Homeowner had been in house for 5 yrs: Cat.I fan assisted 80% gas furnace common vented into a manifold shared with a 40/40 gas DHW heater. Complained the WH pilot kept dropping out. Replaced TC a few times, got fed up, went to Home Depot, bought a GE WH and had "plumber" friend install it into manifold. Pilot on new WH still dropping out. Gas utility tested with match near top of unit blew out match so Red Tagged. Called me to check for obstruction.
When I disconnected vent connector from furnace, I noticed flex. aluminum liner from manifold into breeching but it was 100% blocked with chimney rubble. How did the liner fill with rubble? Pulled breeching and found the liner was about 3 feet long! They turned the end upwards and stuffed it into the breeching. With that furance, it didn't take long to rot the chimney and the debris fell down filling the liner stub. With the vent blocked, both appliances vented out the draft hood. Note, the furnace safeties never tripped because the draft hood allowed it to vent out, thus bypassing primary safety controls. There were no signs of condensation in the exhaust plenum, no signs of flame rollout on the flame shield above the burners and the vacuum, fusible link, and two spill switches were all intact. The plastic bushings on top the new WH were melted.
The two appliances were located in a basement with an 8" masonry curb around them to protect from floods. This curb trapped the CO2 spilling from the draft hood as it settled low and displaced the O2 thus choking off the pilot. I still don't know why they are alive. The "plumber" who installed the WH never inspected the breeching but assumed it was ok because he saw aluminum flex attached to the manifold inside the wall. First of all, never install galvanized pipe in a masonry wall. Always inspect the breeching and flue every time you change out appliances then last, perform a combustion and depressurization test. BTW, the furnace tech never suspected a problem because the furnace was venting nicely through the WH draft hood. Look at how the acids ate up what was left of that aluminum liner.
Before replacing the WH, a simple test with a combustion analyzer would have shown a falling O2 in the pit around the heaters, which would indicate CO2 displacing the O2. CO production isn't far behind.
I recommended a full, thorough inspection and testing of the furnace, esp. HX prior to resumed use and same for WH and a full length listed stainless steel liner. Since common venting will defeat the furnace safeties, I recommended a power vented WH with just the furnace to the liner and add a low level CO monitor .