Drain Valves – To Close or not to Close?

Whether or not to leave one or both of your drain valves open has been a debatable issue ever since we got our RV 7 yrs ago.

7 Tips and Rules of Thumb for RV Waste Tanks
  • Water is good. Use lots of it, as often as you can.
  • Water, soap, and human waste are the only things that go down the drains.
  • When hooked up at campgrounds, keep valves closed until ready to dump.
  • Only dump RV waste tanks when full.
  • When dumping: black first, gray second.
  • Don’t forget to rinse – black tank rinse.
  • Use tank level sensors at your own risk.

This is what we do:

Leave both the gray and the black valves closed at all times!

The only exception is when we are doing laundry. When washing clothes, we will open the gray tank and leave it open until we are finished with the laundry. Then it gets closed again.

Here are the reasons why we keep them closed:

Black Tank:

If you leave your blank tank valve open, the liquid will drain from your tank but the solids will sit on the bottom of your tank and create ‘poo pyramids’ or dried sludge in the bottom of your tank. These piles or sludge dries as hard as cement and are very difficult to remove – and they will clog up your tank.

An valve will also allow sewer smells to permeate into your RV.

Gray Tank:

You need water in your gray tank in order to flush your hose after you empty your black tank. If you leave your gray tank valve open you will need to remember to close it long enough in advance of dumping your black tank in order to accumulate enough gray water to sufficiently flush your hose. I can assure you at sometime you’re going to forget to do that and then you’ll have to wait to dump your black tank which will not be not be good if your black tank is full. You’ll be making trips to the porta-johns or bathhouse.

Also, most people don’t think of gray tanks as being as smelly as the black tank but they are! Small bits of food that escapes past your sink strainer can sit in your gray tank at 90° temps for a day or two and can really create a horrible smell. That smell can travel up your drain lines and enter your RV through your sink and shower drains.

If those aren’t reasons enough to convince you to keep your valves closed here are a few others:

  • Your blank and gray tanks both dump into the same septic system. Leaving either one open when everyone around you has their’s closed makes YOU the path of least resistance – in other words, fumes from the sewer will invade your RV.
  • Sewers attract gnats, flies, bugs, snakes and rats that can travel up your hoses and invade your RV. Snakes and rats won’t get further than your tanks but they can plug up your drain hole and if one dies in your tank you’re going to have a smelly mess.
  • Sewer gnats, flies and bugs can easily get in your tank and then enter your RV through your sink and shower drains and even your toilet when you flush if you have the flapper type toilets.

Here’s a link to Drainmaster’s explanation of what happens when you leave your drains open and a link to the YouTube video mentioned on their page.


Rats in Your RV Tanks


  1. Jimmy Thompson

    Some manufacturers use the inside space of a wall as a vent pipe, The black and grey tanks will have a pipe up the wall a couple of feet above them. Then it’s open space between the walls all the way to the ceiling or roof. Loose fitting wall panels and trim, nail and screw holes left unfilled can let sewer gases and flies into the RV,

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