This 70 gallon gas water heater has a very low flue into the gas boiler flue, at a ‘T’ connection. Is there a min height on the water heater flue height and being so low, the ‘T’ connection concerns me about back drafting from the boiler. Appreciate your opinions on this set up.
That looks wrong to me, I always expect to see a 12" vertical rise before connecting into a wye and not a T. The potential for back drafting certainly exists, did you do the simple mirror test by any chance. I would call it out and recommend a shorter, wider heater allowing a longer vent connection.
The boiler and w.h. were installed in 2000 together. Surprised its been this way for 15 yrs without having caused a problem where it was changed. W.H. is near its end and besides calling it out I am also goin to tell client to make certain when a new one is installed that this set up should be changed if buying same size heater.
I didnt test for backdraft- barely able to get near the heater , there was so much storage ifo it. Couldnt even see the bottom of the tank. Utility room = storage room 
How do you know it hasn’t caused any problems? I mean, you were at the property for, what, a couple of hours? What about the other 131,485 hours you weren’t there?
Regardless, a wye, with lower BTU on bottom, is an ideal arrangement or best practice. I would say unorthodox set up and have further evaluated by qualified professional to ensure proper exhausting of combustion by-products when ideal/accessible conditions exist.
Heck, if I had to report on things that weren’t having current issues, I would produce super-short reports and be loved by all 
Great diagrams. Ty
How did i know as i wrote that, that someone would gig me for ’ no past’ problems. My fingers typed it as my brain said - someone gonna yell at ya Haha.