Duo Therm AC units are sealed units and cannot be charged without modifying the unit. One major problem they have been having is a 1/8â€ capillary tube that sits next to the compressor. If this tube is to close it will result in a hole developing in the tube due to vibration of the compressor. This is hard to detect and the loss of gas can only be verified by checking the load on the compressor. If the unit is more than a year old it was probably made in Mexico where most of the problem units came from. As of last year Duo Therm started getting the units from China so the problem may be resolved.