Picture of water heater vent pipe.

Here is the water hater vent pipe that was installed behind the furnace pipe. Both the furnace and water heater were installed 19yrs ago. Thank for the help on re-sizing pics. It was very easy after installing software.


Water heater vent pipe (Small).jpg

The picture is not surprising.

Looks like a return air duct is blocking a direct approach to the chimney but without getting too much in the math aspect of venting what you have is a condition that is not the best for the smallest appliance venting, the natural draft , natural gas water heater…

It’s called the seven times rule. What this refers to is a condition that the largest flue pipe can not be 7 times larger then the smallest flue pipe .
The water heater mfg specifies the flue pipe diameter commonly (3 or 4 ) inches in diameter per their instructions. If the installer used a 4" on a 3 " hood then then you question: Did they follow MFG installation instructions?
Is the booklet on or next to the appliance?

3" diameter Natural Draft (N.G.) water heater to max 7" diameter flue
4" diameter Natural Draft (N.G.) water heater to max 10" diameter flue

From what I can tell from pic but guessing here:
Your picture looks like a 4" going into a 8" which would be acceptable yet best practice it to get that small flue ahead of the larger one above the large chimney breach. Is that going to happen everytime… nah…:wink:

The other factors to consider:

What is the length of the large flue pipe to the chimney? Is it a short one to the chimney breach (brick)? Is is lined? Horizontal or sloped 1/4" per foot to chimney breach?

I hope it helps.:smiley: