I am finally getting ready to replace my old hot water heater - I asked about supply lines some time ago and received good answers. This question is about the vent pipe. The new heater is shorter than the old one, so I will have to change the existing vent, which was improperly installed 30 years ago. It has a horizontal section of Class A pipe with no slope at all from the elbow on the draft hood through the outside concrete block wall to exterior vertical Class B pipe. I know I will need to slope the new horizontal section at least 1/4" per foot, and the shorter heater will make this easier. I also plan to use Class B for all sections of the vent, even though there is plenty of clearance from combustible surfaces on the inside section. The question is two parts:

1. Should there be a minimum of 1' of vertical pipe from the draft hood
to the first elbow where the pipe turns to the horizontal?

2. Is is OK to use Class B flexible vent pipe from the heater to the
connection to the exterior vertical pipe? The manufacturers of the
flexible Class B vent claim that it is "approved" but I'm not sure if that
Means it would be acceptable to an inspector when the time comes
To sell the house.

I can not answer part 1 of question but someone else will pop in with answer.

Part 2= Check with building department to see if flex is acceptable. A home inspector is hired by buyer to protect them, but if building dept says it is OK that is all u need.

Chuck