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The topics are:-

  • Pre-commission checks
  • Refrigerant gauges
  • Evacuation
  • Refrigerant charging
  • Operational conditions
  • Completion and "Hand Over‟

Refrigerant charging :-

Predominately, today‟s split air conditioning systems are pre-charged. This makes charging a system quite simple. The outdoor unit will have a service valve with a schrader access valve (smaller systems) or 2 service valves on larger systems (usually referred to as the suction and liquid service valves). Once evacuation has been achieved, systems that are pre-charged can have their service valves opened to release the refrigerant into the system. Provided the piping length is within the manufacturer‟s acceptable limits, the system can be considered fully charged. Should the piping length exceed the manufacturer‟s limits then additional refrigerant will need to be added. This can only be done with the unit operating..


Systems that are not pre-charged should have the manufacturer‟s specified quantity of refrigerant charged in. This should be done using refrigerant scales..

The gas bottle is placed on the scales which is electronically weighted

Once the refrigerant has been released or weighed into the system, the system can now be powered up. Where the system can only be connected with one service gauge line, the system should be operated on cooling to ensure suction pressure is being read by the compound gauge.

Operational conditions :-

When the system is first started, it is important to monitor service gauges to ensure correct compressor operation. This is extremely essential for systems that operate with three phase scroll compressors as scroll compressors are directionally sensitive. Should the pressures not show a significant differential (between suction and discharge) then the compressor rotation must be reversed. This is done by simply reversing any two phases supplying the compressor. Another tell-tale sign of incorrect rotation is the sound of the compressor – it has sometimes been referred to as sounding like a „bucket of bolts being mixed‟. In any event, a scroll compressor, rotating in the wrong direction must not be left operating for any period of time – irreversible damage will probably be caused.


Once the system is running, it should be allowed to stabilise few a minimum of 15 minutes. The operating suction pressures (and if possible condensing pressure) should be checked and recorded. Additionally, a suitable temperature probe should be used to measure the suction line temperature (SLT) at the suction service valve. Using a pressure / temperature chart and the operating suction pressure (allowing for pressure drop), the saturated suction temperature (SST) can be determined. The amount of "suction line superheat‟ (SSH) can now be calculated by subtracting the saturated suction temperature from the suction line temperature.


A system that is considered fully charged will being operating with a suction line superheat, SSH range of 5-8K. A higher SSH may indicate additional refrigerant is required; a lower SSH may indicate refrigerant needs to be removed.

Note: When adding "400‟ series refrigerant to a system, it must charged as liquid only. This should be done through the system's suction port and carried out with extreme care, metered in slowly, so as not to "liquid lock‟ the compressor.


It should be noted that ambient temperature, return air or space temperature will affect the amount of SSH and should be allowed for before adjusting the refrigerant charge.

A secondary check of the refrigerant can be done where the operating condensing pressure can be observed. A correctly charged system will generally operate with a range of 4-10K liquid sub-cooling (SC). To calculate this, the temperature of the liquid line (LLT) should be determined; using a pressure / temperature chart, the condensing pressure can be used to determine condensing temperature (CT); subtracting the liquid line temperature from this value will confirm the amount of liquid sub-cooling.


A word of caution – checking the refrigerant charge by simply seeing if the "suction line is sweating‟ doesn‟t indicate the correct charge. A system should always be checked against the operating pressures and temperatures.

Once it has been established that the refrigerant charge is correct, all other operating parameters should be checked and recorded perhaps using a manufacturer supplied or self produced "commissioning sheet‟. Aspects and values that should be checked and recorded may include:-

  • Operating suction pressure
  • Operating condensing pressure
  • Ambient temperature
  • Supply air temperature
  • Return air temperature
  • Supply voltage
  • Supply current draw
  • Indoor fan speed operation
  • Zone operation (ducted systems only)
  • Ancillary items such as louvres
  • Filter access
  • Thermostat operation
  • Condensate drain operation

The system should also be checked in a heating mode (if reverse cycle) with appropriate values checked and recorded. Once all values are within operating parameters, the unit is now ready for continuous operation within acceptable operating limits.

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This page was last modified on 6th June 2010 and maybe out of date with regards to its information at the time of reading this article. The information above is only intended for use a guide and should not be 100% relied upon as a substitute for official manufacturer technical advice. Quality Electrics disclaims responsibility for any damage, claim, or liability any person may incur, whether caused by negligence or otherwise, as a result of anything contained in our articles.


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