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Posted by Yodaddy on July 01, 2002 at 04:38:55:
In Reply to: Furnace corrosion posted by Rod Nissen on June 28, 2002 at 08:06:16:
Rod, the fireside is rusting during shutdown. It happens because damp moist air is entering the boiler and condensing on the tubes where it forms an acid that attacks the metal.
If the boiler has a flue damper, closed it tight after boiler is shut down and has cooled, but is still warm. Otherwise, place boiler in dry layup each time it is shut down or find a way to keep the boiler from 'breathing' in damp air when shut down.
Also examine the boiler room ventilation. determine if the boiler room is in a positive pressure situation. If so, that will force air and humidity into the furnace of a shut down boiler where it will condense it's moistuer on the tubes on it's way up the stack. Especially if the tubes are full of water.
The line you describe represents the path the fresh, moisture laden air takes as it passes thru the furnace...because of this limited natural draft effect, the whole fireside isn't effected, just the areas exposed to full air flow.
You will also notice the the rust is heavyer nearer to where the air is entering(usually at the burner), and gets thinner as you get further away from the source. That's because the moisture in the drafting air is less as it progresses thru the boiler...But not always...if the draft effect is quite heavy, the the rust will be almost uniform.
As a boiler inspector, I have seen this destroy otherwise safe boilers rather quickly.