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Troubleshooting HVAC Systems Using Refrigerant Subcooling And Superheat

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Servicing and troubleshooting HVAC systems can be challenging for experienced as well as entry-level HVAC technicians. Irrespective of your work experience, the size of the equipment you use or location, troubleshooting such a system requires a good understanding of the basics of refrigeration, including the principles of subcooling and superheat. You will also need to have the appropriate tools and understand how to apply both principles to use to tool easily and efficiently.

Most troubleshooting techniques often need simultaneous knowledge of pressure, temperature, current, and voltage values in an HVAC system, which implies that a single-function meter will not allow for a complete analysis of this system. Often, multiple tools are needed.

Refrigeration Cycle

Refrigeration uses the basic principle of heat flowing naturally from warmer to colder areas. This cycle has the following stages: compression of the hot gas, cooling, condensation, subcooling, expansion, evaporation, and superheating. A vapor compression refrigeration system usually consists of four main components; a metering device, compressor, condenser, and evaporator.

Compression energy usually increases the vapor pressure to a boiling point which is below the temperature of the condensing medium. This implies that the compressor rises boiling point of a refrigerant to the point at which water or air moving across the condenser is just low enough to condense this refrigerant to a liquid. Any additional passes through the condenser cool the coolant below its standard boiling point so that it remains a liquid as it experiences a pressure drop as it moves to the evaporator.

A metering device which is located at the evaporator inlet will restrict the flow and drop the refrigerant pressure to a lower boiling point. The new boiling point is below the water or air temperature so that the water or air across the evaporator causes the refrigerant to boil. After the coolant boils to vapor, it picks up the extra heat through the additional passes in the evaporator.

The compressor usually reduces the gas to high pressure while at the same time raising its temperature. This hot gas is then delivered to a condenser where it is cooled, releasing heat and slowly converting this gas into a liquid. And when this liquid at high pressure gets to the metering device, this cycle begins again. When servicing most HVAC systems, a technician will measure the pressure and temperature to assess system performance. Closely monitoring pressure and temperature to verify proper operation and control could reduce energy consumption and ensure longer system life. In most cases, measuring pressure or temperature at crucial places in the system can help pinpoint trouble spots and in this case, superheat and subcooling measurements are vital.

Trouble Diagnosis

Data from the subcooling and superheat measurements could be vital for determining different conditions in your HVAC system, including amount of refrigerant charge and assessing operating conditions of the metering device. Such measurements could also come in handy when you want to determine the efficiency of the compressor, evaporator, and condenser.

And before you can make a decisive conclusion from the data measured, it is essential that you look at external conditions which influence system performance. More specifically, you need to inspect and verify proper air flow in terms of cubic feet per minute across the coil surfaces. You should also remember to look for some obvious issues at the coil surfaces like dirty air filters or outside debris or leaves restricting flow of air to the condenser.

Troubleshooting Using Superheat

The superheat measurement obtained could indicate different system problems like an overcharge, clogged filter drier, undercharge, restricted air flow, faulty metering device or improper blower or fan motor direction. The suction line superheat is the best place to begin your diagnosis since a low reading will suggest that the liquid refrigerant could be getting to the compressor. During normal operation, a coolant getting into the compressor is usually sufficiently superheated beyond the evaporator boiling temperature to ascertain the compression only draws vapor and not a liquid refrigerant.

A zero or low superheat reading usually indicates that your refrigerant failed to pick up sufficient heat in the evaporator to boil entirely into vapor. When a liquid coolant is drawn to a compressor, it results in slugging which could damage mechanical components and compressor valves. Besides, the presence of a liquid refrigerant in a compressor could mix with oil thus reducing lubrication and increasing wear, consequently resulting in premature failure.

Superheat readings that are in excess of more than 20 degrees Fahrenheit to 30 degrees Fahrenheit, show that a refrigerant has collected more heat than usual or that an evaporator is being starved of the coolant. The possible causes of such a condition include a metering device which is underfeeding, simply broken or improperly adjusted. Any additional problems with high superheat could mean there is a system undercharge, moisture in your system, a refrigerant restriction, a blocked filter drier or excess heat loads on the evaporator.

Troubleshooting Using Subcooling

Inconsistent subcooling values could indicate different HVAC system problems such as undercharge, overcharge, insufficient condenser airflow or liquid line restriction. Typically, a refrigerant is subcooled between 10 degrees Fahrenheit to 20 degrees Fahrenheit at the condenser. However, a few modern equipment could have subcooling values that are as low as 4 degrees to meet the minimum efficiency standards.

For instance, a low reading of between zero and 10 degrees Fahrenheit shows that your refrigerant didn’t lose its usual heat as it traveled through the condenser. Some of the possible causes of this condition include metering device issues like maladjustment or overfeeding, insufficient air flow or a system undercharge. In many instances, the problem is usually that the condenser coil surface should be thoroughly cleaned to reduce airflow restriction.

Excess subcooling also implies that the refrigerant underwent more cooling than usual. A possible explanation could be underfeeding, an overcharged system, restrictions in the metering device or some faulty head pressure control during some low ambient conditions.

The next time you need to maintain or service your HVAC system, be sure first to check the subcooling and superheat at the unit. Conduct a visual inspection of your equipment to ensure all the coil surfaces are clean and that the fans are rotating in the appropriate direction.

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