Some of the parts that may need to be replaced on a furnace during its lifetime include the thermostat, thermocouples and igniter. The thermocouple is often the first part that needs to be replaced.
Replacing a thermostat needs little in the way of specialist knowledge or equipment and can generally be completed by the homeowner with the help of the furnace installation guide or maintenance manual. Thermocouples can be sourced at most home improvement stores and can also be replaced without professional assistance. The igniter has a lifespan of around five years before it needs to be replaced by either a homeowner or HVAC maintenance technician. Its lifespan can be extended by regular filter maintenance.