You may use a chimney as a common vent for two or more draft-hood equipped or induced draft appliances. Hot air furnaces, hot water boilers, and hot water heaters are common gas appliances.
Hot water heaters that are vented using a chimney as the only appliance are required to have a metal liner, or the chimney will experience condensation and deterioration.
An exterior chimney on the outside wall may not provide enough heat in the flue to allow proper exhaust, and can cause rust, stains or soot around the draft.
Back drafting is also possible, allowing carbon dioxide back into the house.
We recommend annual maintenance of heating appliances by an experienced heating/air conditioner company.
Call me at (518) 577-7707 or send me an email for recommendations.