A high-efficiency oil furnace should be vented through a new duct, or an old chimney should be lined to prevent toxins from re-entering the home. Venting a high-efficiency oil furnace through an existing chimney flue can also decrease the performance of the furnace.
The best way to vent a high-efficiency oil furnace is to create a new chimney or duct and not reuse an old one. New high-efficiency furnaces produce a toxic, acidic gas, and most existing chimneys are not equipped to release it. Changes in air pressure and debris blockage in the chimney can prevent the gases from leaving. New chimneys need to be built with metal double walls to ventilate the gases. Older chimneys need to either be fitted with a metal flue liner or lined with fireclay.
There are many downsides to retrofitting an existing chimney besides a decrease in furnace efficiency. If the chimney is the wrong size, water vapor in the gas condenses, and the draft is insufficient to get rid of the gases. Some older vent pipes and chimneys were built horizontally. These do not produce an adequate ventilation draft and must be fitted with fans to move the gases along through the chimney.