Checking the refrigerant charge of a heat pump in cold weather can be confusing for many technicians, at least partly due to the many different methods recommended by different manufacturers. Although there are many methods, most are described as heating performance charts or heating cycle check charts. A check chart is just that, a chart for CHECKING the charge during the heating cycle. It is not intended to be used for actually ADDING refrigerant. Why? Lower outdoor temperatures cause a lower evaporator temperature and pressure. Lowering the evaporator pressure increases the compression ratio, so the compressor capacity is reduced. Less refrigerant is being circulated in the refrigerant system because of the diminished compressor capacity at low outside ambient temperatures. The refrigerant that is not being circulated must sit somewhere, usually in the accumulator. For this reason, a charge which is adequate to maintain correct pressures and temperatures at 25°F may not be adequate at 35°F. How? Suppose that the unit was undercharged according to the check chart. If you add just enough refrigerant to bring it up to the required pressures, you have only added enough refrigerant for correct operation at that one condition. When the outside temperature rises, the unit will be capable of pumping more refrigerant. However, since you didn't add any extra refrigerant, there won't be any more to pump. The system will be undercharged. Just adding a little extra is not a great ida either because you really have no way to determine how much extra to add. This is why manufacturers say that the check chart cannot be used to CHARGE the system, only to check its operation. If the system is undercharged, the recommendation from many manufacturers is to recover the refrigerant in the unit and weigh in a total system charge according to the manufacturer's instructions.