HVAC and Water Heater Maintenance Service- Performed preventative maintenance on an oil furnace and oil water heater. Brushed and vacuumed. Replaced the oil filter, nozzles and strainer. Started and checked the safeties. Performed a combustion test. System is operating properly. Inspected the heat exchanger on the furnace and found a crack on the back of the heat exchanger. There is a hole on the back side of the chamber allowing excess heat hit the metal of the heat exchanger. Recommend replacing the furnace.