In the context of developing heuristic rules for troubleshooting a cryogenic liquefier using an expert system, effects of heat leak location on performance are presented. Though heat leaks into the cryosystem from the surroundings are undesirable, they are unavoidable. The present calculations show that the liquefier＇s liquid yield is affected differently depending on the location of the heat leak. The liquid yield decrease is enhanced when heat leaks into the liquid reservoir instead of the heat exchanger. Heat leaks into warm process streams are more detrimental than those into cold process streams. Two heuristic rules are presented.