1-1/2" or 3" Diameter Drain Valve for Grey Water Tank

[Gary Hatt - Publisher BCM]

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

Most RVs and bus conversions are built using a 1-1/2" diameter Grey Water Drain Valve.  This is sufficient to get all of the water and other stuff out of the grey tank in your rig.  However, a 3" diameter drain valve will do it 4X faster. Remember your Arithmetic?  Area = Pie are Squared? (I would always argue with my teacher saying "Pie aren't squared, Pie are round. Cornbread are squared".  ) 

Besides, you already have a 3" diameter drain hose connected up anyway, so why not take advantage of all of it.  This also has the advantage of flushing your drain hose better as you have more gallons per minute going through the hose. That is of course, if you drain your grey water tank AFTER your black water tank as most people do to flush out the hose before putting it away for travel.

A larger grey water drain valve gets you in and out of a dump station faster, so the next rig in line can get in and dump their tanks. Yes, I realize, it takes longer to fill your fresh water tank than dumping your grey and black water tanks, but the sooner I can get this nasty job done and everything put away the better in my opinion.  Besides some campgrounds have fresh water at each site, but only one dump station in the park for everyone to share.

So how would you plumb your next bus?  Would you use?

1-1/2" Diameter Plumbing and Dump Valve?
or...
3"       Diameter Plumbing and Dump Valve?

We had an 8" drain valve on a Tanker Fire Truck I used to drive to fill a portable pond.  That would really drain your tanks fast, but may be overkill for this application.   

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?

Gary

[Van]

This is why I chose to use a large combined  black/grey tank. Less plumbing less valves. Very rarely boondock but we have the capacity to do it for a few days. We mostly do parks with full hookups and don't have to wait in line to dump or fill fresh tank, works fir us. To each his own.

[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, speed of flow through the stinky slinky is critical for 'solids' to not 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.

I do however have fresh water fills and 120VAC inlets on each side to avoid having to drag my hose or cable under the bus.

John   

[dtcerrato]

We have & love the same tank set up. Ingenious idea of power & water on both sides but got pretty good at slinging the hose &/ or cable to the other side! 

[peterbylt]

We have both Grey and Black tanks and 3 inch pipes and Valterra valves attached to a single outlet.
Works very good, as suggested I drain the black first and then the grey to rinse the Slinky.

Peter 

[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 that will be pretty optimal to flush the black tank, though it will clearly take more time to dump than a 3" valve would, and 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 just in case a dump station is not available.

Jim

[buswarrior]

I hear some old busnut 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?

happy coaching!
buswarrior

[windtrader]

I'm in the use one size 3". It is the standard. If you have specific need for smaller, then insert a reducer but keep as much 3".

[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 lose the flushing action. It just becomes another drain instance. If the outlet is larger the water will flush through without slowing down. And a 2" outlet from the gray tank will drain that tank pretty quickly. Both 2 and 3" valves are common sizes so not much need to carry spare parts.

Jim

[Gary Hatt - Publisher BCM]

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 lose the flushing action. It just becomes another drain instance. If the outlet is larger the water will flush through without slowing down. And a 2" outlet from the gray tank will drain that tank pretty quickly. Both 2 and 3" valves are common sizes so not much need to carry spare parts.

Jim

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.

[Gary Hatt - Publisher BCM]

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 having potential issues with your black water tank, but using grey water to flush your black water tank still helps.

Another advantage of having a macerator toilet is that I can drain both tanks through a 5/8" diameter garden hose which comes in handy when I stay at the Elks Lodge in Garden Grove, CA.  My 100' hose will reach the dump station from any space from #1 to #7.  So I can more easily drain my tanks. 

One of the disadvantages of a macerator toilet, if you lose power, or your pump fails, you can't flush your toilet. 

[dtcerrato]

We have a back-up macerator pump we seldom use that connects to the main 3" dump valve. It is a portable set up in the case its needed to pump waste longer distance through a smaller diameter hose like Gary mentioned. It will also pump up hill which is a plus.

[plyonsMC9]

On the topic of : 
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?

I would say that 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 try to work that.  There have been a few times where that would have been helpful.  So that thought has crossed my mind.  Would need the dump valve and some kind of water flow as well to wash things down on that side.  But as our bus is now, space is very limited and I'd have to through the basement a/c with the sewage.  It doesn't sound promising.   

Kind Regards, Phil

[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 & 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 (almost never) the black tank.  One rig we owned had the capability to empty grey into the black to make more room.  I thought that was a handy feature.

Kind Regards, Phil

[windtrader]

Hi Phil,
It's a topic that ranks low on the long list of topics related to the bus. First, it's in and changing over from separate tanks would be as project and the benefits are marginal for me.


Having two tanks, gray feeds into black when the valve is opened then through the outlet valve.


pros/cons
our gray fills faster too so I like having the black downstream as an overflow to the gray. Gray fills then let some into the black, giving me a warning reminder and time to start planning a dump.


the way mine is plumbed the gray can not be emptied independent from the black. dumping black alone is rare if ever so a trip to the dump station is alway for both. A bit more work as an extra valve needs to be opened, not a big deal.


If designing a new waste system, a single would be easier and less costly to install. You lose the overflow feature; other  than that it's a wash.


Dump on both sides. Eh. If installing new system I'd probably do it if there is an open pathway. I'd say it is a toss up what has caused connecting issues; wrong side or placement on the passenger side is inconvenient.