The blockage of the secondary heat exchanger is internal to it. Quite a bit of iron oxide (rust) seems to have discolored the white plastic of the drain trap. Since the inducer wheel fell off its shaft about 14 months ago, because its sheet metal collar was completely rusted and had desintegrated, the debris from it had the same color and did not make me suspicious. Some of the heat exchanger have a polycarbonate coating on the inside that can desintegrate, peel off, flake off and add to the blockage of the trap. I did clean the trap last year and at the beginning of the troubleshooting (leading to this thread) and found little black/gray particles in it. Maybe the particles came from the heat exchanger?