Have you ever physically measured your gray tank capacity?

krsmes · June 2023

I was doing my regular post long-trip cleaning (4+ months) and maintenance a month ago, flushing everything out really well, sanitizing, etc.

In doing so I decided to see how much water I could put into the gray and black tanks, gallon by gallon to see when the ⅓, ⅔, and full lights came on.

I have a 2021 T@B 400 which the specs show the gray tank at 18 gallons and the black tank at 12 gallons.

The black tank worked as expected and went to "full" between 11 and 12 gallons and I poured 13 gallons in with no issues and then drained it.

But on my gray tank the full light came on before hitting 11 gallons and by the time I got to the 14th gallon (I kept going as the capacity stated 18) my kitchen sink was backing up. I had flushed the tanks several times before trying this and the light indicated "empty" when I started (and it was completely empty). This surprised me so I drained it and tried again. Same exact thing happened, full light came around 11 gallons and sink started backing up around 13 gallons.

My initial thought was maybe there was air getting trapped in it, but I don't think that would explain the full light coming on at around 11 gallons. Until the sink backed up at 13-14 gallons I though maybe it was just a very conservative placement of the "full" sensor. But I'm 5 gallons short of stated capacity. Trailer was perfectly level during the test.

Anyone else run into this? Or even tried measuring your actual capacity?

Sharon_is_SAM · June 2023

I found that since our 320 gray tank inlet is tongue side, to add capacity to the tank required tongue up (both fresh and gray tanks). The tanks are shallow and elongated and it seems they can’t be fully filled.

Binghi · June 2023

krsmes said:

I was doing my regular post long-trip cleaning (4+ months) and maintenance a month ago, flushing everything out really well, sanitizing, etc.

In doing so I decided to see how much water I could put into the gray and black tanks, gallon by gallon to see when the ⅓, ⅔, and full lights came on.

I have a 2021 T@B 400 which the specs show the gray tank at 18 gallons and the black tank at 12 gallons.

The black tank worked as expected and went to "full" between 11 and 12 gallons and I poured 13 gallons in with no issues and then drained it.

But on my gray tank the full light came on before hitting 11 gallons and by the time I got to the 14th gallon (I kept going as the capacity stated 18) my kitchen sink was backing up. I had flushed the tanks several times before trying this and the light indicated "empty" when I started (and it was completely empty). This surprised me so I drained it and tried again. Same exact thing happened, full light came around 11 gallons and sink started backing up around 13 gallons.

My initial thought was maybe there was air getting trapped in it, but I don't think that would explain the full light coming on at around 11 gallons. Until the sink backed up at 13-14 gallons I though maybe it was just a very conservative placement of the "full" sensor. But I'm 5 gallons short of stated capacity. Trailer was perfectly level during the test.

Anyone else run into this? Or even tried measuring your actual capacity?

That would explain why we’re constantly having to drain the grey tank on our 2021 400 BD. Thanks for the analysis, @krsmes!

marleecamps · June 2023

I measured my gray tank capacity on my 2021 400 a couple years ago because I also felt that on a trip with a second person we had prematurely filled the gray tank. I recall hitting the "full" indicator at about 13 gallons.

manyman297 · June 2023

I’ve never measured our tank capacity but it does seem to fill fast. We almost always have water left in our fresh tank after a 2-3 day trip but find ourselves having to dump a bit of our gray sometimes because the tank is full.

Doug_Edwards · October 2023

Wouldn't the shower drain, back up before either of the sinks?

Horigan · October 2023

The 2021 400 may have the Hepro valve for a P-trap in the shower, which can seal off the back flow of water from the tank.

Image: https://us.v-cdn.net/5021717/uploads/editor/nf/a17hh8rsreb4.png

Deb55 · October 2023

Doug_Edwards said:

Wouldn't the shower drain, back up before either of the sinks?

Our kitchen sink was always the first to back up when the gray tank was full.

ScottG · October 2023

Yep, it depends on the model and year. In mine (2015 320) the shower has a regular drum trap and it's the first thing to back up. In fact, since I have no electronic level indicators, it's the only way I know my tank is full! :-o

I thought the Hepco valves were used on the sinks as well. Makes me wonder why the sink would back up, but not the shower.

dsfdogs · October 2023

@krsmes thanks for the experiment; I too have noticed how fast my gray fills. I swear it didn't fill as fast in the 320 with a 17 gal gray tank. Any chance you've reached out to nuCamp for an explanation?

MuttonChops · October 2023

Interesting . . . . . .

On my '18 320S I did verify the black tank gallons vs SeeLevel percentage full
(which was/is very accurate BTW) but never did the fresh or gray.

manyman297 · October 2023

I've had my sink back up (with the hepvo valve) and my shower drain be fine. I know I have a traditional p-trap on the bathroom drain but maybe the air in the trap is providing enough "cushion" to keep water from coming out of that drain. It is weird that the kitchen sink would back up first.

pthomas745 · October 2023

Is it possible that the gray tank does not expel air like the fresh water tank fill can? And that "bubble" prevents the tank from filling all the way? Has anyone tried "bleeding" the air from the tank with the tongue up and then opening the gray tank waste valve? Essentially...burping the tank?

I know this idea may not...."hold water"

. And, a 5 gallon "air bubble" sounds pretty huge.

Binghi · October 2023

The idiot lights in our 2021 400 are pretty much worthless. I know the gray tank is full when the kitchen sink won’t drain (which is all too often). I know the black tank is full when I push the flush handle down, shine a flashlight into the void, and see the dark liquid too close for comfort (which is much less often).

ScottG · October 2023

The gray and black tanks are vented to the atmosphere (just like the waste plumbing in your house). If they weren't, the air in them would quickly compress, impeding draining and causing odiferous gas to bubble up through the drains.

That said, an oddly shaped or misshapen tank could theoretically trap a couple gallons of air in a pocket where it couldn't escape out the vent. Seems unlikely, but who knows.?

Measuring the actual functional capacity of the gray and FW tanks has been on my project list for years. Maybe next year... :-/

rh5555 · October 2023

We've also noticed that the grey tank seems to fill "too fast". We have started to chock up the wheel opposite the grey drain as high as we can with our Lego blocks when dumping. This seems to help some. Residual water in the grey tank after dumping would obviously reduce our capacity. (It also drains faster).

A word on the sink Hepvo valve: It stops opening properly to drain the sink if the back pressure on the valve matches or exceeds the head of water from your sink. If the sink is nearly empty, this could be just a few inches of water. And that back pressure need not be water, it could be compressed air. That pressure wouldn't be enough to force water up into your shower.

So; a theory: If the vent into the grey water tank was badly designed so that it doesn't end flush with the top of highest point of the grey water tank, but protrudes in like this:

Image: https://us.v-cdn.net/5021717/uploads/editor/27/xcergeg808ad.jpg

Then when the tank is filled above the bottom of the vent pipe, pressure will rise (as the air cannot escape) and quickly stop the Hepvo sink valve from allowing more water in.

You can test this out: When the sink starts to back up, just pour more water into the sink. Does any of that water drain away? If it does, then the tank isn't really full - you just have back pressure stopping your Hepvo valve from opening. If it doesn't, your tank really is full and now you have a bigger mess on your hands!

atlasb · October 2023

When we first got our 2018 400, we had the water back up in the shower. That let us know to watch how much water we use. I installed the Hepo valve in the kitchen sink and checked the shower drain. I found that we have one there already. Even with the hepvo in the shower we got standing water one time.

marchule · October 2023

Obviously, I have too much time on my hands because I decided to measure the grey water capacity.

To start with, my 2022 400 Boondock trailer is level.

My first experiment was to pour water in the kitchen sink. At 5 gallons the readout said 1/3 full. At 10 gallons it was 2/3 full. At 15 gallons - you guessed it - it was full. I was able to pour about another 1/2 gallon and then the sink was backed up.

I drained the grey water.

My second experiment was to pour water in the shower drain. Same results regarding the readout but I was able to get a little over 16 gallons before I saw it was draining no more. Why could I get more down the shower drain? I'm clueless.

This is disheartening. It's just one more thing where I "believed" what was represented by the nuCamp specification sheets and the sales person. I should have not trusted, but verified. (It's like the other sales line, "It has a queen size bed, too.")

Every inch and ounce of space in a Tab is a precious resource and we need to get the most of each. A trailer that can only last 2 to 3 days out in the "boondocks" is not a great selling feature for that environment.

I see that nuCamp now specifies the size of the bed in inches. I would recommend they do the same in terms of actual capacities for all the tanks.

rh5555 · October 2023

@marchule - once the sink started to back up, did you try adding more water to the sink to see if some of that drained away? That would be the expectation if there is an air pocket trapped in the top of the grey water tank.

A further experiment would be to repeat but with the bathroom-side of the trailer elevated on Lego blocks (or equivalent). The vent pipe is on that end of the grey water tank, and needs to be at the highest point.

Sharon_is_SAM · October 2023

I do remember that nuCamp did reduce the reported capacities to reflect insitu volumes. I suspect going tongue up may allow for more capacity. If someone can try that, it may prove helpful. I know that is true for our 2017 CCS.

marchule · October 2023

In the spirit of exploration I decided to repeat with pouring water into the shower and adding a tongue up/down element to the exercise. With the trailer at a level position I was able to get 16 gallons and almost a quart. On adding more water it did not drain. I raised the tongue 4 inches and this only succeeded in making the water slosh to the back of the shower pan. I know, right? Lowering the tongue 4 inches from the level position just makes the water flow to the front of the pan.

I guess I was hopeful because if it did work then it would be a non-major fix to getting a few more quarts by changing the tongue height. A left-right approach with the Lego blocks would require hooking up your vehicle which would be way too much work.

On the humorous side I was puzzled as to why I could get 1 more quart down the drain than on my previous test. Turns out I had forgotten to close the grey water valve. Oops.

rh5555 · October 2023

I suggested raising the bathroom side of the trailer as a diagnostic, not a solution. If raising the bathroom side helps, then we know we have an air pocket and then we can explore ideas to fix that.

marchule · October 2023

Let's say that the diagnostic shows there is an air pocket. What would we explore next? If there is a good possibility that we can make progress I may consider putting in the effort to doing a "Lego" test (grumble, grumble).

rh5555 · October 2023

I would open up the cabinet below the sink to see how the vent pipe is installed. I'd cut it and probe the inside to see how far it extends into the tank. If it is problematic, I'd see if it can be easily rectified else I'd consider drilling a small vent hole through the top of the tank. This is grey water, not black. Fumes are not so much of an issue.

Thank you for doing all the work while I enjoy camping out at the seashore of the Olympic National Park!

Binghi · October 2023

As far as I’m concerned, the 2021 400 has a 15-gallon gray tank. Weak spot for boondocking, but at least the black and fresh water tanks hold their own.

manyman297 · October 2023

Yeah, the gray tank is what always fills first. I've never run out of water nor filled the black tank completely up. But the gray tank consistently overfills. I know it's a no-no for some but ocassionaly I'll have to dump a gallon or two of gray water at the campsite in order to keep using the sink for cleaning. Pretty rare but I've done it a handful of times.

scott14 · October 2023

We carry a 5 or 6 gallon auxiliary gray water tank in our tow vehicle. I have on occasion emptied 5 gallons out of our gray tank and walked it to the dump station. We have gotten better about gray tank capacity management, so we don't use it every trip, but bring along every time. The trip I don't bring it, will be the time I need it! It may have been mentioned already here, but we have found it helpful to put a bowl in the kitchen sink to capture water with doing dishes. We use Dawn detergent, so it can be dumped outside.

marchule · October 2023

@rh5555 - you're a genius! I guess this is why you get to enjoy the seashore while I'm stuck doing the heavy lifting.

While the trailer was level I was able to pour in just a little more than 16 gallons. I added one more gallon in the shower pan. It pooled. I then got distracted and went away for several hours. When I came back it appeared about 2 quarts had drained all on their own. Hmm. I then pulled the trailer upon 3 blocks and presto! The remaining water drained. I added another gallon and it nearly drained, too, maybe a pint or so on the shower pan. Definitely 17.5+ gallons in the grey water.

Now what do "we" do??

rh5555 · October 2023

I get home on Thursday. Other than sorting out why our batteries don't seem to take a charge anymore, I'll get right onto figuring out how to vent the grey water tank better. Thanks for doing all the work!

krsmes · January 3

marchule said:

I then pulled the trailer upon 3 blocks and presto! The remaining water drained. I added another gallon and it nearly drained, too, maybe a pint or so on the shower pan. Definitely 17.5+ gallons in the grey water.

I’m assuming the blocks were on the bathroom/drain side? This was my curiosity as well. The air vent inlet to the gray tank is actually just to the right of the bathroom door in that little triangle corner on the left side under the front seat. You can see it if you open the A/C filter cover on the seat and look left with a flashlight (it is a one way air intake valve on the top of a pipe).

Optimally, I believe it should be located on the other side of the trailer under the kitchen sink.

We aways like our head side of the bed to be higher vs lower but only .5 degrees, not three blocks.

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Have you ever physically measured your gray tank capacity?

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