I know there are times when this is ok and when it isn't ok. Currently, both run seperately to my chimney stack and the chimeny isn't lined. I want to move my gas hot water heater to a different location and it'd be much easier to then duct into the vent that feeds the furnace, but I want to insure it is ok to do. I believe I asked a housing inspector once and that he said it was ok, but it's been a while and I want to double check.
My water heater is atmospheric-vent, the furnace appears to have a small blower on it's vent, it is about a 7-8yr old unit if that makes a difference. The water heater is currently a piece of crap wirlpool, but when the move is made will probably be replaced with a Bradford White unit. I can provide exact model if need be for the furnace and the water heater.
Once I make the change, I plan on adding a flexible stainless steel (?) liner to the chimney that will terminate the top of the chimney. I could run them seperate, but then I'd have to run two liners out of the chimney, but prefer a single liner for now to allow room to possibly add a gas fireplace to my firebox and would need the remaining room for that liner!
No problems running both to the same liner. It will need to be the right size. Sizing info is in the furnace and water heater installation instructions.
Thanks, I'll try to find that info. It looks like the flex liner I would add is 6" and I believe the ducts in place currently are 4" coming off the furnace and the hot water, so if all I'd need is to increase that, no big deal....