Maintenance plan. Completed three system maintenance. Basement system is 25 years old along with 25 year old mast/ upstairs system. Recommend replacing those due to major repairs necessary. Main floor system is 10 years old carrier infinity. System has air flow issue from poor duct design. Recommend reducting system with more direct air flow supply instead of excessive amount of wyes. Flushed and pumped drains, added enzymes, visually inspected filters and equipment. Checked refrigerant levels, performed carbon monoxide inspections. found master/ upstairs to have cracked heat exchanger. Advised customer not to use heat do to safety concerns, left thermostat in off positions. Recommended varying repairs on all systems.