The oil filter should be changed or cleaned at the start of the heating season and about midway through the season -- three or four times a year [source: NationalFurnace.com]. Here's how to clean or replace the filter:
Step 1: Close the oil shutoff valve between the fuel tank and the filter.
Step 2: Unscrew the bottom or cup of the filter housing and remove the filter.
Step 3: If the filter is disposable, insert a new one of the same size and type. If your furnace has a permanent filter, clean the filter according to the furnace manufacturer's recommendations.
Step 4: Replace all old filter gaskets with new ones.
Step 5: Screw in the bottom of the housing and open the oil shutoff valve.
Some oil furnaces have a pump strainer, which is located on the pump attached to the burner/ blower unit. Clean this strainer when you clean the oil filter. Here's how:
Step 1: Unbolt the cover of the pump housing (where the oil line enters the burner) and lift off the cover.
Step 2: Remove the thin gasket around the rim. Find and remove the strainer, which is a cylindrical or cup-shape wire mesh screen.
Step 3: Soak the strainer in kerosene for several minutes to loosen any built-up sludge. Carefully clean the strainer with an old, soft toothbrush.
Step 4: Inspect the strainer. If it's torn or badly bent, replace it with a new pump strainer of the same type.
Step 5: Set the strainer into place on the pump, place a new gasket on the rim and then bolt the cover of the pump housing back on.
Next, we'll take a look at the switches and stack control on the furnace.