HVAC Contractors’ Guide to Troubleshooting Cooling Systems
Differences exist between R-410A and R-22, but best practices remain the same
R-410A has been the refrigerant of choice in residential and light commercial cooling systems since 2010, when federal government regulations mandated that new air conditioners and heat pumps could no longer be manufactured using R-22. Since then, the production of new R-22 has been slowly winding down and will stop completely at the end of 2019, although reclaimed R-22 will be available to service existing systems long after 2020.
While many of those R-22 systems have been replaced with R-410A units, a large installed base still remains, meaning that technicians will have to know how to troubleshoot and service both types of equipment for many years to come.
Although both refrigerants are used to move heat from one place to another, there are a few major differences between R-22 and R-410A. For example, R-22 is a single — or azeotropic —refrigerant, while R-410A is a 50/50 mix of two refrigerants — R-32 and R-125 — which makes it a zeotropic blend, according to Doug Bates, senior product support specialist, Rheem Mfg Co., AC Division.
“Each of the refrigerants in the R-410A blend boil and condense at different temperatures, so there is always a possibility of boiling one refrigerant before the other,” he said.
For this reason, R-410A must be charged as liquid only — it cannot be vapor charged. And the R-410A cylinder must be inverted when charging the system.
Another major difference between the two refrigerants is that R-410A systems use polyolester (POE) oil, while R-22 systems use mineral oil. POE oil is hydroscopic, meaning that it absorbs moisture quite easily, which is why it is important that technicians keep systems closed as much as possible during the installation process, said Tom Johnson, product service coordinator, residential cooling, Lennox Industries.
“Technicians should properly evacuate the system during startup,” he said. “They also should make sure that there is a filter drier installed to remove any moisture that may be in the current system. That’s important not only during the installation, but also when servicing the equipment. Any time the system is opened, whether it’s to change out a component or for other maintenance, technicians should definitely replace the drier to make sure the system remains moisture free.”
Mineral oil and POE oil should never be mixed, which is why technicians must have separate refrigerant gauge sets for R-22 and R-410A systems.
Refrigerant recovery and reclaim equipment also needs to be checked for compatibility in order to avoid cross contamination when switching from one refrigerant to another, said Bryan Rocky, director of residential technical services - ducted systems, Johnson Controls Inc.
“There are also real differences in safety concerns due to the higher operating pressures found in R-410A systems,” said Rocky. “Experienced technicians know that the typical operating pressures of R-410A systems are usually much higher (by about 50 percent) than the pressures found in R-22 systems. Those higher pressures could surprise a less experienced technician when servicing equipment.”
FOLLOW THESE BEST PRACTICES
Before technicians ever start troubleshooting a cooling system, they should first take the time to interview the customer. This step is crucial in understanding what types of problems the customer may be experiencing, but unfortunately, it is frequently overlooked.
“Ask good leading questions to understand the symptoms and their concerns,” said Rocky. “See if the homeowner made any changes that could impact the HVAC system operation (e.g., closed off rooms, covered up registers, redecorated or added rooms, etc.). Understand their problem, and the answers will usually help the technician diagnose the real root cause.”
Technicians should be aware that when a system is initially designed and installed, the load calculation reflects the reality of the home or space at that time, said Rocky.
Changes such as physical additions, new windows, additional occupants, heat-generating equipment (e.g., large flat screen TVs), and the loss of shade on the home all play into the successful performance of a system.
“Even the type of air filters that a customer uses will affect performance, because high-efficiency air filters have higher pressure drop and can significantly change the airflow through the system,” Rocky said.
After interviewing the customer, technicians should look over the equipment to see if there is anything that is obviously wrong, said Bryan Orr, co-founder of Kalos Services, Clermont, Florida, and founder of HVACRSchool.com.
“I advocate for doing a very detailed visual inspection before using fancy instruments,” he said. “The majority of issues that occur are common and fairly obvious when the technician takes the time to look carefully at the whole system. Checking filters, coils, capacitors, contactor points, drains, pans, connections, ports, wires, etc., will often lead to noticing abnormalities, which will lead to a full diagnosis.”
Johnson agrees, noting that technicians should make sure the indoor and outdoor units are properly matched and that the correct metering device is being used — usually a piston metering device for R-22 systems and a thermostatic expansion valve (TXV) in R-410A systems.
The indoor and outdoor coils should be inspected to ensure they’re clean, and then technicians should verify that the system is operating with the expected level of airflow.
THE IMPORTANCE OF AIRFLOW
It is absolutely crucial to verify proper airflow before putting gauges on a unit, added Bates, particularly if the system has performance issues. That’s because without the right airflow, the refrigerant cannot do its job, nor can the system be properly charged.
“The problem is that airflow is not as easy to measure as refrigerant pressures or voltages, and as a result, it’s usually the last thing checked,” he said. “These are sealed systems, so if they are charged properly at commissioning, and assuming there is no leak, then the charge is always right. Airflow restrictions, such as dirty or restrictive filters, dirty indoor coils or blower wheels, the opening or closing of registers, or something otherwise wrong with the duct system, will make the charge appear to be wrong.”
That’s why technicians should correct any problems with the system first, whether it’s cleaning the coils or setting the correct fan speed; otherwise, it will be more difficult to troubleshoot the system, noted Johnson.
“For example, if the suction pressure is low, technicians may just start adding refrigerant, without realizing that a plugged filter or a really dirty evaporator coil could be causing that low pressure reading,” he said. “Then, the system will have a dirty coil and be overcharged, and technicians will have to spend a lot more time troubleshooting.”
Because improper airflow is the No. 1 cause of problems in a/c and heat pump systems, techs should never assume that it is correct or even adequate for the system, said Rocky.
“They think, the air handler is rated for 1,200 cfm, so that must be what is being delivered, right?” he said. “This is the most challenging problem for most technicians, as they may not have the right equipment [manometer] or the knowledge to use the manometer to check static pressure, and they may not understand the importance of proper airflow to system performance.”
Once the visual inspection is finished, proper airflow is established, and any other corrections are made to the system, it is time to start taking measurements.
Orr recommends starting with the six primary readings: suction pressure, liquid pressure, superheat, subcooling, compressor amperage, and evaporator delta T (see Figure 1).
“From there, technicians can test temperature differences across the lines and driers to help find restrictions,” he said. “Remember that operating pressures are impacted by indoor and outdoor temperatures, and system efficiency will change accordingly.”
On older R-410A systems, for example, head pressures may be up to 450 psi when outdoor temperatures are over 95°F, said Orr. With newer equipment, which typically has higher-efficiency condensers, pressures are rarely over 400 psi, even on the hottest days, provided the system has proper condenser airflow.
When troubleshooting a system, it is important to keep in mind that the evaporator should be cooler than the air passing over it, so that it can absorb heat, said Bates. The greater the difference between the coil temperature and the air temperature, the better the heat transfer. That means that in a residential setting, an evaporator temperature of around 40° is usually low enough to pull both the sensible heat and latent heat from the air; however, it depends on how much moisture is present.
“A 30° coil would be better at removing both sensible and latent heat, but just imagine what water condensed from the air would do when sitting on a 30° metal surface,” Bates said. “A 50° or 60° coil may do some sensible work but will probably not remove as much moisture. The coil temperature has to be below the dew point to pull moisture from the air.”
On the condenser side, the coil must be warmer than the air passing over it in order to reject the heat, said Bates. So if it’s 80° outside, the coil must be warmer — around 90° to 110°.
In order to have a 40° indoor coil, suction pressure would need to be 118 psi, and to have a condenser coil with a temperature of 100°, the high-side pressure would have to be 317 psi.
“It’s worth noting that not every system should have these pressures,” he said. “It’s variable, dependent on outdoor temperature, outdoor airflow, indoor temperature, indoor humidity, indoor airflow, and surface areas of the coils.”
SUPERHEAT AND SUBCOOLING
Using superheat and subcooling measurements can also help technicians when troubleshooting cooling systems.
Superheat is measured on the low side of the system and is the difference between the actual refrigerant temperature and the saturation temperature of the refrigerant, explained Bates.
Subcooling is the same temperature difference on the high side.
“In the example given earlier, suction pressure of 118 psi converts to a saturated temperature of 40°. If the suction gas is 50°, then there is 10° of superheat,” he said. “Too much superheat means the compressor is not being cooled properly and can overheat. But it also means either the evaporator coil is operating at reduced capacity, or it is absorbing too much heat in the suction line, which is supposed to be insulated. Too little superheat means the compressor could be exposed to refrigerant in the liquid state.”
This could end up damaging the compressor, which is why it is so important for technicians to refer to the manufacturer’s data, which will show the acceptable range of superheat under various outdoor temperatures, said Johnson. This is particularly important with fixed orifice metering devices, as these systems are charged based on superheat.
“On an expansion valve system, there is typically more focus on subcooling, which is basically the charge in the outdoor coil,” he said. “Since the expansion valve keeps the superheat between 10° to 15°, as long as there is adequate charge, the expansion valve will control the superheat.”
With a fixed orifice metering device, technicians may assume that 10° to 15° superheat is the correct measurement, and it may be when it’s warmer outside, like 90° or 95°, but in lower outdoor temperatures, the superheat could be in the neighborhood of 20°, said Johnson.
That is why technicians should not only read the appropriate materials from the manufacturer, they should look at all measurements before making adjustments to a system. If they’re just looking at one particular value and acting on it, it is likely they will be pointed in the wrong direction, noted Johnson.
The bottom line is that technicians need to learn to diagnose the whole system before coming to any conclusions, said Orr.
“Often, we find that the real issue is simpler than we initially thought,” he said.
Publication date: 1/9/2019