How to Extend the Life of Your HVAC Equipment – This Summer and Beyond (Part 3)

Michael W. Dickenson, P.E., CFEI

Checking Refrigerant Charge:
Leave it to the Professional

As you may recall, in parts one and two of the blog series, we discussed several preventative steps you can take to extend the life of your HVAC system. But, there is one last and very important area to be checked on your HVAC system: the refrigerant charge. Proper refrigerant charge can ensure your system will perform at its listed/design capacity. Until now, the maintenance checks we discussed were tasks most homeowners could tackle on their own.  However, checking refrigerant charge is a complex process which should be conducted by a trained, licensed professional.  Even a small misstep in checking the refrigerant charge could result in damage to the HVAC system.

HVAC systems, both split-type and self-contained packaged units, contain refrigerant circuits which are considered closed-systems. This means the refrigeration circuits should not be opened by anyone other than a trained technician who is using the proper equipment and following proper procedures. However, despite being a closed-system, issues such as leaks may take place within the refrigeration circuit. In today’s post, we will discuss checking refrigerant charge on a split unit in cooling mode (summer) which operates on R-22 (commonly known as Freon) refrigerant.

HVAC systems contain a metering device which modulates the quantity of refrigerant entering the cooling coil (or evaporator). The metering device of the system, as well as the refrigerant used, will determine the method of charging required. Typical metering devices are a fixed-orifice type, or a thermal expansion valve (TXV). It is common to see the fixed-orifice metering device used with R-22 systems, whereas modern refrigerant systems use the TXV metering device. R-22 systems are being phased out due to EPA regulations; most modern systems use less ozone-depleting refrigerants, such as R-410a (sometimes called Puron). However, R-22 systems are still quite common in residential use because most R-22 systems are just now reaching the end of their service life. This means it is not uncommon to encounter refrigerant leaks with this type of system, and it is relevant for technicians to repair leaks as necessary.

A trained technician will use refrigerant manifold gauges as well as multiple types of thermometers to check refrigerant charge. Ideal conditions for checking refrigerant charge are hot and humid weather. The first step in the technician’s process is to set the system to provide cooling throughout the process. The technician will then measure the indoor/outdoor temperatures, the humidity conditions, and the refrigerant piping line temperature. The technician will also measure the refrigerant line pressures using special gauges suited for the particular type of refrigerant that is being checked. 

These temperatures and pressures are required for the technician to determine what is known as superheat – the temperature rise of the refrigerant without a rise in pressure. The refrigerant, by design, changes states from a liquid to a vapor (boiling) then from a vapor back to a liquid (condensing) during the refrigeration process. The superheat is the amount of energy in the refrigerant above the temperature at which the refrigerant changed state from a liquid to a vapor. While it’s important for the professionals to understand all the factors and variables related to superheat, what the average homeowner really needs to understand is that the superheat of the system being checked determines the quantity of refrigerant charge required.

Properties of refrigerant, such as temperatures and pressures, are known and tabulated for ease of access and are required to determine the system’s superheat. Once the system’s superheat is determined, the technician will compare that data with the known data for the refrigerant, (i.e., what the superheat would be under perfect, lab conditions).

We know that HVAC service calls can be expensive, and it is not uncommon for a system to be misdiagnosed. Below are some “rules-of-thumb” to keep in mind when talking with your HVAC technician.  Hopefully, this information will provide some insight into your HVAC problems and avoid unnecessary costs. (Note:  the conditions of each HVAC system will vary and the topics below are included for reference purposes only).  

  • If the system being checked is found to contain too little superheat, the system may be over-charged (too much refrigerant); alternatively, if it is found to have too much superheat, the system may be under-charged (too little refrigerant).
     
  • One of the two refrigerant piping lines is referred to as the suction line (sometimes called low or liquid side). If the temperature of the suction line is too low for its conditions, the evaporator may not be absorbing as much heat as it should. This may indicate the evaporator requires cleaning, the return air filter is dirty, or there is poor airflow, in general, across the evaporator.
     
  • The second refrigerant piping line is referred to as the vapor line (sometimes called hot gas, or high side). Elevated vapor pressure, coupled with low suction pressure, may indicate a blockage or restriction within the metering device.

In summary, checking refrigerant charge is a complex process and should be conducted by licensed, trained professionals. It is, however, valuable for homeowners to understand the common terms we’ve discussed here, and to have a high-level understanding of what the professionals will be looking for in your unit. If the HVAC system is suspected of not performing as intended, homeowners should not attempt to check the refrigerant charge themselves. A variety of other factors may need to be considered prior to checking refrigerant charge.

There is no doubt that performing routine maintenance on your HVAC system can save you time and money. As we have discussed in this three-part series, the HVAC system is quite complex with many components working together to provide heating/cooling to your home or place of business.

We hope, this series of articles has served to make you more comfortable with performing routine checks that can not only extend the life of your HVAC system but, just as importantly, better help you know when to call a professional and what steps they will take to repair and restore your system.

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You can read part 1 here and part 2 here.