Why is it sometimes necessary to add or remove refrigerant from a pre-charged air conditioning system?
Many systems are supplied pre-charged with a refrigerant charge (in the outdoor unit) suitable for correct operation of the system for a given maximum pipe run, for example 7.5m. If the actual pipe runs on site are longer, additional refrigerant must be added to the system to ensure correct operation. This is sometimes called a “trim charge”, and is specified in grams per additional meter of liquid line (expansion line) pipework above the maximum for which the unit is pre-charged. For example, if the pipe run separating the indoor unit and the outdoor unit is 17.5m, the unit is pre-charged for 7.5m, and the trim charge is 25g/m (this information is usually found in the installation manual) then the additional refrigerant that needs to be added is: 17.5 – 7.5 = 10, x 25 = 250 grams When pipe runs are very short, for example 2m or less, it is often recommended that charge is removed. The calculation is the same as before, but removing refrigerant. For example, if the liquid line pipe run
