Moderator Post (s) for this thread: >Consult the insultbyHKestenholz on 02/01/2008
It sounds like the seperation between your vent pipe and the upstairs wall is not sufficient creating a dangerous condition I know your probably thinking it has been that way for 25 years what makes it dangerous now but that kind of thinking burns alot of houses, the suggestion to consult your local codes officials is a good one if you live in a municipality that has a mechanical inspector.
Before you spend much time and energy trying to correct this situation let me suggest looking at it in a different way.
For every dollar of energy (oil, LP or gas) you put in that 25 year old furnace 50 to 60 cents of that dollar is going up that hot vent pipe and not in your house. The least efficient gas furnace you can buy today is 80% that means only 20 cents of each dollar is wasted and some better models get in the 90% of efficiency. Using what you pay in fuel bills every winter and figuring how much can be saved of each dollar you spending and fuel prices increasing annually you can see that upgrading your system will actually pay for itself in a very short time.
Remember dollar for dollar there is no better way to green up your home than upgrading an old HVAC system.