Venting two appliances into the same flue saves a homeowner from investing money in a second chimney and can speed up the installation of appliances, but there are restrictions.A wood-burning stove or fireplace is always required to have its own, unshared flue. This is largely because there is no practical way to connect another wood, gas or oil appliance to that flue without causing air infiltration into the flue. Air infiltration slows the rate of exhaust travel up the flue while simultaneously cooling the wood exhaust gases, causing excessive creosote condensation and an increased incidence of chimney fires. The codebook is quite clear on this point: every wood-burning appliance must have its own flue. The only way you can safely vent an oil-burner and a wood-burner into the same chimney is if you install two stainless steel chimney liners, one for each appliance.When dealing with oil exhaust intrusion into a chimney venting wood exhaust, other factors come into play. The acids contained in oil exhaust blend with a colorless, volatile liquid compound found in wood exhaust to create an extremely corrosive mixture inside the flue. This mixture attacks both the bonding agent in the mortar and the actual chimney structure itself, drastically reducing the usable lifetime of the chimney.Finally, the combination of oil soot and wood creosote in the flue presents the most dangerous of chimney fire hazards. Oil soot ignites at extremely low temperatures, and wood creosote burns at extremely high temperatures. When ignited, this mixture "spits" burning balls of oily creosote out the top of the chimney in every direction. Thus, a chimney flue that is coated with a combination of oil exhaust and wood exhaust deposits is much more likely to experience repeated chimney fires, and those fires are much more likely to destroy the chimney and / or burn down the neighborhood.Review local building and fire codes to determine any local limitations on gas and oil burning appliances together into the same flue. Replace the chimney liner with a liner large enough to meet the venting requirements of the two appliances if the current liner is not adequate. Attach a wye (pronounced "y") connector to the chimney liner, or a double snouted tee with two take offs, which is designed to allow you to connect two appliances to one chimney liner. Some codes require that the appliance with the lowest BTU input should be attached in a position higher than the appliance with the greatest BTU input.