Checking AC Refrigerant Charge

• In order to check the refrigerant charge on an air conditioning system, the saturated temperature on the vapor line must remain above 32 °F.
• The superheat method, total superheat method, subcooling method, or another method specified by a manufacturer can be used to check the refrigerant charge.
• These methods are also used for troubleshooting the system.
• Always check the indoor airflow before attaching the manifold gauge set.

Basic Procedure Prior to Checking Refrigerant Charge

• If the AC system can't lower the temperature and humidity fast enough, verify these things before connecting the manifold gauge:
  • Is the building envelope tight? Close any open doors, windows, or attic entrances.
  • With the system off, check and replace the filter if it's dirty to ensure it is clean.
  • Visually inspect the condition and size of the outdoor unit, indoor unit, and ducts.
  • Verify the size and amount of ducting, return grilles, and supply registers.
  • Ensure the airflow volume matches the capacity of the outdoor unit and indoor coil.
  • For split systems, turn off the outdoor unit at the electrical disconnect and turn on the indoor thermostat in AC mode to check airflow.
  • For packaged units, turn on the blower in fan mode to check airflow, but note it's likely lower than in cooling mode.
  • Ensure proper airflow comes from all supply registers (see Chapter 15).
  • Outdoor and indoor temperatures should both be at 70°F or above.
  • To check the charge, attach the manifold gauge set, turn on the outdoor electrical disconnect for split systems, or turn on the thermostat in AC mode for packaged units.

Frozen Evaporator Coil

• Diagnose a frozen evaporator coil using total superheat and subcooling methods to determine refrigerant charge and heat transfer at both coils.
• If the saturated temperature of the refrigerant on the low side gauge doesn't rise above 32°F after three minutes, there is a problem causing water vapor to freeze on the evaporator coil.
• If the evaporator coil is already frozen, melt the ice completely before turning the compressor on for diagnosis.
• Consider potential water damage from melting ice, even with an emergency condensate pan.
• Low side saturated temperature, total superheat, and subcooling measurements are helpful to diagnose the actual problem.
• The type of metering device must be known before interpreting these measurements.
• Frozen evaporator coil issues can be caused by:
  • Low Indoor Airflow
  • Low Refrigerant Charge
  • Liquid Line Restriction
• Each problem will have different superheat and subcooling combinations, but all three will exhibit a low side saturated temperature below 32°F.
• Diagnose the underlying problem before the evaporator coil freezes.
• Three scenarios may have a correct refrigerant charge level.
• Technicians may find the system overcharged while having low airflow or a liquid line restriction, caused by previous technicians adding refrigerant to raise the low side sat temp without addressing the underlying problem.
• This leads to system inefficiency and possible compressor damage, indicated by very high subcooling.

Low Indoor Airflow

• Indicators of a low indoor airflow problem are based on the type of metering device.
  • TXV: Vapor Sat Temp Below 32° F, Normal Superheat, Normal to High Subcooling
  • Fixed Orifice: Vapor Sat Temp Below 32° F, Low Superheat, Normal to Low Subcooling
• Low indoor airflow can be due to:
  • Undersized or collapsed ducts
  • Undersized or blocked grilles/registers
  • Clogged air filter
  • Dust clogging the indoor coil or secondary heat exchanger
  • Low blower speed
  • Dirty blower wheel
  • Broken blower motor
  • Overall high static pressure for the blower motor

Low Refrigerant Charge

• Indicators of a low refrigerant charge are based on the metering device type:
  • TXV: Vapor Sat Temp Below 32° F, Normal to High Superheat, Low Subcooling
  • Fixed Orifice: Vapor Sat Temp Below 32° F, High Superheat, Low Subcooling
• If an AC system that previously worked is now low on refrigerant it has a leak that should me fixed.
• If a new system is low on refrigerant, add it slowly into the suction port while the system is running until the superheat and subcooling are correct.
• A low refrigerant charge may cause the manifold gauge set to indicate a hunting action with fluctuating pressures as the TXV tries to maintain superheat.

Liquid Line Restriction

• Indicators of a liquid line restriction are not based on the metering device type.
  • TXV or Fixed Orifice: Vapor Sat Temp Below 32° F, High Superheat, High Sub-cooling
• A liquid line restriction could be due to a clog in: the strainer, filter drier, distributor, capillary tubes, or TXV.
• It may also be due to a bad TXV failed to a closed position such as a loss of bulb charge.
• Verify the liquid line service valve is fully open to diagnose.
• Take a temperature reading across the filter drier; a substantially lower downstream temperature indicates blockage.
• After determining the problem, pump down the unit or recover refrigerant to address it.

Result of a Frozen Evaporator Coil

• Frost on a standard AC system's suction line indicates the refrigerant sat temp in the evaporator coil is too low, due to either a lack of refrigerant or of heat for the refrigerant to absorb.
• Any water vapor crossing a 32°F or lower evaporator coil is freezing.
• A low airflow problem on a system with a fixed orifice metering device will damage the compressor faster and more severely than a system with a TXV.
• The coil doesn't need to be frozen before liquid refrigerant enters the compressor in a system with low refrigerant charge or liquid line restriction.
• Since compressors are designed for vapor only, any saturated refrigerant adds strain and damages it.
• A TXV with a low airflow problem at the evaporator will only allow a small amount of refrigerant into the evaporator coil helping to partially safeguard the compressor.
• A low refrigerant charge or liquid line restriction can lead to saturated refrigerant damaging the compressor once the coil is almost frozen and the superheat.
• Icing of the evaporator coil insulates it, preventing heat absorption and resulting in saturated refrigerant entering the compressor.
• Melting of ice from a frozen coil can cause water damage in situations where it drips outside the condensate pan.
• A secondary pan with a safety switch or drain is a safeguard.

Overcharged System

• Indicators of an overcharged system are based on the metering device:
  • TXV: Normal Superheat, High Subcooling
  • Fixed Orifice: Low Superheat, High Subcooling
• An overcharged system leads to higher discharge pressure, electrical usage, and lower lifespan for the compressor.
• Fixed orifice systems can experience compressor damage due to saturated refrigerant.

Overcharged System With a TXV

• In an overcharged system with a TXV, the TXV only allows the correct amount of refrigerant into the evaporator coil, sending extra refrigerant to the condenser coil.
• Increased subcooling and excess liquid refrigerant reduce space in the condenser coil, making the compressor work harder and raising high side pressure and sat temp.
• The outdoor condenser coil cannot reject heat well leading yo higher a higher than normal temperature in the subcooled liquid.
• This reduces electrical efficiency, compressor lifespan, and heat removal capacity.

Overcharged System With a Fixed Orifice

• If a system with a fixed orifice is overcharged, the superheat will be lower and the subcooling higher than normal.
• High and low side system pressures will be higher than normal, which lowers heat removal capacity and electrical efficiency.
• The more important result is possibility of saturated refrigerant in the compressor which can lead to system breakdown.
• Total superheat means refrigerant is vapor ensuring compressor safety.

Excessive High Side Pressure

• Indicators based on the type of metering device:
  • TXV: Normal Superheat, High Subcooling, High Vapor Pressure, High Liquid Pressure, Low Delta T
  • Fixed Orifice: High Superheat, High Subcooling, High Vapor Pressure, High Liquid Pressure, Low Delta T
• Excessive high side pressure decreases electrical efficiency as the compressor works hard to reject heat.
• It causes faster wear on system components due to higher pressures for a longer time.
• The system will experience a higher vapor saturated pressure and a lower Delta T.

Contaminated Refrigerant

• Indicators of a refrigerant charge that is contaminated with air or nitrogen stay the same whether the metering device is a TXV or fixed orifice.
  • TXV or Fixed Orifice: High Superheat, High Subcooling, High Vapor Sat Temp
• If a system doesn't have very good heat transfer and the refrigerant pressures are high and not aligning to the proper sat temp levels of the expected refrigerant in the system, there may be a mix of two different refrigerants, an incorrect refrigerant, or air or nitrogen in the system.
• If air/nitrogen is there, the high side pressure will be higher than the normal pressure but will not allow the subcooling to be accurate.
• The pressures may rise and fall dramatically back and forth while running, wether the unit is equipped with a TXV or a fixed metering device.
• To test, turn off the system until equalized.
• Verify the equalization of the refrigerant pressures until the low and high side pressures are the same, it may takes hours.
• Convert guage pressure reading to its saturated temperature.
• Compare this sat temp reading with the surrounding DB temperature.
  • If they don't match the refrigerant may be contaminated.
  • If the sat temp is higher the outdoor DB temp there may be air, nitrogen or a higher boiling point refrigerant mixed in.
  • If the sat temp is lower than the outdoor DB temp there is likely different refrigerant listed on the rating plate with the same sat temp.
• Another test for a very low refrigerant charge is the refrigerant pressure wont align to its saturated temperature with a P/T chart.

Weak Compressor Valves or a Bad Reversing Valve

• Indicators of Weak Compressor Valves or a Bad Reversing Valve Regardless of the Metering Device: TXV or Fixed Orifice: Normal to High Superheat, Normal to High Subcooling, Very High Vapor Sat Temp
• Leaking compressor valves, a leaking internal pressure relief valve, or a high heat load can increase vapor pressure.
• If high pressure is not due to heat load the compressor may be leaking discharge gas into its suction.
• The problem can be diagnosed by performing a partial pump down procedure.
• This procedure will determine if the compressor is capable of pumping the refrigerant into the condenser coil.
• If the compressor's valves are bad it wont move refrigerant to the condenser and the pressures on the vapor and liquid lines do not go down.
• if a loud noise occurs and the pressures rise again, the problem is the internal pressure relief valve blowing prematurely.
• A faulty reversing valve may leak discharge gas into the suction line accumulator and back to the compressor inlet on a heat pump.

A. Reversing Valve not Sealing or Moving Properly

• The reversing valve may be the problem if the vapor pressure on the suction port is high while building heat load is normal.
• When the valve doesn't separate discharge tube pressure and section this occurs it creates a recirculation of discharge gas back to the inlet of the compressor.
• Mount one temp sensor ont he tube going from the reversing valve to the accumulator.
• Mount the second temp sensor on the vapor line tube that connects the evaporator to the reversing valve in cooling mode.
• Mount the second tenp sensor on the vapor tube that connects the outdoor coil to the reversing valve in heating mode.
• There is a reversing valve problem if the temperatire split is more than 6d F and the valve isnt moving of sealing.
• Even if the refrigerant charge is correct, the coil voltage is good, the coil is energizing the pilot valve as is should the reverse valve may be defective.
• Raise the high side pressure to free the valve if seized.

TXV Problems

• TXV Metering Device has Lost Refrigerant Charge from the Bulb, is Clogged, or is Stuck in the Closed Position: TXV Bulb Charge Lost = High Superheat, High Subcooling
• Any of these TXV problems is considered a liquid line restriction which prevents the devide from allowing sufficient refrigerant into the evap coil.
• Place the bulb into a container filled with hot water to verify charge is lost by disconnecting the straps that secure to suction line.
• If the pressure on the low tide of the system rises and superheat decreases then a low charge on bulb is not a restricition.
• If system pressure it doesn't react when placed in hot water then ether it is out of refrigerant or another liquid line restriction is present.

TXV Bulb is Not Insulated, is Only Loosely Attached to the Vapor Line, or is Not Attached to the Vapor Line at All

• Uninsulated or Detached TXV Bulb = Low Superheat, Low Subcooling
• For an air conditioning system, the TXV bulb should be located on the vapor line right at the outlet of the evaporator coil.
• Always insulate the bulb whether inside or outside the coil box.
• When the bulb is mounted outside the coil box, problems such as the bulb being loose, uninsulated, or off the vapor line will cause the refrigerant inside the bulb to absorb extra heat from its surroundings and increase the pressure onto the TXV head which results in a lower superheat.

Troubleshooting Guide

The troubleshooting guide was created during the testing and study of running HVAC units in cooling mode and does not encompass all scenarios and results may differ from system to system due and the units tested were split type air conditioning and heat pump systems with a single speed compressor.