Originally Posted by NottieMatt
Isn't the black tank vented to the roof stack? Why would this pressurize the tank enough to hurt it?
I have to ride to his defense here.
1) While the tank IS vented to the roof, the angle iron supports that hold the tank in place are at the absolute minimum gauge to support a full tank of water. As the tank bulges due to the weight of water in the stack, it can break (or slip) free damaging the tank and plumbing.
2) The water is coming in at 40-60 PSI pressure and could be higher since no one uses their potable water pressure reducer (I hope) to limit pressure when you are flushing your black tank. (IMO higher is better!) However, if the tank is already full to the stack, the water coming in has to push against that column of water in the vent. This will certainly raise the pressure inside the tank well past its designed limits.
3) Something similar DID happen to my friend (a former C-130 Nav I flew with and currently camp with) when he dropped a garden hose down his vent stack to flush out a slow tank in his Montana. While admittedly he reduced the effective vent capacity to relieve water when the tank filled with the diameter of his hose, he felt that air and water should have been able to clear the vent if it should overfill. Yet, when he closed the dump valve to let the tank fill for another "flush" he was distracted by a phone call. Next thing he knew black tank water was running over his roof from the stack and gushing out of his burst tank under the camper.
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