Chimney line and flue

An inspector found carbon monoxide backdrafting from the water heater in my house. The inspector said may be the ventilation (the flue or chimney line) needed to be clean. So I hired a chimney service company to take a look. They say, the reason for backdrafting is not because of blockage but instead the chimney line is not big enough. Currently the chimney line is 5" in diameter and they suggested 6" or 7" and the cost is $75 per ft to replace the line all the way to the roof. My questions are these:
1. I am surprise that is not big enough since the house is only 6 year old. At least it will not pass the county code then isn't it ? 2. The water heater flue and the furnace flue share the same chimney line. Only the flue that connected to the water heater has some corrosion/residue on its joint. Is this cause by water condensation or leaking carbon monoxide ? 3. Can my water heater produce more carbon monoxide now than 5 year ago ? 4. Is there any other possibilities on what is the problem ?
Thanks.
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Talk to your plumber or your HVAC tech. and they can install a booster blower on the flue pipe.
Do you see any black streaks on the side of the water heater? If you do then call out the plumber.
How old is the water heater? You may have to replace it.
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Moe Jones
HVAC Service Technician
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