You can replace certain parts of a household furnace by shutting off the power and fuel to the furnace, removing the faulty part, installing the correct parts according to the manufacturer's specifications, and restoring any hardware or wiring removed while taking out the broken part. Most furnaces list a model or serial number that can help you locate the correct replacement part.
FamilyHandyman.com lists three common furnace part replacements that the average homeowner can accomplish: a spent ignitor and flame sensor, a clogged filter, or a high-limit switch.
Everyday use exhausts the ignitor and flame sensor over several years. Remove the device from its location in the combustion chamber before installing a new one and re-establishing the electrical connection to the new device.
A clogged filter is another problem that -- if left unaddressed -- can lead to a broken high-limit switch. Remove the filter from its mount and install a new one. If the furnace fails to work properly after the new filter is installed, replace the high-limit switch by unscrewing the switch and installing a new one.
FamilyHandyman.com advises leaving the replacement of the inducer fan or gas filter to the professionals, as these repairs require specialty training and equipment.