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?