5 Install Mistakes that Kill Systems

When we install systems, we should have three main goals in mind: maximizing longevity, efficiency, and capacity. We want our units to work as long as possible, use the lowest amount of energy, and move the greatest amount of BTUs possible with a proper mix of sensible and latent.

Unfortunately, some installation practices can severely reduce these three qualities. Even small installation mistakes and oversights can affect a system’s performance, from evacuating without a micron gauge to forgetting to check for more leaks after finding one. 

This article will take an in-depth look at some of the worst installation practices that affect residential systems. We’ll also offer alternative best practices to help you with your future installations.


1. Improper brazing, flaring, and leak testing

Unsurprisingly, a good chunk of the system’s functionality relies on its piping system. Brazing, flaring, and leak testing are the main procedures we perform on pipes, and there are plenty of mistakes that can make you either create or fail to detect leaks.

Brazing: the basic mistakes

In HVAC work, brazing copper is a critical skill that takes lots of time and practice to hone. As a result, some common beginner mistakes can lead to poor installs. The inexperienced tech may move the torch around too much and may not heat the copper enough to draw the alloy into the joint. In cases like those, it’s best to focus on being steady and deliberate, which will come naturally with practice. Overheating of valves and components is also a common issue and is easily resolved by being more deliberate in prep to keep cold rags or heat blocking putty on components we are working on. 

You can avoid longevity and efficiency issues by sanding copper before cutting it. As always, you’ll also want to make sure to clear your workspace, slip on some gloves, and wear properly tinted safety glasses before you start brazing. 

To prevent contamination, seal off open tubing and don’t allow burrs to fall into the copper. The best thing you can do is hold the tube sideways or tilt it down during deburring or reaming. It also helps to have some practice under your belt and a good deburring tool.

Brazing: torch mistakes

You’ll want to heat the copper between 1200° F and 1300° F, and it will likely be a dark to medium cherry color. You can see a color chart below to determine the corresponding colors for temperatures. (Note: this chart is for pure copper. Mixtures may deviate from this table.)

When it comes to the actual torch, you’ll want what’s called a carburizing flame. The flame has layers, which we call “feathers.” All flames will have at least two feathers, but a carburizing flame has three. You can see that in the image below.

As you can see, the carburizing flame has an acetylene feather, which is an aqua-colored middle layer between the innermost white cone and the main blue feather. Even a neutral flame without an acetylene feather is okay, but you don’t want an oxidizing flame (no acetylene feather and a shorter cone). When copper oxidizes, it creates weaker bonds.


Creating flares with proper depth requires lots of skill and practice. If you don’t have a lot of flaring experience and don’t feel confident with the procedure, we recommend using a modern flaring tool to help you.

As with brazing, proper deburring is also a concern. Instead of worrying about copper pieces falling into the pipe, you must be careful not to overdo the deburring and thin out the line. Of course, you also want to make sure you deburr in the first place; otherwise, the flare might not seal well.

Cross-threading is common with inexperienced techs. The only way to overcome cross-threading is with patience and practice. Get used to pushing the two surfaces together gently and threading on carefully unit it's aligned properly and threads with ease, not just jamming and cranking. 

Over- and under-tightening are both quite common mistakes, especially among new techs. We highly suggest using a torque wrench to help you feel out the tightening and locate a sweet spot. 

All you need is a little dab of assembly lubricant, so you don’t need to seal the threads with a big gloop of sealant. Put just a little on the back of the flare and the surfaces you’ll be connecting. Some technicians don’t use thread sealant at all, but we find that assembly lubricant (particularly Nylog) works wonders at helping to get a better connection. 

Leak detection: pressure testing

Proper nitrogen pressure testing is a crucial part of good leak detection. The most prevalent mistakes tend to happen because techs get impatient and move too quickly missing leaks.

For example, some techs don’t hold the pressure test for at least 20 minutes. Honestly, 20 minutes isn’t even enough time to find small leaks in most systems; we recommend holding a test under nitrogen for at least an hour, even up to a day or two for very large piping systems like VRF and Grocery. Twenty minutes is the absolute bare minimum for a small residential system with a very accurate digital gauge. 

We don’t recommend using gauge manifolds, as they may leak and lead to confusion. Instead, we suggest using probes to reduce leak points and minimize confusion during the testing phase.

When you perform pressure tests, be sure to do it at low side test pressure. Installations typically require you to test the line set and evaporator coil, so you’ll want to check your equipment and test the recommended low side test pressure. Many techs test at lower pressures than recommended, such as testing at 100 PSI for every system (even though the specs may list 300-350 PSI). When you test at lower pressures, it takes longer to find leaks and increases the probability that you’ll walk away from the job with undetected leaks. 

Electronic leak detection

The previous section on pressure testing dealt heavily with nitrogen. It’s worth noting that the electronic leak detectors we’re about to cover CANNOT detect nitrogen. It will only respond to whatever it’s supposed to react to, which is usually refrigerant. If you want to detect nitrogen, you’ll have to either mix some trace refrigerant into the line (per EPA standards) or perform a bubble test. (Note: if you do a bubble test, make sure to remove those bubbles before going through with an electronic leak detector.)

The first electronic leak detection mistake is pretty self-explanatory: moving too fast. When you perform any task too quickly, you may skip over critical information. When you use an electronic leak detector, you must move very slowly over the coils, pipes, or whatever you’re testing. 

There are also a few places where techs can get confused. For example, some techs don’t remember the differences between the heated diode and infrared detectors. These detectors work differently, and you may miss a leak if you treat an infrared detector the same way you’d treat a heated diode one. Heated diode detectors continue to go off when you stop them over a leak, but infrared detectors’ signals will shut off when you stop moving them; you must continuously move an infrared detector over the leak.

Some techs also mistake port and gauge leaks for system leaks, especially when they detect leaks on the outdoor condenser coil. Refrigerants are naturally heavier than air and will settle in those areas. You’ll get a false positive test for a leak if you aren’t aware of your equipment and can’t distinguish a port or gauge leak from a system leak.

You also want to make sure that you don’t test for leaks while your indoor fan is running. The coils and lines should be as still as possible, as additional movement from the fans can make it difficult to locate leaks.

You can avoid these mistakes by purchasing a good leak detector (prepare to spend at least $300), knowing your equipment, and being diligent as to avoid making oversights that muddle your test results. Also, don’t walk away after finding one leak! There could be more in the line, so check the whole line slowly.


2. Failing to flow nitrogen

Oxidation can also occur when techs braze without nitrogen. Cupric oxide forms when oxygen and high temperatures mix, and the cupric oxide can fall away from piping and clog valves or be an all-around nuisance. 

Before you flow nitrogen, be sure to purge the lines. Moisture in the lines may condense during pressurization, which isn’t good for the system. After you remove your cores, you’ll use relatively high-pressure nitrogen to purge the lines before you flow.

As a best practice, we recommend flowing nitrogen at 3-5 standard cubic feet per hour (SCFH) while you braze. This pressure is quite a bit lower than purge pressure.

Bert made a highly informative video about flowing nitrogen on the HVAC School YouTube channel, which you can watch here. It reinforces the information in this article while providing some extra tips and footage of real-life field experience.


3. Poor Evacuation

Core removal is necessary for proper brazing, so this is not an issue if you keep the cores removed. However, some techs add them back before evacuating and don’t take them off again. Evacuation is a lot harder with Schrader cores still in, and they make your evacuation setup heat up pretty quickly. Overall, it’s best to keep the cores off until the system has a refrigerant charge.

Some techs say that they don’t need a micron gauge because they can read the suction gauge. While suction gauges can technically give you an idea of your vacuum, the scale is far too large to provide you with an accurate idea of your vacuum. It’s easier to have a more precise understanding of the vacuum when you use a micron gauge, so don’t toss it aside. The best way you can use a micron gauge is to attach it at the system’s furthest point, not the pump.

Connecting and taking care of your vacuum pump

The vacuum pump does all the heavy lifting for you. However, you can still make some mistakes that slow down your evacuation. Some errors can also result in leaks and take their toll on your vacuum pump’s health and longevity.

We don’t recommend evacuating with a manifold because it may make the process take longer, and manifolds often leak. Using a manifold is technically not incorrect, but it can present you with avoidable problems. If possible, it’s best to use core tools instead and hook the system directly up to the vacuum pump. 

It’s also not a great idea to use refrigerant hoses for evacuation. We use refrigerant hoses to transport the refrigerant, and it’s not a good idea to get oil, moisture, and other gases into those hoses. They are also relatively prone to leaks. Instead, use dedicated hoses (the shorter and larger, the better).  

Leaving a pump open to the atmosphere when it’s not in use presents a contamination risk to the oil inside the pump. Luckily, this blunder is easy to avoid if you stay attentive. You just cap the pump or shut it off when it’s not in use. Of course, be sure to perform proper pump maintenance as well.


4. No Airflow Setup

As many of you already know, maintaining proper airflow isn’t as easy as replacing the filter once every few months and making sure the duct design is ideal. You must make sure you have an appropriate fan and that the static pressure stays in an acceptable range. We see many mistakes that result from substandard airflow setups (or simply not making an effort to check the airflow in the first place).

While duct design may be the most obvious indicator of poor airflow, the filter also has a massive impact on airflow. The main mistake that technicians (and ordinary homeowners) make is assuming that a filter works because it fits. A filter’s suitability depends on more than just the MERV rating and size dimensions (though those are also important). 

To combat these challenges, choose your filter wisely as not to be too restrictive. It also helps to be fluent in the airflow rate/initial resistance charts. That way, you can understand how much of a pressure drop you can expect from a filter with a given airflow rate. I’ve attached one of those charts from a MERV 11 filter below.

CFM targets

CFM (cubic feet per minute) targets are moving targets. The mere thought of having a “target” CFM is a point of contention among scientists and educators, but that’s not what we’re looking for. Many techs can benefit from at least being aware of a typical CFM target range under design conditions.

Some techs assume that 400 CFM/ton is a rule of thumb, but it’s not. On a typical Florida summer day, our targets are typically around 350 CFM/ton, but that changes by model and by season. 

Take some extra time to assess the operating conditions, desired temperature, and ambient temperature to calculate the desired BTU output. You can get a more precise target CFM if you do the math, which will help you understand and achieve your target total external static pressure. (Of course, we think the industry would see some significant improvement if manufacturers could be more transparent about the targets. However, that’s just a dream right now.)

Total external static pressure (TESP): installation and testing 

An alarming number of techs don’t bother checking the TESP and adjusting the blower speed accordingly. In many cases where systems have incorrect airflow, it’s been wrong since the beginning. 

When you install a system, you’ll want to make sure the blower speed and airflow are ideal from the get-go. According to one of our articles written by Neil Comparetto, you can determine the blower speed by measuring the TESP and cross-referencing it to the manufacturer’s literature.

You’ll want to measure static pressure drops across multiple parts of the system. TESP is NOT an especially helpful measurement by itself because it won’t tell you where the source of the problem is. So, we’d consider it a mistake to not take  static pressure readings in the return/supply individually as well as across the coil, filter, and air handler/furnace. The solution, of course, is to take those measurements. That will be harder than you might think, but the best thing you can do is approach each job with a trusty manometer in hand.

As a general best practice, it also helps to ensure that the wiring and settings are correct before you start looking for pressure drops all over the system.

It’s also worth mentioning that some techs mistakenly assume that the design TESP will always be 0.5”. That’s not true for every case, so you’ll want to be familiar with your system and understand its unique TESP needs.


5. Compressor overheating and flooding

Mistakes on other parts of the system may have a catastrophic impact on the compressor. Nobody wants compressor failure, so we’ll talk about the errors primarily responsible for fatal overheating and flooding.

We just talked about airflow and won’t go back into it, but poor airflow can damage the compressor over time via poor oil return and flooding. We’ve already described the common airflow mistakes and best practices, so please consult section #4 for those.

Some techs don’t weigh their charge in or out of the system. If they don’t use scales, they have no way of keeping an accurate account of the system’s charge. Both excessive and insufficient amounts of charge strain the compressor over time. To avoid this issue, simply weigh the refrigerant charge in and out. That way, you’ll know what’s in the system and if it’s an appropriate amount or way out of line.

Flooding: crankcase heaters and superheat

Flooding occurs when the refrigerant migrates to the crankcase during the off cycle and condenses to a liquid. When you start up the compressor again, the refrigerant boils off with the oil, foaming it off and removing it from the system. It’s horrible for the compressor and leads to failure a lot more quickly than normal operation does.

The manufacturer may recommend using crankcase heaters to keep your compressor crankcase warm during the off cycle and prevent flooding from the liquid refrigerant. We’ve included a picture of your standard bellyband type of crankcase heater below.

Manufacturers may also recommend using liquid line shutoff solenoids and hard shutoff TXVs. All of these can be good defenses, but it’s best to check the manufacturer literature to see what they recommend for their systems.

Low superheat can also indicate an increased flooding risk. The superheat gives you a clue about the TXV condition, and the TXV is one of the barriers against flooding. Sometimes, the TXV’s bulb will be improperly mounted, and the TXV can flood back if that’s the case.

Overheating: temperature, airflow, and hard starts

There are three main high temperatures you must look out for: ambient, superheat, and suction line.

A system running through a hot attic will be hotter than one kept in a relatively cool closet. Obviously, the system in the attic will be more prone to overheating. High ambient temperatures make it easier for a compressor to overheat.

Low superheat indicates an increased flooding risk, but high superheat can indicate a compressor overheating problem. It could be set too high and demand too much work from the compressor, or maybe the TXV could be set improperly. Either way, it will strain your compressor and reduce its lifespan and efficiency. Compressors are refrigerant-cooled, and they need the proper temperature to function correctly.

You’ll ideally want to keep the suction temperature around 65° F. There will be cases when it’s higher, such as during a hot pull-down, but it shouldn’t otherwise exceed 65° during normal operation.

Another way to prevent overheating is to use the factory hard starts where recommended. Many techs don’t use those hard starts, especially in long-line applications. If you don’t use them when your compressor is under load, it may not start at all, or it could go into thermal overload. 


Even though we’ve only listed five install mistakes, it feels like we’ve covered a lot more than that. There are so many things to consider to avoid making mistakes, and we understand that it can be overwhelming to take all of this in at once. 

Nevertheless, we hope this article can help you perform some self-reflection and guide you in practicing better installation habits.

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