Charging an air source heat pump during cold weather has always been a bit of a problem. The problem is that the amount of refrigerant circulated decreases as the outdoor temperature drops. Why is this? Well, as the outdoor temperature drops, the evaporator temperature has to drop in order to be able to absorb heat from the outdoor air. The lower evaporator temperature produces a lower evaporator pressure. The lower evaporator pressure increases the compression ratio because there is now a greater difference between the suction pressure and the discharge pressure. The higher compression ratio means that the compressor does not circulate as much refrigerant.
At a 45°F outdoor temperature, a typical air source heat pump produces a heating capacity roughly equal to its nominal cooling capacity. At 17°F outdoor ambient, it produces about half as much heat as it does at 45°F. This difference in capacity is directly related to the amount of refrigerant being circulated. The rest of the refrigerant is just sitting somewhere – normally in either the accumulator or the charge compensator. So a system operating at 17°F outside could have perfect pressures even if it only had half of its factory charge. That is why you can be way off checking a heat pump by pressures in the heating mode.
Some manufacturers provide heating performance pressure charts, but refer to them as “check” charts. They are intended to check the system operation at specific conditions, but are NOT intended as guides for adding refrigerant. The problem is that you don’t have a good way to judge how much refrigerant is stored out somewhere in the system. I can hear a bunch of you saying that measuring superheat and/or subcooling solves that problem. While I AM a fan of checking both, they still just measure the refrigerant that is circulating.
There have been some interesting methods used, such as measuring discharge superheat. For discharge superheat, you measure the temperature and pressure of the discharge line right as it leaves the compressor. It should be somewhere around 60°F warmer than the discharge saturation temperature. So if you have a 410A system running at a discharge pressure of 318 psig (saturation temperature 100°F), the discharge line should measure 160°F. A lower temperature reading indicates an overcharge and a higher temperature reading indicates an undercharge. The surest way to charge a heat pump in the winter is to recover the refrigerant, evacuate the system, and weigh in the correct charge. If you have performed a repair on the refrigerant system, then this will save you time and insure a correct charge.