We will be installing a new mid-efficiency furnace and air-conditioner. We are told that we will need a chimney liner. The old furnace is still using the chimney, but the new furnace will have a vent going out to the side of the house and the only appliance that will still utilize the old chimney will be the water heater. So what's the point in installing a liner? Are we getting charged for something that doesn't need to be done?

Thanks

Depending upon the size of you chimney you could have a draw problem when the chimney is cold. This will cause it not to draw and the flue gas to come back into the house. I would go with the recommended the liner or switch to a electric water heater.

This could also depend on the condition of the current liner. It could already be leaking. Frequent headaches in the family? Could get worse. Make whoever is saying you must do it explain why. If their answer doesn't sound right, post back. The water heater is already the only appliance feeding the chimney any time it is on and the furnace isn't. If the furnace never feeds it anymore, that shouldn't make much difference. Got along without the furnace before, should continue.

The most likely reason is the water heater exhaust would condense in the chimney and form acid. Also water may leak out of the chimney and into the house. With a liner the vent temp would higher and not condense. When the chimney does not condense it’s called a healthy chimney.

As I said, if you didn't have condensation when the water heater was on, but not the furnace, you shouldn't without the furnace. If you did have condensation and the liner is damaged, yes it must be replaced. It is also possible code may call for a smaller vent for the heater alone, even if the current one is working fine.

Thanks guys. Great info you provided. :)