Arkansas HVACR NewsMagazine May 2025 Issue

HVACR NewsMagazine May 2025

Tech News

Adding more refrigerant will increase the subcool reading, and recovering refrigerant will decrease the subcool reading.

Approach Method

Lennox factory information asks that we charge by the approach method on TXV systems. I suggest charging to at least a 6 degree subcool before even attempting to calculate the approach method. The approach method is a calculation based on the relationship of liquid line temperature to the outdoor temperature. To calculate the approach, subtract the outdoor ambient from the actual liquid line temperature. The outdoor temperature used to calculate the approach should always be taken in the shade and away from the hot condenser discharge air. To increase the approach differential, you would remove refrigerant; you would add refrigerant to decrease it. Some Lennox heat pump systems come with a subcool chart next to the approach chart. This subcool chart is for <65˚. This means the subcool chart is only valid when the outdoor temperature is below 65˚. Follow the instructions on the unit carefully when ch arging in subcooling in <65˚ temperatures. The method requires that you block sections of the coil to achieve higher head pressures before setting by subcooling.

First, make sure you switch your hoses so that the suction gauge is reading off of the “common suction” port that taps in between the compressor and the reversing valve. You may put your high side gauge on either the discharge or liquid (on most systems), depending on what you are checking. Before doing any heat mode charging, use common sense. If installing a new system, the best bet is to calculate line distance and weigh in any additional charge before moving on to the detailed testing phase. The first one is the 100˚ over ambient discharge temperature rule. The general rule is that a properly charged unit will have a discharge line temperature of 100˚ above the outdoor ambient temperature, but this is only a rule of thumb and cannot be relied upon. If the discharge line is too hot, you would add refrigerant, which would lower the discharge temperature. Alternatively, if the discharge line were too cool, you would remove some refrigerant to raise the discharge temperature. This rule is to be used only as an indicator and, in some instances, may not be accurate given some other factors such as dirty coils, excessive superheated refrigerant entering the compressor, etc.

Heat Mode Charging for Heat Pumps

In most, if not all, cases, you will charge a unit in heat mode according to the manufacturer's recommendations. In those cases where no information is available, there are other indicators that you may use to set a proper charge in heat mode.