I can answer some of your questions:
Pictures of burner tray: Here they are. Took off a couple of orifices at each end of the gas supply pipe: clean inside the pipe. No spider webs or corrosion. Blew out pipe from the dead end with compressed air: nothing came out of the other end. All orifices clean and rodded out with copper wire anyway. Also photo of gas supply pipe and orifices.
Maybe obstructed burner tubes: Removed all tubes and found them clean and unobstructed.
Picture of Heat exchanger: Will send tomorrow. Was unobstructed from the top. Moved the baffle plate things aside one at a time and vacuumed out what little corrosion there was with a brush. I did this after the first time it exploded, and also replaced the circuit board (some guy on YouTube had the same problem and same heater and replacing the board fixed his problem): I started it up, it fired normally (making the familiar WHOOSH it has always done) and it ran for about 10 minutes then exploded again, shut off power at the house breaker panel. Currently disassembled waiting for new Igniter.
There may be variable gas pressure causing surges. I don't know what you mean by surging. It would periodically (every 5 - 10 minutes or so) explode. Did not get near enough to observe operation after the first EXPLOSION (not a whoosh or banging) ...that blew off the access panel...and caused folks two houses away to call 911 was afraid to go near it.
May be a bad gas valve or gas pressure regulator: I have a new gas valve/regulator and waiting delivery of new igniter.
Gas supply pressure low or high: I can't check this. No equipment do do so: I'm just the homeowner....
Suspect it could be a bad ground on the igniter or dirty igniter causing the flame sensing circuit to be intermittent: So installing a new igniter and have wire brushed all points of ground between the igniter, mounting bracket, burner frame and frame of the heater itself. Will run a new ground wire from the burner frame to the ground lug on the heater frame.
If you have more suggestions, I'll try those .....Not much else to replace.....Igniter arrives next week....will reassemble and try again.
Will send heat exchanger pictures tomorrow (Thurs Dec 5) , will try to get one looking up from inside.
Thank you for your ideas and questions.
