Monday, November 28, 2022


Quick RV Tech Tip: Do some of RV’s drains go into the black tank? Is it legal?


Dear Dave,
I have read articles stating that gray water from a kitchen, bath or shower might be routed to the black tank rather than the gray tank. Is that legal? I would think that there should be a check valve of sorts if it is. Do you know if my rig has that design? —Frank, 2022 Nash 18FM

Dear Frank,
“Legal” is a very loosely defined word when it comes to the RV industry. The only codes that apply are regulated by the Recreational Vehicle Industry Association (RVIA) and those codes are not as strict as housing or automotive regulations. I have seen many cases over the years where a sink drains into the black water tank due to the location of the sink and tanks, especially in half-bath models that are just toilet and sink.

The “P” trap is the check valve that is designed to keep odors from coming from the tank.

The only manufacturer that I know has plumbing diagrams is Winnebago. Viewing the Northwood website I don’t find any information about how the plumbing is routed or even how many tanks there are. It just says “total grey” and “total black.”

You will have to physically run water down the sinks and see what tank fills up to verify where the water drains to. Not the best scenario, but there is very little documentation available. Even if you call the factory, you’ll likely get a customer service rep that is fresh out of school.

I do have a call in to RVIA to determine if the code has changed, so I’ll keep you posted.



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Shop Rat 00
25 days ago

I know the answer about why the bathroom sink may/probably does drain into the “Black Lagoon”. It is to insure that there is sufficient water to keep waste & toilet paper from staying solid & not draining properly from the “Black Lagoon”. Before they did this people were shorting on how much water they used to flush the toilet because the waste tank is smaller & has to be dumped more often. And places like Camping World don’t want to have to deal with having to take care of an issue like that. And it is expensive. So the manufacturers changed the routing for the bathrooms sink.

Mark Nelsen
26 days ago

For what it is worth, I have a 2019 Winnebago Spirit 26A. The bathroom sink and toilet drain into the black water tank. The kitchen sink and shower drain into the gray water tank. 😉

Last edited 26 days ago by Mark Nelsen
Dave E
26 days ago

If it ALL went into one tank the extra water would eliminate a lot of poop pyramid issues. The traps would stop any sewer smell from getting the RV, just like any fixture in a house, they use the same traps. And as a couple have said, it all ends up in the same place, again just like a house.

Don Nedrw
26 days ago

Way back [late 70’s and early 80′] there was only one tank for all drains.

Ruth W
26 days ago

I had a 2004 Triple E 5th wheel and the sink drained into the black water tank. We also had 2 grey water tanks. One only for the shower and one for the kitchen sink. It was a great design for waste liquids.

Daniel Merkovsky
26 days ago

I forgot to add, bathroom sink goes to the black tank. Initially I was annoyed about it. But it sort of makes some sense since it adds some liquids to the black tank on short trips, so the tank actually drains better.

Daniel Merkovsky
26 days ago

We have a 2022 Thor Quantum Class C. All the schematics for plumbing and wiring are available to owners, directly from Thor.

26 days ago

Amazingly, I recently managed to reach a Jayco tech who sent me the plumbing diagrams to my 2015 5th wheel!

Bob p
26 days ago

My first TT a 1978 TEC Travelmate, it’s easy to check after emptying the tank shut off the toilet valve water, open the flush valve and listen for water draining in.

26 days ago

I don’t see how or why it would be illegal as long as proper traps and venting are used on the sinks. Not any different than your house. Everything eventually ends up in the same place.

Marlene Syversob
26 days ago

I called Northwood and got the schematic for my fresh water, grey and black tanks and electric. I have a Arctic Fox 29L.

26 days ago

It all goes to the same place. Legal is a very loose definition.

26 days ago

Thor sometimes deliberately runs the bathroom sink into the black tank for the alleged purpose of ensuring sufficient water in the black tank to help avoid blockages.

Joseph Testa
26 days ago

I have water schematics for my Thor MH also, I think anyone with a Thor can create an account and get access to their model schematics, water, electrical, etc.

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