corrosion lime build up

[Andy T. Bishop]

When you guys see corrosion lime type build up on the flues of the HVAC, what do you say or call out? It has the water heater also in the same flue but the water heater flue looks fine.
Thanks
Andy

[Patrick Bolliger]

It maybe an indication of how the FAU "exhaust" is poorly functioning.
I have seen very bad corrosion on draft hood /flue connections. Major rust...very bad due to a open humidifier in the FAU closet...

The other question is how is the flue / chimney draw and what is effecting the movement of products of combustion. Poor dilution air supply, oversized chimney, weak draw, improper connections, length of flue and so on.
Natural gas combustion will release moisture as one part of the process.

The flue pipe / chimney heats up and a "draw" is created. Now if the moisture stays within the flue what is causing that? Excessive moisture in the location of the FAU (humidifier) or the operation of the appliance itself... Put on your CSI cap...

Where is the water heater flue installed with respect to this flue pipe? At the chimney breach or "tee" connection into a "common" flue to the chimney.

Smaller flue pipe (connections) are recommend "on top" if two different sizes are connected to a chimney breach.

[Andy T. Bishop]

AWESOME explanation.
That is what i thought but your wording is better than what i would have said. I just wanted to make sure i should recommend a HVAC person to review/repair.
Thanks
Andy