This tech tip is based on the podcast with Joey Henderson, “Using Refrigerant Troubleshooting Charts w/ Joey Henderson.” You can listen to that podcast HERE.

The HVAC trade is very much a visual one; I wouldn’t recommend doing a drinking game where you take a shot each time you read “Do a thorough visual inspection” in a tech tip. However, our eyes aren’t just focused on wire rubouts, corrosion, or oil spots—or even our gauges. We’re looking at P-T charts, psychrometric charts, and diagnostic charts (like the one below). 

Just like galvanic corrosion and oil spots on a coil, those charts provide only part of the picture. You don’t just look at a chart and see your answer, just as you don’t automatically diagnose a leaky evaporator coil when you see a bit of galvanic corrosion. They help us take mental shortcuts, but we need to be able to fill in the gaps first.

Diagnostic charts are useful, but they cannot help unless you have the right mindset to use them.

Quick Checks vs. Setting the Charge vs. Troubleshooting

Before anything else, we need to know exactly what we’re trying to accomplish. Are we trying to set the charge after a major repair? Troubleshoot a system? Or just do a quick check on a PM or after a service procedure?

When we’re just doing quick checks, we probably won’t use diagnostic charts—at least not to their full extent. The diagnostic chart just narrows down possibilities if you do notice issues. Wet-bulb and dry-bulb temperature splits are good quick-check items to keep in mind on any service call. If those are in range, it’ll spare you the need to hook up your gauges.

If you’re setting the charge, you’ll want to use the manufacturer’s charging charts. (Make sure your airflow is dialed in first. Remember ABC: Airflow Before Charging!) Sure, a diagnostic chart can help point you to a possible undercharge or overcharge condition, but it’s not going to help you dial in a charge precisely. Most manufacturers have charging charts you can consult, and THIS tech tip breaks down the differences in charging methods and tools. 

The diagnostic chart is really meant to be used for troubleshooting—you have your gauges, psychrometers, and electrical meter at the ready and just need some direction to make sense of all of your readings. If you have the Five Pillars—superheat, subcooling, suction pressure, head pressure, and delta T (temperature split)—then you’ll get a much clearer picture. (Compressor amps also help with diagnosis.)

Measurements Mean Nothing Without References

Let’s say you do a non-invasive test on a suction line, read 50 degrees, and take a look at the 5 Pillars chart (re-pasted below for your convenience).

It’s pretty easy to diagnose the system, right?

Of course not! 

That reading is meaningless by itself; you can’t see the picture with just one piece of the puzzle, especially when HVAC diagnosis is an art with so much nuance.

There are so many gaps we’re missing here. Let’s start by doing the quick math to get the superheat. First, we’d need to know the suction pressure—let’s say we measure it and get 110 PSI. Then, we’d have to know the refrigerant type. If it were R-410A, a PT chart or app would show us that the SST would be around 36 degrees Fahrenheit, so that would leave us with a 14-degree superheat. 

What does that even mean, though? Even with the superheat at our disposal, we need to know whether it can be considered normal, high, or low so that we can use our diagnostic chart. 

Frustrating as the answer may be, it really depends, especially when you consider:

• What is the indoor ambient temperature?
• Does the system use a TXV or a piston for a metering device?
• If doing non-invasive testing, where is the temperature clamp on the suction line—at the evaporator outlet or right before the compressor? 
• Is the line set 15 feet long or 200 feet long? 
• Is the unit a traditional split system or a ductless system? (Some ductless systems have very low superheat values—almost zero.)

Diagnostic charts won’t be much help if we don’t get the full picture by figuring out all those other things. The charts let us know about possibilities if we have high or low superheat, subcooling, suction pressure, head pressure, and delta T—all five pillars, not just one or two. 

The biggest danger of diagnostic charts is putting blind faith in them to lead you to the right answer if you aren’t thorough, intentional, or nuanced in the way you gather and interpret data in the field. This danger is especially true if you rely heavily on outdated rules of thumb or unreliable (or misused) test instruments.

Rules of Thumb & Evolving Equipment

Some of the rules of thumb we may have used in the past no longer apply. Beer can cold is just one of them; that never was an accurate means of measurement, but it was way more likely to help you on R-22 systems than R-410A systems. 

The same is true for pressure readings. This issue won’t be as prevalent as we switch from R-410A to the two most prevalent new A2L refrigerants (R-32 and R-454B) because those operating pressures are similar. However, the suction and discharge pressure rules of thumb we used for R-22 completely went out the window when we went to R-410A, which has significantly higher operating pressures.

The rules of thumb for condensing temperature over ambient (CTOA or “condenser split”) have also gradually changed over the years. When HVAC systems were far less efficient in the 80s and 90s, a 30°F CTOA was considered a reliable rule of thumb. Nowadays, even standard-efficiency equipment (14-SEER) will have a lower CTOA. High-efficiency equipment could have a CTOA of 15°F or less.

Tool Quality, Usage, and Calibration

When it comes to HVAC tools, you tend to get what you pay for. The more accurate tools tend to be more expensive than cheap ones. (However, you can get a good discount on some quality tools at trutechtools.com with the discount code GETSCHOOLED.) That’s especially true of equipment like psychrometers, which pick up the wet-bulb temperatures for use in your evaporator TD (and help you determine your target superheat).

However, tool quality isn’t the only concern. Tools are only reliable when they’re calibrated correctly. Otherwise, our readings will be off. In the case of temperature clamps, we can calibrate them using ice water (water that’s mid-phase change will always be 32°F or 0°C).

We also have to be mindful of where we place our tools, especially when it comes to temperature clamps on long, uninsulated line sets. The superheat at the compressor inlet will be higher than at the evaporator outlet. 

Conclusion

The diagnostic chart is a tool to help you interpret your readings—but it’s more like an interpretation of several interpretations. You have to know whether your superheat is high, suction pressure is low, your tools are calibrated, you’ve put the temperature clamp on the right part of the suction line, and so on.

So, where do we begin? We start by getting references and understanding the nuances of the system we’re working on. What’s high for this particular system and diagnostic setup? What’s low? 

These things ultimately take field experience and pattern recognition, which develop with time. Once you get a feel for what’s normal in your market and notice those patterns as you work on more equipment, you can use the diagnostic chart to take mental shortcuts.