I recently bought a older house. Yesterday I started to install new roof vents. While in the attic I found that the gas water heater was not properly vented, so now that heater is turned off. The previous owner was a little old widow woman who had a 30 gallon heater installed nine years ago. I figure to end up replacing the unit fairly soon. I also want to go with a much larger unit. Maybe a 50 gallon high efficiency unit. After all, it is a 3 bedroom, 2200 sq ft house.

I went and bought the material to install a totally new stainless steel vent. The water heaters I am looking at use a 4" vent system. However the current heater uses a 3" vent. I really do not want to get a new heater just yet and the old one works fine. But I do not want to put in all new vents just to replace them in 6 months.

So my question is this-can I use 4" vent for the section that runs through the roof and then reduce it down to 3" for the last 4-6 feet?

My concern here it the brackets and braces that run through the roof. That section is a total pain in the backside to reach due to very low roof clearance. But I still want the heater to draft properly.

By the way, as near as I can figure, the heater has been dumping hot flue gasses into a severely under-vented attic for a least 11 years!