Low suction pressures can cause a higher compression ratio and lower system capacities. There is no set suction pressure to charge your unit to every time; this is why it is important to use the superheat and subcooling methods. They take into effect your actual current temperature and pressure for your system at the time you are working on it.
Many reasons can cause low refrigerant suction pressure, i.e.: low indoor temperature, dirty filters, restricted ducts, undersized ducts, closed dampers, frosted coils, restricted refrigerant line, restricted piston, incorrect piston, restricted strainer, bad indoor blower motor ect. Resist adding refrigerant until more information can be attained.
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