this week i had to go check a problem for a long term customer that she was having with her furnace. she had had some company come in and do a furnace service, and a few days after they left, she was having water leaking from near the furnace. i just assumed they broke something or knocked it loose.
i went and had a look and was shocked to find that a brass nipple (1/2" MIP to 1/2" pex barb) had partially dissolved. yes, actually dissolved. it had actually lost about half of its total mass (compared to another identical fitting) and the dissolved portions matched identically to the probable flowpath of the liquid condensate. the fitting was acting as a connector between the furnaces condensate collection port and a 1/2" drain line to a floor drain. it was threaded into a nylon collection tray and connected to a plastic drain line.
i have never in my life seen anything (outside of process equipment) like this. whats even crazier is that the furnace and all its plumbing was replaced 3 years ago. my thoughts are:
a) dielectric dissolution (galvanic corrosion) – if there is a small current leak through the furnace condensate to ground in the drain (current flowpath is the water), its theoretically possible there could be some low level ionization in the water, and thus some corrosion. this seems so unlikely as to be almost unmentionable.
b) the furnace cleaners used some sort of acid to clean out the furnace and it ate through the fitting. i know of lots of acids capable of doing this but why would they use it considering how much other damage it would do inside the furnace. i just cant see this either.
c) could the condensate water be acting like distilled water (i guess the interior of a furnace is really is like a distillation tower if you think about it). over time distilled water can dissolve just about anything.
d) could carbon dioxide from the combustion chamber be dissolving into the water, making dilute carbonic acid – causing the corrosion?
if anyone has ever seen or heard of this or knows whats going on, i would appreciate the input
thanks
PPA
Best Answer
I would think electrolytic corrosion to be unlikely due to the intermittent nature of condensate flow and the fact that the brass fitting is isolated between nylon and PEX.
Your thoughts around acidic condensate are intriguing. Boiler feed water for industrial steam systems is treated with inhibitors to prevent yellow metal corrosion in the condensate collection systems, and industrial closed-loop cooling systems are treated for the same reason, so it is a known fact that yellow metals are susceptible. Pure water easily absorbs carbon dioxide just from being exposed to air, so the gas-rich environment of the combustion chamber may be accelerating the process.
I would replace the nipple with a plastic component, that should take care of the issue either way.