To replace an oil furnace flue pipe, measure the pipe between the oil furnace and current flue pipe. Take the screws from the flue's cover to determine the diameter. Purchase pipes according to those measurements, including two elbow pieces of the same diameter.
Begin installing the pipe by placing an elbow pipe against the flue and securing it with screws or silicone meant for high temperatures. Using both ensures a sturdy hold. Wait an hour for the silicone to dry.
While the silicone dries, begin to piece together the remaining pipes. Use the same screws and silicone procedure to attach the second elbow pipe to the end of the pre-cut pipes to form a pipe with an "L" shape. Again, wait an hour for the silicone to dry, and then attach the L-shaped pipe to both the elbow pipe on the flue and the oil furnace. Seal with screws and silicone, and wait a minimum of 12 hours before using the furnace.
Flue pipe replacements can often be avoided with yearly inspections and maintenance. Visually inspect the flue pipe for any holes that could leak carbon monoxide into the house; they are most common where the pipe and furnace intersect. A small hole is easily patched with foil tape. Flues that appear corroded, however, require replacement.