Actually there could be a few problems with using galvanized vent pipe for exhaust on the water heater that has been designed to utilize only PVC vent pipes for that purpose.
First issue is the PVC vent pipe installation requirement in the manufacturer’s manual (unless this particular one permits using galvanized pipe). There are no choices for the installer between galvanized or PVC pipes, I have a few manuals from various US power vented water heater manufacturers and the only listed vent pipe material choices are PVC, CPVC, or ABS pipes.
Another explicit requirement in all those manuals is that all of the venting connections must be leak checked upon initial startup of the water heater. In other words, they have to be air tight which cannot be achieved on a galvanized connector sections.
Third issue is the condensation. Although the condensation does not occur in all power vented water heater installations, sometimes it can actually happen.
The reasons behind condensation could be:
ambient temperature and humidity in a particular water heater installation location
humidity and ambient temperature of venting space
vent pipe length and slope
usage of the appliance
In case one or combination of the above would occur in a galvanized vent pipe, the slightly acidic condensate will cause its corrosion, and leak through the vent pipe joints.
This should be more than enough reasons for any installer to do it “by the book”.
501.14 Category II, III and IV appliance venting systems. The design, sizing and installation of vents for Category II, III and IV appliances shall be in accordance with the appliance manufacturer's installation instructions.
503.4.2 Special gas vent. Special gas vent shall be listed and installed in accordance with the special gas vent manufacturer's installation instructions.
I hope this helps. Let me know if you have more questions but please provide the make / model# of your appliance.
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