Thanks for the comments... let me clarify my wording, now that I have had a chance to deep think a bit. My chimney has 2 routes up the stack. One is dedicated to exiting the fumes from the wood burning fireplace- yes, creosote city that we pay to clean out every year because a wood fire 4-5 nights a week from late October to early March is a pleasure we enjoy- I put way more effort into it than I need to, but my wife feels the family room is not complete on nights we don't have a fire.
The second shaft that ends at the chimney top starts in the basement, runs parallel to the chimney, and handles the vapors from the HWH only. The gases are clean, but contain water vapors which enter the shaft at high temp which lowers as it moves up to the cap. If its -0 outside, it starts to give off moisture which is not good for masonary. I believe the service tech is giving me a valid concern- but I also know that Frank Lloyd Wright designed over 1600 different fireplaces, some terribly inefficient but no 2 alike... Think I will call my HWH installer first for an opinion.