Performed heating maintenance on three systems. Replaced 16 x 25 x 1 filter for basement furnace. Replaced 20 x 25 x 1 filter for main level furnace. Replaced 20 x 20 5 51 filter for upstairs furnace. Cleared and treated condensate drain lines. Removed and cleaned flame sensors. Inspected electrical connections and components. The upstairs furnace has a bad pressure switch. Customer informed me that he does not use this furnace for heating because of the heat rise in the house. The main level and basement furnaces are heating properly.