When you say "charcoal particles/chunks" is it possible that you are actually seeing rust flake/scale?

It's black. It leaves marks on my hand like charcoal.

Here is a link that might be useful:

I think your water filter media is coming apart. Charcoal is one of the medias used in water filtration.

This post was edited by Fireballsocal on Sun, Oct 5, 14 at 20:33

So, that's a good thought, but the "whole house filter" is just a particle filter, no charcoal in it. Also then should have then been in the cold inlet as well.
We personally installed the bathroom, the plumbing within the bathroom is all new copper pipe from our reno 5 yrs ago. There was never any house filter until I installed it at the well entry to prevent junk clogging up the new fixtures!!

When we opened up the fixture, the hot orifice was filled with it- so probably a full 1/2 measuring teaspoon (that was quite a lot from that tiny place). Plucked some out but then had to tap out the rest from the mixing channel. The pic is just a few of the bits that I scraped up from the sink after cleaning it.

This post was edited by pepperidge_farm on Sun, Oct 5, 14 at 20:47

It looks like rust likely from your oil burner coil...