We are in the process of getting a new split-system and during the visit by one of the possible contractors he mentioned that the we might not be able to install a high efficiency furnace, since that would orphan the existing 40k BTUH water heater. The obvious question is - is the current vent (6" OD Type B) too big? The run from the water heater to the vertical vent is about 5 feet and is 4" single walled pipe. The vent itself is is about 35'-40' long (furnace is in basement of a two story house). Looking at this: http://www.hartandcooley.com/Librari...?download=true (Page 9, bottom right), it would seem that the current vent is not too big (10 inches being the max). Having done a search on this site, I see that others have had to have a smaller pipe liner installed, which I really don't want to do if I don't have to beaciuse of the added cost. My first inclination is to trust the contrator's word, but I don't want to limit myself to another 80% furnace if I don't have to.
The other contractor that has visited mentioned nothing about the venting system.
On another note - As a new member, thanks to you all for contributing on this site and answering the questions of us homeowners. I have been lurking here for the past few weeks and it has been eye-opening to learn all the in's and out's of the process needed to replace our current system. Seems trying to get a contractor to do a Manual J calulation is like pulling teeth. I have done my own using HVAC Calc, but so far neither contractor that have visited have offered to do one. In fact the second asked why I wanted one done (but that's another story yet to be completed...).