I just had the aqua hot system worked on a few weeks ago. The burner had a crack in it so it was replaced, also replaced the motor inside that circulates the fluid and replaced the mixing valve and put on a new circuit board. I called the shop today that did all the work and told them about the chunks coming out. They told me it was more than likely residue left over from the prior owner before I had it worked on. They had me check to see if there is any smoke coming out the exhaust and there was not any, in fact it was all clear.