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Plmqwk, Master Plumber
Category: Plumbing
Satisfied Customers: 920
Experience:  20+ years in plumbing field. Licensed master plumber, commercial plumbing inspector
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After 5-10 seconds of running any hot water tap the water comes

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After 5-10 seconds of running any hot water tap the water comes out almost black for a second or so and then clears. If you stop running the tap, leave it for a bit and run it again the same happens but for a shorter time. We have an unvented system with a water softener.

Welcome to Just Answer. I will do my best to fully answer your question(s).


The black color that appears is likely from a build up of deposites somewhere in your water piping system. This is usually temporary, harmless, and caused by a disruption in water pressure from a system shut down, maintenance, and re-start. Air entering the system from it being drained down can cause this to happen.


If you recently had some work done to your water system I recommend running the taps, both hot and cold for about 5-10 minutes to see if that clears it up.


If you have not had any work done then something else is going on, possibly the softner not working properly, or as you mentioned, the hot water loop pump not working properly.


For the latter situation I would have a plumber come out to see whats causing the black water. Just to be certain it is safe to use and drink.

Customer: replied 6 years ago.
Relist: Answer quality.
Our first answer did not tell us anything we did not already know. Our question did not mention any previous work being done recently so that part of the answer was irrelevant. I would like to know what the potential sources are of a very dark grey to black coloured slug of water coming out of our hot taps as explained in the original question. There are no bits in the water so whatever it is is extremely fine grained. Could it be corrosion/dissolution of a metal component in the system, can rubber parts dissolve this finely, could it be from something inside the water softener, could it be from something inside the hot water tank (we have a tank in tank cylinder)??? Is our water too soft? Pressure in our unvented system remains at 1.5 bar, as usual. There have been no broken water mains in this neighbourhood.
To answer your question, yes rubber can deteriorate into a fine, "paste" is the best way to describe the feel of it. My first thought is that is is a mold that grows inside water systems. Don't worry, (if this is what your problem is) the mold is harmless and grows in all water systems everywhere in the US. What I'm not sure about is why it would be getting flushed through your system. It usually hangs on the walls of the piping until something disturbs it. Have you seen any pressure variations or water-hammer lately?
Customer: replied 6 years ago.

We get minor water hammer everytime the washing machine runs, which is almost every day. I also topped up the pressure on the heating side of the water system about a week ago, only by a very small amount. The heating/boiler hot water and the hot water that comes out the taps are two separate circuits, so it shouldn't be those, unless there's a hole in the hot water cylinder connecting the two. Its an expensive Belgian tank-in-tank cylinder, so I can't imagine it would have corroded. If there was leakage between the heating circuit and the hot water coming out the taps then wouldn't you expect more of a continuous discoloration of the water?


I replenished the salt in the softener almost a week before the black water appeared....? This morning I tried to bypass the softener but it turns out that the bypass valve is defective and cannot be opened fully, so I can't isolate the softener until our plumber fits a new valve next week. I will try again then. There are no larger black pieces associated with the stained water so I'm not sure it is either mould or rubber which you might expect to produce a range of grain sizes rather than a homogeneous grey/black solution. We checked the small header (pressure maintenance??) tanks associated with the boiler and hot water cylinder to see if the bladder had gone in either of them, but they appear to be fine.


After isolating the softener to see if that makes any difference, my next step will be to get the boiler serviced - the plumber that does this may be able to shed a little more light on things.

Dear Customer,


I have been following your post since I opt-ed out to another expert. The next expert said basically the same thing but in different terms. BotXXXXX XXXXXne your doing exactly what I suggested you do. We both agree there is a form of something deposited in your system, where it is coming from is going to have to be determined from a service call to your location. The water hammer "may" be causing this turbidity to a degree but as I suggested, please get a plumber to check it out. It appears you are close to doing so. Better safe than sorry.

Plmqwk, Master Plumber
Category: Plumbing
Satisfied Customers: 920
Experience: 20+ years in plumbing field. Licensed master plumber, commercial plumbing inspector
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