Noritz NRCB199DV: E54/E57 again. domestic expansion tank was on the hydronic loop for years.

Photos are from last year, BEFORE the repairs described below. I know
that's not ideal. I'm back at the unit Wednesday and will post current
photos and pressure readings then. Posting now because I'd rather know
what to look for before I'm standing there.

SYSTEM

• Noritz NRCB199DV, manufactured Oct 2018. Space heating only - the DHW
  ports are capped, domestic hot water comes from a separate gas tank.
• Rating plate: heating capacity 120,000 Btu/h.
• Supply setpoint 160-165F.
• Four zones on an Argo panel - zone valves, one circulator, plus the
  boiler's own internal pump.
  1. Basement baseboard
  2. 1st floor baseboard
  3. Owner's suite baseboard
  4. 2nd floor apartment, forced hot air through a 12x15 water-to-air
     coil in an attic furnace
• Boiler is in the basement. The attic coil is above the 2nd floor -
  call it three levels of rise.
• Mix of old iron/galvanized and copper. Municipal water.
• No record of the heating side ever being flushed, cleaned or treated.
  No real maintenance for many years.

THE FAULT
Intermittent shutdowns on Error 54 (low heating water pressure) and
Error 57 (auto feeder open more than 600 seconds). Power cycling resets
the timer and it runs fine for a while, then does it again.

WHAT WAS FOUND AND DONE LAST YEAR
A plumber replaced the expansion tank and the boiler's internal
auto-feeder inlet valve. Two things from that job that I only recently
understood:

1. The old feeder valve's internal black rubber filter screen had
   completely collapsed, squeezed down and turned INSIDE OUT.
2. The invoice reads, word for word:
   "Replace expansion tank that's for domestic water with proper
   hydronic expansion tank #30. And proper pressure gauge. Pressure was
   low so turned up fill valve/prv to 15 psi. Boiler running at 20.
   That is correct to pressure."

So a DOMESTIC water expansion tank had been sitting on my hydronic loop,
along with a domestic 0-160 psi gauge (photo 4). Both were replaced with
an Amtrol #30 and a new gauge. Nothing on the invoice about setting the
tank pre-charge. The relief valve was not touched.

Less than a year later the E57 timeouts are back.

The expansion tank sits on the floor in the corner, several feet below
the boiler (photo 1, bottom centre). The new one went into the same
spot, just larger.

NO VISIBLE LEAK
Nothing wet, no puddles, no staining on the floor. The only thing I can
see is rust-brown streaking running down the side of the OLD tank in
photo 3.

PHOTOS

1. Near-boiler piping. Argo panel on the left, old expansion tank on the
   floor at bottom centre.
   2. Same wall with the boiler's front cover off - taken by the plumber
   during that visit.
   3. The OLD expansion tank, the domestic one. Note the rust staining
   running down the shell.
   4. The OLD pressure gauge. Everpure, 0-160 psi.
   5. Zone valve manifold, on the wall opposite the boiler.
   6. Rating plate.
   7. Other side of the room - separate gas DHW tank, Argo panel, the
   circulator, and the old expansion tank on the floor at right.
   1-Near-boiler piping. Argo panel on the left, old expansion tank on the%0A floor at bottom centre.jpg
   

QUESTIONS

1. What makes a feeder screen collapse and turn inside out? I assume
   flow went backwards through it. What drives that?
2. If a domestic tank (40-ish psi pre-charge) was on a loop running
   15-20 psi, am I right that the diaphragm never moved and I effectively
   had NO expansion capacity for years? And would that have had the
   relief valve lifting and reseating every cycle - dumping water, the
   feeder making it up, over and over? If so, is the relief valve seat
   likely wrecked by now and weeping on its own, even with a correct tank
   fitted? It was never replaced.
3. Is 15 psi cold fill enough with the attic coil three floors above the
   boiler? My arithmetic says that leaves the coil at roughly 2 psi,
   which sounds like air vents up there ingesting air rather than
   purging it.
4. The tank is on the floor several feet below the boiler, and there's
   both an internal boiler pump and a separate circulator (photo 7). How
   do I confirm the tank is on the right side of the pump? If it's
   wrong, could the pump be pulling the top of the system toward zero
   every time it runs?
5. Could years with no flushing in an iron system silt up the boiler's
   pressure sensor and give me a false E54 rather than an actual leak?
6. A contractor has proposed capping the internal auto-feeder entirely
   and fitting a standard external fill valve with backflow preventer on
   the loop, to get past the 600-second timeout. Reasonable, or is that
   just hiding a leak I haven't found?

Tell me what to photograph and measure Wednesday and I'll get it.

Thanks.