A couple of weeks ago we noticed that black specks were coming out of the bathroom hot tap. Whatever it is is soft: if you trap a piece you can smear it across the plastic of the bath and then easily wipe it clean. At first, there were only a few and we just fished them out but now we're getting a lot and the bath is no longer usable.

Testing suggested that it was only coming out of the hot tap, not the cold. It seemed confined to the bathroom hot tap but by removing the filters from other hot taps and the shower and running them for a long time, one or two tiny specks of what might be the same stuff came out but they're so small it's hard to tell. I've not seen any sign when running cold.

I had a plumber come out and take look who seemed baffled. His theory is that it was due to the heat exchanger in the combi boiler (it's not that old but was last serviced in 2019) and that it's only appearing in the bathroom because the flow rate through the tap is higher. So now he's saying we need to replace the heat exchanger but he's not totally confident it'll fix the issue.

I don't really want to pay to replace an entire part on my boiler if it isn't going to sort out the problem, but I have no other options to go with - another plumber I had to look had no idea and left without even charging me. The majority of the pipework in the house is plastic, with only small lengths of metal (presumably copper) running from the boiler and into the tap itself.

I checked this question - which sounds similar - but I didn't really understand the solution. I don't know if it's a UK/US thing (we're in the former) but we have a gas-powered combi boiler, not a water heater so as far as I know it won't have an anode of any kind that can corrode.

Can anyone confirm whether this is likely to be the heat exchanger, and that the plumber's theory about higher flow on the bath tap is correct, or whether there's something else I can check for or try to fix the issue?

  • Do you know if the pipes going to that tub have a larger size/diameter? That is the only reason to have/get a larger flow from that tap, most US homes have the same size pipes to all taps. If only coming from one tap with the others on the same hot water system, the problem should be located between that tap and where it connects to the the other plumbing pipes. If from the boiler it would be in all hot taps.
    – crip659
    Apr 19, 2023 at 16:27
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    I can't speak specifically to the black specks in your situation, but I have experienced a failing boiler heat exchanger and the initial sign we noticed was specks of metal and calcium clogging the shower and tub faucet strainers. This went on for months before water started weeping out of the heat exchanger itself. My theory for only noticing it in the bath/shower was that they use so much more hot water overall than really any other plumbing fixture. Clothes washers/dish washers have dedicated hot lines, but they are generally used much more infrequently.
    – Chris O
    Apr 19, 2023 at 16:36
  • @crip659 No, I don't know. The plumber had a look but I wasn't present. I can raise it when they come to quote for the heat exchanger replacement though.
    – Bob Tway
    Apr 19, 2023 at 20:12

1 Answer 1


I had a similar issue in my bathtub (black specks that smeared) and ended up determining it was an O-ring that had gotten old and started degrading. Replaced it and no problems since.

Perhaps you have a similar cause. If it is only coming out of the hot tap for the faucet, an easy test is to check the water directly from the hot supply line to the faucet by filling a large (preferably white) bucket and seeing if you are getting black flecks.

  • The tap is a mixer - I should have mentioned this in the question - and there's nothing coming out when it runs cold. Wouldn't a mixer tap use the same O-ring for both hot and cold water?
    – Bob Tway
    Apr 19, 2023 at 20:12
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    @BobTway I would expect so, but it could also be that the cold water hardens the O-ring and prevents pieces from flaking/breaking off. Apr 19, 2023 at 20:27
  • @BobTway If you have a multi-turn shutoff valve on the hot water supply line, it is likely the black rubber washer in that disintegrating. High temperature speeds the degradation process, so this happens years sooner on hot water vs cold water shutoff valves that have rubber washers/gaskets. Cheap valve rebuild kits are available for such multi-turn shutoff valves.
    – Armand
    Apr 20, 2023 at 4:36
  • Related question with info about multi-turn shutoff valve rebuild: diy.stackexchange.com/questions/246134/…
    – Armand
    Apr 20, 2023 at 4:41
  • @Armand I don't think I've got one of those. UK homes generally have a stopcock, a single tap that turns to cut off the whole water supply to the property.
    – Bob Tway
    Apr 20, 2023 at 7:56

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