Wrong size vent pipe?

[David Block]

Why is it when a home owner replaces a mid efficiency furnace with a high efficiency furnace, the vent pipe that use to vent both the water heater and furnace is left in place to now only vent the water heater? Isn't the water heater vent pipe suppose to be the same size as the opening on the drafthood? I see this often and can't figure out why nobody else seems to have a problem with it.

[John Arnold]

You're right to have a problem with it. The flue is now oversized and that's probably why the stain is there - excessive condensation because the WH can't heat up that flue pipe.

[Nick Ostrowski]

You're right David. I see this all the time as well. Either the installer of the new heating system doesn't recognize it or the homeowner can't understand why they'll need to drop $1,000+ on relining their chimney to ensure the water heater will draft properly. As your picture shows, there is a condensate/corrosion issue on the bottom of that flue pipe and the wall.

My guess is that the service people installing the systems are not thinking about how any changes they make with the current setups will affect future performance of the now-orphaned water heater and flue.