As computing power escalated and inverter technology kicked in, diagnosing air conditioning problems became a lot more complex. Mechanical engineer Steven Mantine explains why the service market is changing and how to deal with the high level of frustration from end users complaining it is all too hard.

If these complaints referred to a large complicated HVAC system it would be understandable but I often hear the same complaints time and again for a simple single split unit.

This is usually due to a prolonged (and sometimes incorrect) diagnostic procedures that lead to a waste of time and money (and frustration for both the end-user and the service agent).

Air conditioners have evolved and that is exactly what we need to do when it comes to managing service problems. Here are some tips to help ease the pain.

It is not a system, it’s an eco-system. Compare your HVAC system to any other appliance in your building and there are significant differences. For example, a TV is installed and users get the same level of service whether they are in Victoria or on a Queensland. An air conditioner’s performance is determined by installation and conditions on-site, pipe length, duct work (which affects airflow), roof space temperature for a ducted unit, and the list goes on. All of these factors must be taken into consideration.

It’s a computer with a compressor. In the good old days, it was simple - a couple of thermistors, an on/off compressor, fan motors and that’s it. If there was higher demand the compressor would kick in, and when you got below the desired set point the compressor would cut out.  Today things are different.

The compressor is controlled by software embedded on the PCB. All the thermistors, valves, pressure transducers, pressure switches etc. are connected to the PCB. Information gathered from the system operates according to the software inside. Many times, the software has protection protocols (algorithms) that prevent the unit from ramping.

The system will go to protection mode several times before it will produce an error code. To learn more about the algorithms users must read the manuals. Unless you have a monitoring tool there is no way to identify the protection protocols.

Use the Engineering and Service manuals. To understand the system, we need to be able to understand how it should work. A good example is in heating operation.  If the ambient temp (outside) is above 20 degrees the building could be cold inside although the outside temperature is rising.

Despite this some systems will not ramp up. This is normal. You will be able to see the operation limit in the Engineering Manual and save yourself time. Since the system's behaviour today is based on software, many algorithms and procedures run in the background. The algorithms that the unit follows are described in the manuals. It is easier to tell if the unit works properly if you understand what it should do in the first place.

The manuals will also provide you with the best practice for diagnosing a specific unit, and how to make a proper error reclaim from the unit.

The system doesn’t see temperature and pressure only resistance and voltage. Sounds very simple but it might be very challenging. The thermistors are alternating the temperature to resistance, the pressure transducers transform pressure to voltage. Based on that information the HVAC system makes decisions. And here is where it becomes tricky. If the component is 100 per cent faulty the unit will give you an error code, however, if it is misreading the temp by just a couple of degrees (Ohms) or the pressure by a few Pascals (Volts) the unit will work but not as expected, and an error code will not appear.

And here it becomes even more complicated, in some cases bench tests can give you the right value (the value you should see according to the service manual), but the component is faulty as it might be falling under certain conditions. For example, you have a hairline crack on a thermistor shell, but issues will only arise when moisture gets inside.

To be able to properly diagnose this you need to have access to a monitoring tool. All HVAC companies offer a diagnostic device that connects to the system on one side and your PC / Cell phone app on the other side enables you to see what the unit can see and compare it to your actual measurements on-site (your pressure gauges and temp readings). In many cases, just by connecting the device, you will be able to identify a faulty component.

Get used to testing run/trial operation. The basic reason for the system to increase the compressor frequency is the difference between the return air thermistor to the setpoint (different systems have other factors as well, but this is the basic one that I will use for this example). If you want to diagnose the system on a mild day, it might be idle in low frequency and will not increase the compressor speed. To force the unit to ramp we need to use the test run/trial operation.

The unit will force itself to increase the compressor speed, and if it won’t, the unit is diagnosing itself by doing it. You have to allow the unit to ramp properly (depending on the system it might take a while). Finish the procedure to get a full picture of the unit operation. If you have a monitor tool connected it will provide you with valuable data about the system behaviour/protection.  Every manufacturer and every model has a different procedure so please refer to the specific system manual for instructions.

Allow the unit to stabilise. Inverters systems take between 15 to 30 minutes (and sometimes even more) to stabilise. Let’s say you suspect a refrigerant shortage and decide to add refrigerant. The system will need time to digest the refrigerant and show some kind of change on your gauges. If you don’t see the change, wait. The same goes to testing operations. It takes time. Don’t rush. On a side note, many times I am asked to analyse a system’s behaviour, and there isn’t enough data to get any conclusion.

Get a monitoring tool and learn how to use it. All the major HVAC companies have monitoring tools. This tool allows you to connect and see how the system will behave. If the system is in error it is much harder to identify the problem without a monitoring tool. It will also allow you to record data and produce graphs that allow you to analyse the system while also referring to the service manuals. Connecting your gauges to the system, without being able to know what the unit sees, is like trying to diagnose the system blindfolded.

Check the airflow. HVAC systems are designed to perform under certain airflow conditions.

Iif the unit tries to maintain a Super-heat of 5K over the heat exchangerby controlling the opening ratio of the expansion valve – and the airflow is below a certain level - the system will decrease the expansion valve opening to keep a proper Superheat and prevent the risk of wet-operation. Another thing you might discover is an airflow difference between the Inlet and the outlet that might indicate air leaks or untreated air going to the unit which impacts performance, and leads to a dirty heat exchanger. Again, do the test after you have allowed the unit to stabilise.

Check the inlet and outlet temp from the unit. Especially in ducted units, we might have a major difference between the energy the unit transfers and what is introduced to the building. We have to check (again, after the system has stabilised) the Inlet and outlet air temp and compare it to the temp of the return air grille and the diffusers. We might discover that we lose energy to the roof space.

This was a very brief explanation about the basics of HVAC service diagnostic there is so much more to cover. However, these tips will improve service skills and may save time, money and a lot of frustration.

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