i have 3 hot water heaters and 2 gas boilers in a back room in the basement. directly above that are dryers for a laundromat. i have backdraft problem on one of the water heater, it gets worse if all the dryers are running. if i stand on the chimney on the roof i can feel the hot air from the dryer vent. would that be the cause of the backdraft?
are the vent pipes on the water heaters properly install?
Doesn’t look like they are. Get a qualified person to look at the whole thing.
Looks like the pipe is partially disconnected at left edge of the picture.
Also looks like they went from B vent to single wall instead of single wall connector into B vent, but that may be your picture.
Too much horizontal, not enough vertical on the connectors maybe.
Roof is a mess as well.
Is there a make up air provision when the dryers are running and sucking all the air out of the building?