Completed homecare maintenance for two systems in home. Fully tested the system and heat and cool from thermostat. Found that both furnaces have poor combustion and inspected heat exchanges. Found that both have multiple cracks in the heat exchangers both of the systems have supplied ducks with dampers that are causing airflow issues due to them all being almost all the way closed. Both systems also have a trap that is not up to code and the blower wheels have lots of dust/dirt buildup. Informed customer of the heat exchangers and that the heat cannot be run at this time safely.