Unfortunately with PMTHERM, altitude is selected from a combo box, and -1000 ft is the lowest value accepted. The psychrometric equations behind PMTHERM would have accepted lower values though. But Paul and I didn't think anyone would need them. I did the psychrometric calculation programming for PMTHERM.
in PMTherm -1000 is the lowest value available on the pull-down, but the combo box is not restricted to only values in the list. You can type in what ever you want.