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How to clean a furnace
Locate the access panel on the outside of the furnace.
This is below the return-air duct, between the blower system and the duct. Usually, the filter is found inside the front of the furnace.
You may need to unscrew the front panel from the furnace or remove it from hooks that hold it in place in order to access the filter. Your furnace filter may also have its own access door.