I have a forced hot air / gas furnace. On the top of the unit there is an exhaust pipe that attaches to another exhause pipe from my hot water heater and exit the house through a pipe running out the roof of my home. I am not to familiar with the system. The day I turned on the furnace for the first time this year I thought I smelt something, I believe this is normal. The other day I thought I smelt something again so I check the furnace. I found it odd how hot the exhaust pipe was. Is it suppose t o be extremely hot to touch??
Just fiqure I would ask someone?

Because of the poor efficiency of the furnace and water heater, the pipe will get very hot.

When the efficiency is high, like 96%, pipes can be PVC. But they need to be vented out the wall. If the waer heater is not direct vent then the vent for the water heater needs to be sleaved or made smaller.

The furnace may smell the few times it's started because the cumbustion air is not taken from outside.

This pipe you refer to... does it go into a b-vent type pipe (insulated) at some point before it goes through any ceilings or walls? An un-insulated pipe will get very hot, as KISS said but my concern is that it is properly connected to an insulated pipe where it penetrates walls or ceilings and the roof.