Using the Superheat Method to Charge A/C Systems

August 10, 2001

One of the most important things you can do for your customers is to ensure that their refrigerant system is working properly. Of all the things that can go wrong with an air conditioner¿s refrigerant system, the most common is undercharge.

A study commissioned by Arizona Public Service, an Arizona utility, found that more than 70% of split systems tested were at least 5% low on refrigerant. As you know, low refrigerant charge impacts compressor reliability, as well as efficiency and performance.

I¿m sorry to tell you this, but even if you have proper pressure charts for a system, you can¿t tell if a system is charged properly in mild weather. In fact, if you charge a system up to the pressures listed on the system¿s pressure chart at 95 degrees Farenheit ambient, you can easily be 20% low on charge. If you charge at temperatures below 95 degrees, the problem is exaggerated.

Here¿s some more bad news; you¿d better sit down for this one. The temperature split does not tell you if a system is working properly, especially in mild weather.

The good news is that on most systems we work on, there is an accurate way to evaluate the refrigerant charge during any kind of weather: superheat.

Figure 1. If an evaporator is low on liquid refrigerant, the gaseous refrigerant picks up a lot of superheat.

The Pressure Quiz

When you check the operation of an air conditioner or heat pump, how do you know if it is charged correctly? Many technicians don¿t.

During a refrigerant system troubleshooting class I held in July, I gave 15 service techs a problem to solve. I gave them an example of a particular type of system operating under a particular load condition, and I asked them to write down what pressures they would expect this machine to operate at.

After having them turn in their papers, I discovered that the expected head pressures were anywhere between 225 and 360 lbs. The expected suction pressures were somewhere between 60 and 80 lbs. The purpose of giving this quiz to the class was to get them to realize that at least 14 of them didn¿t know how to charge an air conditioner. If you did this test at your company, you might discover the same thing.

The temptation when working in the field is to avoid a critical evaluation of the refrigerant charge, or to use some arbitrary refrigerant pressures as a guideline. I¿ve had many service techs tell me that an air conditioner¿s pressures should run about 275 and 70 lbs. My first question is, ¿at what load condition?¿

Most technicians know that since the refrigerant pressures fluctuate with changing load conditions, there is no particular set of pressures at which an air conditioner or heat pump will operate. Because of the confusion in the field about evaluating a system¿s charge, many machines are suffering from an over- or undercharge.

Figure 2. If the evaporator has an adequate amount of liquid refrigerant, the gaseous refrigerant absorbs enough heat to increase its temperature 12 degrees over its boiling temperature. The evaporator is operating at 12 degrees superheat.

Consequences of an Improper Charge

An undercharged machine will not only perform poorly with low efficiency, but will also suffer chronic compressor overheating.

There must be adequate refrigerant returning to the compressor to carry the compressor motor heat away to be dissipated into the condenser air. Since the compressor is the last component to receive any cooling, it is the first to suffer when the charge is short.

Compressor oil breakdown comes with compressor motor overheating. Chronic overheating of the oil degrades it, reducing its ability to lubricate the compressor properly. In severe cases, sludge is formed, which can negatively affect refrigerant controls such as expansion valves, check valves, and reversing valves.

Eventually, a system that is subjected to chronic overheating due to low refrigerant charge will have a major failure ¿ usually the compressor.

An overcharged system faces its share of calamities, too. Besides lowering performance and efficiency, overcharging can cause the refrigerant to flood back to the compressor crankcase during start-up, diluting the oil and causing compressor bearing damage.

Overcharging can also aggravate refrigerant migration in a system, causing refrigerant to accumulate in the compressor crankcase during the off cycle.

An overcharged system, like an undercharged system, will suffer a premature compressor failure if not repaired.

Future Problems

Many service technicians fail to appreciate these problems. I often hear, ¿I¿ve been doing this for years and I¿ve never had a problem with the systems I work on.¿

Of course, this is absolutely true. It¿s not the service technician who has the problem; it¿s the customer.

Usually failures that result from improper charging occur months or years after the machine becomes ¿refrigerant-impaired.¿

It¿s understandable that a lot of technicians fail to appreciate the critical nature of the refrigerant charge, because they rarely get to see the consequences. Some other tech or contractor ends up on the job when the machine finally fails.

It¿s not possible to cover all the aspects of proper charging in a single article, so I will cover one of the most significant: superheat.

Figure 3. An evaporator with too much liquid refrigerant; the gaseous refrigerant picks up less superheat.

Superheat and Charging

I, like many others, learned about the superheat method in school. When I went to work in the field, no one else I worked with was using it, so I had no one to reinforce my training and I assumed the habits of my coworkers. Eventually, though, I realized there was much more to air conditioners than I was aware of.

The day I rediscovered superheat was a revelation. Once I became experienced at taking and evaluating superheat measurements, my work became much easier.

Those who are not yet using superheat in their diagnostics will find it will have a profound effect on their ability to diagnose refrigerant systems and will increase their confidence.

If you don¿t believe me, find someone who uses superheat effectively and ask them.

Understanding Superheat

Let¿s do some foundation work to enable us to understand superheat.

Liquid refrigerant is fed into the evaporator of an air conditioner and is boiled off into a gaseous form. The heat used to boil the liquid refrigerant comes from the indoor air as it passes over the evaporator. After the refrigerant turns into a gas, it continues to absorb heat. The heat that the gas absorbs is called superheat.

The amount of liquid in the coil dictates how much evaporator space is remaining for the gas to flow through. This means that when there is a lot of liquid in the coil, the superheat will be low, and visa versa.

Let¿s assume that a machine should have a 12 degrees superheat. If an evaporator is low on liquid refrigerant, the gaseous refrigerant has a lot of evaporator to travel through before exiting, so it picks up a lot of superheat. The suction gas temperature, the boiling temperature of the liquid in the evaporator, and the suction pressure are shown in Figure 1.

Note that by the time the gaseous refrigerant exits the evaporator coil, it has absorbed enough heat to increase its temperature 20 degrees over its boiling temperature. This means that this evaporator is operating at a 20 degrees superheat.

If, as in Figure 2, the evaporator has an adequate amount of liquid refrigerant, the gaseous refrigerant has absorbed enough heat to increase its temperature 12 degrees over its boiling temperature. This means this evaporator is operating at a 12 degrees superheat.

In Figure 3, the evaporator has too much liquid refrigerant and so the gaseous refrigerant has even less evaporator to travel through, causing it to pick up even less superheat. In this case, the refrigerant has absorbed 4 degrees superheat.

If you know what a system¿s superheat is supposed to be and what its superheat actually is, you can determine if the evaporator has enough refrigerant in it.

Note: You may have noticed that in each of these examples, the boiling temperature of the liquid was the same. In an actual application this would not be likely, but it was done here to simplify the examples.

Next month we will discuss how to determine a system¿s required superheat, measure a system¿s actual superheat, and how to use this information to determine if the system is charged properly.

Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

Publication date: 08/13/2001