As long as your facility is heated for inhabitancy, the PMG-60 would be unaffected whether ON or OFF. The PMG-100 and PMG-200 have exhaust fans that keep warm air moving out the vent when they are ON. Because of this, it is recommended that the heater be left ON at all times when the outside temperature is below 32°F (0°C) to counteract the unlikely occurrence of cold air migrating into the vent pipe.