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Phil and Ginni Lyons
June 27, 2024
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Recommended Drain Valve Diameter for Black and Grey Water Tanks, and Should they be combined?

If you were to build another bus, would you use a 1-1/2" or a 3" Drain Valve for your Grey Water Tank?

Most RVs of any size use a 3” diameter black water plumbing and dump valve, and most RVs and bus conversions are built using a 1-1/2" diameter grey water drain valve.  This is sufficient to drain all the water and other "stuff" out of the grey tank in your rig.  However, a 3" drain valve will do it 4X faster than a 1-1/2 diameter dump valve. Remember your arithmetic?  Area = Pie are Squared? (I would always argue with my math teacher, saying, "Pie aren’t squared; Pie are round. Cornbread are squared").

Consider the practical benefits of a larger drain valve. With a 3" diameter drain hose already connected to your black water tank, why not leverage its size? A larger diameter drain valve not only drains your tank faster but also flushes your drain tank and hose more effectively, thanks to the increased gallons per minute flow rate. This is particularly useful if you drain your grey water tank AFTER your black water tank, a common practice to flush out the flexible drain hose before travel.

One key advantage of a larger grey water drain valve is the time it saves you at a dump station. By getting in and out faster, you allow the next rig in line to dump their tanks sooner, preventing you from holding up the line. While it may take longer to fill your fresh water tank than to dump your grey and black water tanks, the sooner you can complete this task and put everything away, the better. This is especially beneficial in campgrounds with limited dump stations.

That being said, how would you plumb your next bus? Would you use 1-1/2" diameter plumbing for your grey water and a dump valve, or 3" diameter plumbing and a dump valve for both tanks?

Another thought: would you combine your black and grey tanks or keep them separate?

And one last question: Would you design your rig to dump grey and black water out both sides of the bus or only on the street side, as is the standard?

Post by: Van 

I chose to use a large, combined black/grey tank. Less plumbing equals fewer valves. I rarely boondock, but we can do it for a few days. We mostly stay in parks with full hookups and don't have to wait in line to dump or fill a fresh tank. This works for us. 

Post by: Iceni John 

I have 3" dump valves for both the black and grey tanks and a third 3" valve to backflush the black tank with grey water.   Yes, flow speed through the stinky slinky is critical for 'solids' not to get trapped in its inner convolutions.   This is also why I made my black tank 24" high, so its contents will flow out with some force.   The grey tank also has a 1.5" dump valve that exits straight down on the passenger side just ahead of the rear wheels, "just in case...", but it will very rarely, if ever, see any use.

However, I have fresh water fills and 120 VAC inlets on each side of the bus to avoid dragging my hose or cable under the bus. 

Post by: dtcerrato 

We have and love a single-tank setup. The ingenious idea of power and water on both sides is great, but I’ve gotten pretty good at slinging the drain hose and/or shore power cable to the other side! 

Post by: peterbylt 

We have grey and black tanks, 3-inch pipes, and dump valves attached to a single outlet. This works very well. Drain the black first and then the grey to rinse the slinky as suggested.

Post by: Jim Blackwood     

I have a 2" valve on the gray, dumping into the black tank on the opposite end from the black tank dump valve. I think it will be pretty optimal to flush the black tank, though it will take longer to dump than a 3" valve.  Because the black tank dump valve is 3", the flow through the black tank will not be impeded. I also have a garden hose dump on the gray tank in case a dump station is unavailable.

Post by: buswarrior 

I hear some old Bus Nut voices hollering "spares" from the Great Beyond.  One size means one spare, two $ize$ double$ that.  Keep the part count down. One size fits all?

Post by: windtrader 

I'm in the “use one size, 3 inches" camp. If you have a specific need for smaller, insert a reducer, but keep as much 3" piping as possible. 

Post by: Jim Blackwood 

Well, here's the thing, though: If you use the gray water to flush the black tank, then the outlet needs to be larger than the inlet so that the water will flush through, picking up any debris and carrying it along. If they are the same size, the water will pool in the black tank, and you will lose the flushing action. It just becomes another drain instance. The water will flush through the outlet without slowing down if the outlet is larger. A 2" outlet from the gray tank will drain that tank quickly. Both 2 and 3" valves are common sizes, so there is not much need to carry spare parts.

Post by: Gary Hatt 

Yes, that brings up another good point. If you place your grey tank above your black tank, you can use your grey water to flush out your black water tank.  Having them both at the same level works, too, and is almost as effective, but with gradually less force, as the grey tank approaches being empty. 

Post by: Gary Hatt

On a similar topic, I have a macerator toilet.  This grinds everything up, so you end up with a slurry of stuff in your black water tank.  This avoids potential issues with solids piling up inside your black water tank, but using grey water to flush it still helps.  Also, with this setup, unlike using a conventional RV toilet, I can leave my gray and black dump valves open until I move again, as both tanks drain just fine with no solids involved.  But I generally close the grey water valve a day or two before departing, so I have some grey water in the tank to do the final flush before leaving. 

Another advantage of having a macerator toilet is that I can drain both tanks through a 5/8" diameter garden hose, which is handy when I stay at places like the Elks Lodge in Garden Grove, CA. My 100' hose will reach the dump station from any space from #1 to #8, so I can drain my tanks without having to move the bus. 

One caveat of a macerator toilet is that it can't flush if it loses power, as it takes power to run the macerator pump in the toilet.  You can, of course, when installing your macerator toilet, wire it to your house battery system as usual, and also run a separate set of wires from your start batteries with a 2-way toggle switch in case your house batteries get too low as with regular batteries, or drop out all of a sudden if you have lithium batteries, which has happened to me more than once until I added more lithium amp hours to my battery bank.

Post by: dtcerrato 

We have a backup external macerator pump that we seldom use. It connects to the main 3" dump valve. It is portable in case it’s needed to pump waste a longer distance through a smaller diameter hose. It will also pump uphill, which is a plus. 

Post by: plyonsMC9 

Regarding: “And one more question, would you build your rig to dump both grey and black water out either side of the bus or only on the street side?”

If I could get away with the dump valves on both sides without giving up too much real estate in the opposing bay, I would do that.  There have been a few times where that would have been helpful.  So, that thought has crossed my mind.  It would also need the dump valve and some kind of water outlet to wash things down on that side.  But as our bus is now, space is very limited, and I'd have to go through the basement A/C with the sewage, which doesn't sound promising.

Post by: plyonsMC9 

And another thing now that you've got me started, are the separate grey and black water tanks a remnant of the early RVing days when you could dump the grey water on the ground and not be arrested?  Is there a big reason to have separate tanks vs. one large tank?  It helps the black tank to have more liquid in it anyway.  We always seem to fill up grey first, then (rarely) the black tank.  One rig we owned had the capability to empty the grey tank into the black tank to make more room.  I thought that was a handy feature.

Post by: windtrader 

Having two tanks, gray feeds into black when the valve is opened and then through the outlet valve. Our gray fills faster, too, so I like having the black downstream as an overflow to the gray. The gray fills, then lets some into the black, giving me a warning reminder and time to start planning a dump stop.

The way mine is plumbed; the gray cannot be emptied independently from the black. Dumping the black tank alone is rare, if ever, so a trip to the dump station is always for both. It's a bit more work, as an extra valve needs to be opened, but it's not a big deal.  A single tank for both grey and black water would be easier and less costly to install. You lose the overflow feature; other than that, it's a wash.


Post by: Gary Hatt

I, too, like combined tanks; it just seems like a better use of space. But one thing to consider: When combining grey and black water in one tank, after you dump your tanks, you are more than likely to have to flush out your hose with the freshwater supply, which I seldom do. My hoses are in a separate sealed bay, so I am not too concerned if they are stinky when I finish dumping them. My current tanks are separate, so the grey water flushed out the hose last. However, if either of my tanks fills up, the full one drains into the other tank, so, in essence, I can end up with mixed fluids.

Post by: windtrader 

Having dump valves on both sides increases convenience as you spend time on the road and off the grid. If you are in parks with hookups all the time, then the issue of tanks is moot. If off-grid all the time, dumping is more frequent to the extent of how much you are on the road. The fewer trips one makes, the more incidents of inconvenience lessen. So, it all depends on your lifestyle, yet again. 

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Article written by Phil and Ginni Lyons

Phil Lyons has been a Bus Nut and moderator of the BCM forum for many years. He and his wife Ginni live in the central highlands of Arizona.  Phil’s day job is in IT Security and Ginni is a retired Registered Nurse.

They are the proud parents and grandparents of daughters, granddaughters, and two spoiled dogs.

Phil and Ginni are part of a bluegrass/gospel trio called Copper Mountain StringAlong, are members of Bethel Baptist Church in Prescott Valley, and volunteer and serve in various capacities in the church and the community.

RVing has been part of their lives for over 35 years, and they both hope to enjoy the bus lifestyle for many more years.

You can contact Phil via email at

Phil@BusConversionMagazine.com

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