Back pressure on black tank flush

Black Bark · 07-12-2021, 10:29 AM

Is it normal for there to be back pressure when flushing black water tank? Even when I shut off the water and wait a bit with tank drain open, some water will come out when I disconnect the hose.

**flybouy** · 07-12-2021, 10:34 AM

Back pressure no, some back flow yes. The anti-siphone valvenis typicall 18" or so above the hose inlet. So when you turn off the water that column of water will drain out when you unscrew the hose.

**JRTJH** · 07-12-2021, 10:48 AM

My gray tank, black tank and even the fresh water tank will all "leak a bit more" for several minutes after the majority of the contents are drained. Then, all I need to do is "move the trailer a tad and it'll usually change the position of the tank bottom enough that it will drain some more. After towing, if the tank valve is open and the terminal valve closed, there will almost always be a couple gallons "extra" waiting behind the valve".....

It's been said that once you put water in a holding tank, it's going to have "something in it" until you sell the trailer.... From my experience, that's pretty much a true statement.... You will always have "something left to drain" no matter how hard you try to drain it all the first time.....

Black Bark · 07-12-2021, 11:34 AM

Thanks- if I wait a bit it just runs out, but once I opened right after shutting off supply and it sprayed out. Do you think I have an issue with anti-siphon?

**JRTJH** · 07-12-2021, 12:25 PM

Quote:

Originally Posted by Black Bark

Thanks- if I wait a bit it just runs out, but once I opened right after shutting off supply and it sprayed out. Do you think I have an issue with anti-siphon?

An issue with anti-siphon ?? No, I don't think that "sprayed out" is associated with the back flush nozzle in your black tank. I'd suspect that your black tank, like mine, is "behind a series of angled fittings and the spray nozzle is mounted on the side of your black tank, probably 3' or more from that small 3" opening that then "turns at least 45 degrees before it reaches the actual opening where you connect your sewer hose. So, the chances of "a low pressure spray of water deflecting off the sides of the tank, making a turn in the plumbing and actually having enough force to 'spray out" is pretty remote....

Chock it up to some "strange event"... Your black tank flush system can't "spray you after you disconnect it from the city water supply"... I'd be very surprised if you could even see more than a "trickle of water flowing out of the dump port on the side of the trailer... Most black tank nozzles deliver less than 2 GPM of flow thorough the spray nozzle, and that's a "trickle at best" coming out of the 3" opening on your trailer dump.

**flybouy** · 07-12-2021, 12:30 PM

Quote:

Originally Posted by Black Bark

Thanks- if I wait a bit it just runs out, but once I opened right after shutting off supply and it sprayed out. Do you think I have an issue with anti-siphon?

The easiest way I know to check the blank tank rinser: connect a clear fitting between discharge hose coupler and the drain hose. You should have one anyway to monitor the draining and how well the tank rinser is cleaning. Drain the black tank and wait until jotjing is flowing, this could take a couple of min. Then with drain valve still open turn on the water supply hose to the tank rinser. In a min or two you should see water flowing thru the drain line. You should also hear the water spraying inside the tank.

Also if the toilet drains straight into the tank then turn off water pump or city water, open drain valve, turn on tank rinser, the hold open the flush on the toilet and look down thru it with a flashlight. You should see water moving around in the tank.

CedarCreekWoody · 07-12-2021, 02:59 PM

Your anti-syphon valve is doing what it is intended to do. It closes under pressure but is releasing under the negative pressure of that line draining. If it didn't it could possibly allow the line to syphon water from the tank.

GaryUT · 07-12-2021, 03:35 PM

Quote:

Originally Posted by Black Bark

Is it normal for there to be back pressure when flushing black water tank? Even when I shut off the water and wait a bit with tank drain open, some water will come out when I disconnect the hose.

Disconnect the hose from the faucet and let it drain before disconnecting from the flush port.

Gary

Black Bark · 07-13-2021, 04:56 AM

Thanks all! Appreciate the good advice. To be clear, the back spray was only from the hose connection, not the drain. I do see clear water flowing through drain 'window' when flushing so guess everything is ok.

**sourdough** · 07-13-2021, 05:36 AM

Quote:

Originally Posted by Black Bark

Thanks all! Appreciate the good advice. To be clear, the back spray was only from the hose connection, not the drain. I do see clear water flowing through drain 'window' when flushing so guess everything is ok.

If you see water flowing indicating the flush system is working but getting some water push back at the exterior flush connection you might just replace (for grins), or disconnect (replace with L connections) the anti sipon valve located on the flush line probably under the bathroom sink. Trailer before last mine would give me a shower when I disconnected the flush hose; removing the valve fixed that and I just placed the antisiphon valve at the end of the hose.

rlh1957 · 07-25-2021, 09:12 AM

Follow the back flush connection along the tube to where the anti siphon valve is. The line is behind panels, sometimes goes up under sink connects to valve then back down to black tank.
On the valve will be marked the direction of water flow. It may take careful look to determine which line is inlet and outlet.
Turns out mine was installed backwards so I was only pressurizing that part of the line. Couldn’t hear water flowing into black tank and no level increase.
Cut PEX valve out, tested to be certain of operation, put back in with PEX connection and it worked! Could hear flush water going in. It is more of a black tank fill than spraying. More water in tank better the flush when dumping.

Spletbr · 07-25-2021, 12:39 PM

How about black stuff coming from the drain when I take the cap off? No matter how long I let it drain after closing everything up and traveling when I take the cap off to connect again there’s always “stuff” that comes out. I know to hold the hose under it to catch it but it’s still gross. Sometimes it’s a cup or more by my estimation.

**flybouy** · 07-25-2021, 12:49 PM

Then the valve is leaking. May be some "debris " stuck on the valve blade or the seal is leaking by.

rlh1957 · 07-25-2021, 02:32 PM

Quote:

Originally Posted by Spletbr

How about black stuff coming from the drain when I take the cap off? No matter how long I let it drain after closing everything up and traveling when I take the cap off to connect again there’s always “stuff” that comes out. I know to hold the hose under it to catch it but it’s still gross. Sometimes it’s a cup or more by my estimation.

I thought we were talking about the back flush connection, which is higher than the black tank so black tank contents can’t flow out.

If you’re talking about the black cap on the lower black pipe where sewer dump hose attaches… if you get any liquid when removing cap, then the black or grey waste gate is leaking. As mentioned debris is likely stuck in the gate… ie: toilet paper, partially dissolved plastic from drop in treatment packs. Some valves just start leaking. If you can get to the valve body you can replace it. Not a clean task.
Many valves are buried during the manufacture and just have the extended cable controlling the open/close gate.
My black tank started to seep like that and back fill the pipe. Yuck!
Valterra and other brands have a Twist On Waste Valve you can just twist on where your sewer hose goes on, then your sewer hose twists on that.
The extra Twist On Valve stays in place and only adds about three inches to the end.
It solved my problem and the seep stays on the backside of the new valve.

Example:

https://www.amazon.com/dp/B000BGHYJS?ref=detail_main_image_block_ivx_share

Hblick48 · 07-26-2021, 04:40 AM

Quote:

Originally Posted by rlh1957

I thought we were talking about the back flush connection, which is higher than the black tank so black tank contents can’t flow out.

If you’re talking about the black cap on the lower black pipe where sewer dump hose attaches… if you get any liquid when removing cap, then the black or grey waste gate is leaking. As mentioned debris is likely stuck in the gate… ie: toilet paper, partially dissolved plastic from drop in treatment packs. Some valves just start leaking. If you can get to the valve body you can replace it. Not a clean task.
Many valves are buried during the manufacture and just have the extended cable controlling the open/close gate.
My black tank started to seep like that and back fill the pipe. Yuck!
Valterra and other brands have a Twist On Waste Valve you can just twist on where your sewer hose goes on, then your sewer hose twists on that.
The extra Twist On Valve stays in place and only adds about three inches to the end.
It solved my problem and the seep stays on the backside of the new valve.

Example:

https://www.amazon.com/dp/B000BGHYJS?ref=detail_main_image_block_ivx_share

I used abs glue to make sure the extra valve so it can't come off. Adding this valve also allows you to flush your black tank using grey water, and "joining" both grey tanks to better utilize your grey capacity. Highly recommended.

travelin texans · 07-26-2021, 07:44 AM

Quote:

Originally Posted by Hblick48

I used abs glue to make sure the extra valve so it can't come off. Adding this valve also allows you to flush your black tank using grey water, and "joining" both grey tanks to better utilize your grey capacity. Highly recommended.

So now you have 2-3 black tanks! If you tied them all together you no longer have grey tanks, now all are black.

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