being wrong with the refrigerant, and connecting gauges is not free. Every connection is an opportunity to lose charge and introduce air or moisture.

The sequence that saves the most time on a no-cool call in a commercial kitchen:

• Confirm the unit is in a running cycle and not in defrost
• Look at the evaporator for frost and at the fans for rotation
• Look at the condenser for grease, dust and clearance, and at the ambient around it
• Check the door seal and whether the box has just been loaded
• Then take temperatures, and only then connect gauges

On the kind of equipment a kitchen runs, the first four steps resolve a large share of calls. The equipment cares about the air it is given more than it cares about anything a technician does with a manifold. The wider mechanics of that are in how commercial refrigeration systems work.

What is worth recording so the next visit starts ahead?

A superheat reading is only a diagnosis on the day. It becomes a trend when the same number from the same point exists from 6 months ago, and a trend is what justifies replacing a unit instead of repairing it again.

• Superheat and subcooling with the exact point each was taken at
• Suction and liquid pressures, and the refrigerant in the system
• Ambient at the condenser and box temperature at the time
• Whether the coil was clean when measured, because that decides whether the reading is comparable
• Refrigerant added or recovered, which the federal rules also require per appliance

Those belong on the equipment record rather than in a job note. A reading attached to a job is attached to a day. The same reading attached to the unit builds a history that the next technician reads before getting out of the van, and the compliance side of the same records is covered in EPA 608 and refrigerant handling.

Roopairs runs a product tour where you can see equipment records per asset, service history and technician documentation in one place.

What service companies in the trade say

“My favourite thing about Roopairs is the equipment tracking. Being able to have the history at the spur of the moment when the customer requests it. The old school version would be me flipping through my day minder going, when were you there last and worked on the bar cooler. With Roopairs I can just pull open the equipment, type in the serial number, and the repair history is there. Flawless.” — ProTemp Solutions

Also worth reading

• Commercial Refrigeration: How the Systems Work
• Commercial Ice Machines: Why They Fail
• Technician Notes That Are Worth Reading Next Time

Frequently asked questions

How do you calculate superheat?

Read the suction pressure, convert it to the saturation temperature for the refrigerant in the system, measure the actual line temperature at the same point, and subtract the saturation temperature from the measured temperature. The result describes the evaporator at the coil outlet and the evaporator plus the suction line at the compressor.

How do you calculate subcooling?

Read the liquid line pressure, convert it to its saturation temperature, measure the actual liquid line temperature, and subtract the measurement from the saturation temperature. It is taken on the liquid line, and it tells you whether solid liquid is arriving at the metering device.

What does high superheat with low subcooling mean?

It points at an undercharge, and in a sealed system an undercharge means refrigerant left through a leak. The leak gets found and repaired before anything is added, because topping up a leaking system returns the same call in a few weeks with the same reading.

What does high superheat with high subcooling mean?

A restriction between the condenser and the evaporator, or a metering device that is closed too far. Liquid is stacking up on the high side and not enough is reaching the coil. Adding refrigerant does not correct it, and the usual places to look are the filter drier, the liquid line and the valve.

What superheat should a system run?

Whatever the manufacturer specifies for that model at the conditions you are measuring in, taken from the data plate, the service literature or the charging chart. Published general ranges vary because the correct figure depends on the equipment, the ambient and the load, so a chart beats a remembered number.

Does a thermostatic expansion valve change how superheat is read?

Yes. A thermostatic expansion valve actively controls superheat, so on that kind of system the figure is what the valve is holding rather than a direct reflection of the charge. A superheat that does not move while subcooling climbs during charging is the signature of a valve problem rather than a charge problem.

Why are my readings different right after a defrost cycle?

Because the circuit is not at steady state. For a period after defrost, after a door has been held open or after warm product has been loaded, pressures and temperatures are still moving and any reading is a snapshot of a transient. Let the unit settle into a normal running cycle first.