Anytime we have a compressor failure, we can’t just pull out the bad compressor and drop a new one in. We have to put our detective hats on and find the culprit so that it doesn’t happen again. In many cases, acid is a prime suspect. Whenever there’s acid in the system, the next compressor failure is a matter of when, not if, so acid testing should be a part of every compressor replacement protocol. 

When you have a bad burnout, acid may be obvious as soon as you hook up your gauges; it has a pungent odor. However, you can’t always rely on the sniff test, especially if there is only a small concentration of acid or you need a deeper analysis of the oil in the system. And what if there isn’t a burnout, but you have some moisture contamination? 

There are three main methods for acid testing, and they all have their ideal use cases. We’ll go over each one and explain how they work and when to use them. 

A Brief Review of Acid Formation

There are a few ways acid can get into the system, and it’s usually because of chemical changes in the oil. 

In any type of compression refrigeration system, excessive heat can cause oil breakdown. We can get excessive heat whenever there are high discharge temperatures (hence the “225 stay alive” rule for discharge temperature), but it can also happen in burnout situations. Electrical burnouts happen when there is arcing in the compressor. An arc introduces sudden high heat in the windings, which will lead to oil breakdown. 

However, systems that use R-410A, R-454B, and R-32 will typically use POE oil, which breaks down into acids when it reacts with moisture in the system. POE oil is hygroscopic, meaning it attracts water. Some oils, like PVE and PAG, also attract water, but POE is uniquely problematic because water makes it decompose to acids and alcohols.

Large concentrations of acid can often be detected by smell, especially in burnouts, but we should really have hard evidence of contamination. That’s where acid testing comes in. 

Port Testing

The simplest and quickest form of acid testing is port testing, such as with the QwikCheck. These port testing kits have tubes with built-in core depressors and a paper inside that changes color when exposed to acid. You can perform the test by pressing the core depressor up to the suction service valve port and sampling the refrigerant that comes out. (Any venting that occurs is considered de minimis, so the EPA doesn’t condemn the use of these tests in good faith.) If there is acid in the system, the indicator will change color, such as from yellow to orange or red in the case of the QwikCheck.

These port tests can give you an idea of whether acid from severe oil breakdown, particularly from burnouts, has been detected in the refrigerant. It does NOT test the oil itself, and the packaging of the QwikCheck even states that the product isn’t designed to detect mild organic acids from POE oil breakdown. However, it does test for residual acid in the system and is accurate when used correctly in residential and light commercial applications where there is just one compressor.

If you’re out in the field and dealing with a compressor failure on a residential call, a port test kit will typically suffice. 

Field Testing

Companies like Sporlan also offer field testing kits that sample oil from the system and expose it to a reagent. Sporlan’s Test-All kit uses a liquid solution, but other brands may use litmus paper or other materials. Like the port test, the manufacturer’s reagent will change color if there is acid inside the oil sample. 

Oil field test kits will be more useful than port test kits in larger systems where the oil charge is shared among multiple compressors. The same is true of cases where there is oil breakdown from hydrolysis or high temperatures, not just burnout.

You’ll need to pull oil from the HVAC or refrigeration system for field testing, which is easier in some configurations than others. Most parallel racks make this easy; they have oil drainage ports that you can open and collect a sample from. However, in systems without these oil drainage ports, you will need to install an oil trap with an adapter or hoses to collect the oil. Sporlan explains how to obtain oil samples in Form 40-141.

In the case of Sporlan’s Test-All kit, there are three different vials: one for mineral or alkylbenzene (AB) oil, one for POE oil, and a large test vial with a red-orange liquid. If you wanted to test POE oil, you would pour the POE vial into the large test vial with the colored solution, which will turn the liquid a reddish purple. Then, you’d fill the empty POE vial with the oil sample and pour it into the solution, shaking it and letting it sit. Acids in the oil sample will make the solution turn from purple to orange or yellow.

Lab Testing & Compressor Autopsies

In mission-critical refrigeration applications, especially where oil is shared among multiple compressors, we may need to pull out the big guns. We’ll want to submit an oil sample to a lab and do a compressor teardown to look at the extent and possible origin of the damage.

While field tests can give us general ideas of the acid concentration, a proper lab test can give us exact moisture and acid concentrations. A lab test will also identify particulates inside the sample, which can tell us whether we should be worried about solid contaminants like metal shavings and what that might mean for the system’s overall health (for example, if there is mechanical wear). 

Lab tests can also measure the viscosity of the oil. Some refrigerants that have higher discharge temperatures need more viscous oil to dissipate some of that heat and lubricate components without breaking down. (For example, R-32 has higher discharge temperatures than R-410A and R-454B and requires a more viscous oil.) Viscosity anomalies may indicate other issues, and lower viscosities are more susceptible to oil breakdown under high-temperature conditions.

Compressor autopsies will let you see what happened inside failed compressors so that you can identify and mitigate issues before installing a new compressor. Tearing down a compressor will let you see mechanical wear, metal shavings or debris, copper plating, or other clues as to why a compressor may have failed. 

Sending out a lab sample and doing a compressor teardown will be overkill for typical residential and light commercial applications. However, it’s a high-value test for expensive, mission-critical applications in commercial HVAC/R.

When to Use Each

Ultimately, the decision about which method to use comes down to the application, especially the oil system and the cost of a failed compressor. If you’re already aware of an electrical failure in the compressor of a residential or light commercial unit (or just want to do a quick check during a PM), then there’s usually no reason to go for anything fancier than a port test that samples a little refrigerant. 

When you start getting into large, expensive systems that share oil between multiple compressors, then checking the oil becomes much more important. While a field test kit is usually good enough to tell you if there is acid in the oil, it doesn’t give you the full picture about the oil composition. If you need a thorough breakdown of what’s in the oil, including moisture and particulates, for the future health of the system, then you’d want to go all out. Send an oil sample to a lab and tear down the failed compressor for a detailed analysis. It may seem like a lot, but the future compressor (and your customer) will thank you.