New construction in Georgia.
Does anyone know of a minimum spacing requirement between a Class B furnace flue and a Class B water heater flue above the roof line? Thanks
Note: I have read the install manuals and this is not covered.
16-24 inches, the lower vent can damage the higher vent. The outside wall of a b-vent is usually a galv steel and won’t handle corrosive combustion exhaust gasses well. Depends on the use, of course, etc…
How close were they?
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Good question. I did not walk this roof, views from the ground.
Here are a couple of other photos, the shingles and the passive vents may provide some scale.
That is my concern. I have seen flues closer with the problem you described. Reference material would be helpful or I may just render an opinion.
I’m not aware of any “official” sources you can reference. This is one of those things that never made it into code because it’s not that big of a deal. It’s an inside knowledge by old timers 
Since this is a new construction, contact your local building department and ask them what they think… ask them what would happen when there is damage due to proximity of the two vents. Again, 99% nothing will be done about it because it’s not part of code or a manual. It will just be your opinion and you will end up arguing with the other side because of course… they are right… they did it this way and never had an issue
BTW, I have no idea how close they are from your pics. Unless you can find a way to measure the distance I wouldn’t talk about it. They don’t look VERY close from here. There appears to be 4 exposures of shingles between center of the flues. That would normally add up to about 16-20"
Exactly where this is headed.
I agree. It is marginal. By the way, it was easier to judge on-site and I approximate what you described, 16 - 24", closer to 24.
I appreciate the feedback!