How to Diognose Refrigerant Overcharge, TXV System

When a system with a TXV metering device is overcharged, the suction pressure and superheat may appear normal.  The normal conditionsare a result of the TXV regulating the refrigerant flow independent of the compressor.

The excess refrigerant is stored in the condenser coil and increases the head pressure and liquid subcooling.  If the charge is high enough the suction pressure may be elevated with a slightly low superheat.

Higher pressures increase power required for system operation.

Remove refrigerant in accordance with the manufacturers instructions to obtain the desired or target subcooling.

How to Diognose Refrigerant Overcharge, Fixed Orface

Overcharged systems will experience excessive energy consumption, possible compressor starting problems, internal overload tripping, indoor air humidity complaints and compressor failure due to liquid flood back.

Systems running with high charge will run at pressures above factory required levels.  Abnormally low superheat and high subcooling levels will be present. The compressor will experience liquid flood back problems.

Remove refrigerant to obtain manufacturers recomended level.

How to Diognose Low System Refrigerant Charge, TXV

When a system is undercharged and equipped with a TXV metering device, the TXV may open enough to maintain adequate suction pressure and suction vapor superheat.  The condensing pressure will be low along with low subcooling.  If the system charge is very low, the system will appear with the same symptoms as a fixed metering device.

  • Leak test the system
  • Charge to OEM requirements.
  • If OEM requirements are not available, a subcooling target of 10F can be used until specs are found.

When charging R-410A systems always charge in a liquid state only.

How-to Diognose Low System Refrigerant Charge, Fixed Expansion Device

Low system charge can be caused by a leak or improper charge when installed.
Low charge will result in low capacity, low EER, and a hot compressor.

Systems with a low charge will operate at pressures below the factory recommended pressures. High superheat and low subcooling will be evident. The compressor will run hot with insufficient colling for the motor windings. The low suction pressure may result in sub-freezing temperatures at the evaporator causing ice to form, further inhibiting air flow.

Check for leaks.
Charge the system in accordance with manufacturers instructions to the target superheat level. Make sure when charging R-410A to add refrigerant in the liquid state only.

How to Measure SubCooling

Subcooling is measured to adjust the refrigerant charge on AC/Heat Pump Systems with a TXV expansion device.

  • Run the condensing unit until pressures and temperatures stabilize.
  • Read and record the liquid pressure at the liquid line pressure port fitting at the condensing unit.
  • Place a digital temperature probe against the liquid line near the liquid gauge pressure port.  Read and record the temperature.
  • Reference the face of your high pressure gauge or use a pressure / temperature chart and convert the measured liquid pressure to the coresponding condenser coil saturation temperature for the refrigerant being used.  Record this value.
  • Next, subtract the measured liquid line temperature from the saturation temperature.  The result is your actual liquid subcooling level.

Example:

  • Refrigerant: R-22
  • Liquid pressure:  200 PSIG
  • Corresponding condenser saturation temperature:  100F
  • Liquid line temperature:  90F
  • 100F – 90F =Liquid subcooling
  • Liquid subcooling = 10F

How to Measure Superheat

  • Run the condensing unit until suction pressure and suction line temperature stabilize.
  • Read and record the suction pressure.
  • Place a digital temperature probe against the suction line near the suction gauge pressure port.  Read and record the temperature.
  • Reference the face of your suction gauge or use the temperature/pressure chart and convert the measured suction pressure to the corresponding evaporator coil saturation temperature for the refrigerant being used.  Round this value.
  • Next, subtract the saturation temperature from the measured suction line temperature.  The result is your actual suction vapor superheat.

Example

  • R22
  • Suction Pressure: 70 PSIG
  • Corresponding evaporator saturation temperature: 41F
  • Suction line temperature: 51F
  • 51F -41F = Superheat
  • Superheat = 10F

Refrigerant Charge for a Heat Pump or Air Conditioner

To purchase and use most refrigerants, you must hold an EPA Certification.  It is illegal and there are significant fines for handling refrigerants without the certification.

Heat pumps are air conditioners and need to be charged in the cooling season with an outside temperature above 70 degrees F.  Follow the manufacturer’s directions, usually on a label attached to the inside of the service panel.  The standard charge may be stamped on the model/serial label.  If the system is a split system, additional refrigerant for the line set piping is required.

Methods used to adjust the refrigerant charge:

  • All systems… Weigh-in the charge.  Remove all refrigerant and replace it with an accurate charge using scales or charging cylinder.
  • Systems with fixed orifice.  Measure superheat and add/remove refrigerant to obtain the target superheat.
  • Systems with TXV (thermal expansion valves).  Measure sub cooling and add/remove refrigerant to obtain the target sub cooling for the system.  (adjust the TXV for proper superheat)
  • Approach method (Lennox) See charging by the approach method on this site.
  • Beer Can Method.  Not recommended but very common.  Your energy consumption can double and system capacity can be reduced by 50% using this method.  You add/remove refrigerant until the suction line is as cold as a beer can.  We like 33F beer in Texas.

Superheat … degrees F added to the saturation temperature (temperature/pressure that the refrigerant changes from a liquid to a gas) of the refrigerant in the suction line as measured by the pressure.  A pressure gauge and digital thermometer are required to measure and calculate superheat.  Digital gauge sets are available that will do the calculations.  10F to 13F superheat is typical but follow manufacturer’s recommendations.

Sub cooling.. degrees F subtracted from the saturation temperature in the liquid line leaving the condenser and before the expansion device.  9F to 13F is typical but follow manufacturer’s recommendations.