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Question DetailsAsked on 1/31/2018

I have a gas hot water heater and using hot water only in one bathroom leaving black residue in the tub only

It is only leaving space in this bathtub and only from the hot water. It pears to be graphite. What could be causing this?

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1 Answer


I am going to assume you have not just had an earthquake or had the water in the lines turned off - that can cause release of built-up sediment or corrosion products, as can freezing of the lines. If any of those are possible, try going a couple of days and see if it tapers off.

If a slimy black "ooze" or general coating after the water runs out, more likely from water heater, since only in hot water. If flakes, can be from water heater (usually outlet pipe disintegration) but ore commonly form pipe corrosion or anaerobic iron accumulation in the piping.

First thing I would do is, bearing in mind the water is probably near scalding temperature, drain off 5 gallon or more from the bottom of the tank - if you get just a bit of discolored water or grit, that would be normal, especially if this has not been done annually as it should be.

Assuming incoming cold water is clean, if a heavy accumulation of grit comes out, especially with flaking, then tank is probably corroding out inside and quite near its life. [If on a well, sanding of the well can cause thistoo]. If a black/gray ooze, tank or possibly cold waqter lines have likely gone anaerobic - and most likely is too cool so it is forming anaerobic bacteria (which can also support legionnaires disease and such). If this is the case, if heater is near its life get a new one - commonly $1000-1500 installed for normal size, $1500-3000 for high-efficiency condensing with heat pump on top. If fairly new (well under rated life) a Plumber can drain and flush the tank, dose it with sodium chloride to kill the bacteria, then reflush it and hook it back up - but that is commonly around $400-500 so a dent on the cost of replacing it. If heavy flaking in the bottom of the tank but NOT in cold lines, then I would say trash it or count on it going out soon, because it is rusting inside.

Here are links to a number of previous questions with answers about discolored water or flakes in the water, FYI:

Answered 2 years ago by LCD

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