Today I replaced my 16 year old, leaking 50 gal natural gas water heater with a new 60 gal GE (Rheem) heater. The install went fine and only took about three hours. My problem is that the vent on the new water heater is 4" and the vent pipes are 3", which vet out through the roof. I removed the entire vent system to the base of the roof vent. I can't removed the roof vent as it is secured to the roof with it's flashing glued under the roof tiles.
I know reducers are a bad idea, but could I put a 4" -> 3" reducer at the base of the roof vent? The roof vent is only about 12" in length. From the top of the water heater to the base of the roof vent is about 5 ft and will have all 4" venting.