Changing your furnace to a high efficiency model is a good idea. Just tear out the old one, hook up the new one and you’re good to go, right? Well…….not so fast.
High efficiency furnaces are designed to recirculate gasses so they are re-burned, thereby saving fuel and money. However, the design also causes substantially cooler vent temperatures which can lead to condensation on the flue liner.
In addition, that old chimney of yours with the gaping flue may actually be working against you because the amount of draft is regulated in part by flue size. While your old furnace may have loved that big ol’ flue, your new one may be begging for a venting system that will allow it to work better, with better draft.
All new furnaces will cause serious problems in your masonry chimney. Over the course of a few seasons you may see serious damage on the outside of your chimney. How do you prevent this? Install a stainless steel chimney liner. You can do the job yourself and save a lot of money. Prevent this from happening.—————————————->
This link has some very useful information: