Water Heaters - Using Wye to Connect Hot Water Heater & Furance Flue

Greetings. We have a gas furnace and hot water heater that are both about 15-20 years old but still going strong. Both the furnace and the hot water heater are currently vented into a masonry chimney with a tile liner, each using their own 3" single-wall pipe. I am needing to move these vents higher up in the chimney so I don't have to do the limbo to get around them. While I'm at it, I'm planning to replace both vent pipes with type-B pipes. Two questions:

1. Can I use a wye to join the two 3" pipes into a 4" pipe before it goes into the chimney (so I only have to create one new hole and don't have to buy so much 3" pipe)?

2. Should I be looking into a stainless steel chimney liner? We have not had drafting problems, and we have a carbon monoxide detectors that has not ever gone off. Down the road a little ways, however, we'd like to have a wood burning fire place installed using the same chimney flue that the appliances are connected to.

Thanks,

-Aaron

Most local codes do not allow you to join heaters and water heater exhaust outside the chimney. Each must have its' own metallic pipe(no aluminum), and must angle up to the chimney inlet.
I doubt that a liner is needed at this point. That is usually needed when you go to high efficiency units that may eliminate one of the pipes.

OK, thanks for the response. Looks like I'll stick with 2 3" pipes but just try to move them higher up in the chimney. I've been looking at how things are done in other houses around here and one that was constructed about 15 years ago had a 3" vent and 4" vent (furnace & hot water heater) joined using a wye into a triple-walled 5" vent that went up between two walls and out the roof. But maybe that's a little different than using a chimney.