Confused by black tank flush

With an empty, relatively clean black tank, when I leave the dump valve closed and run water into the flush port, it takes 2 or 3 minutes for the black tank to fill to the 1/3rd level. During this time, when I shine a flashlight down through the held-open toilet valve, I can NOT see any water spray or flow, although after a while I see the level start rising into view across an apparently sloped bottom or pipe connecting to the toilet location.

I would have thought that the flush nozzle would create a spray like a regular garden hose nozzle, and that this spray would be directed at the offensive location directly under the toilet. But this seems to not be the case. That is, the flush seems to be nothing more than a refiller, with no spray intent upon cleaning the most likely dirty part of the tank.

What's up with that?
 

RoadJunkie

Well-known member
Don't ignore unusual symptoms regarding your holding tanks, but you can ignore the tank level indicators on all but fresh water tank. The remaining tank indicators are notoriously unreliable.
 

sengli

Well-known member
I know when I use my black tank flush, I can hear the spray in the empty tank whooshing. Not a lot of flow, but with my head in the front compartment near the UDC...I can hear it spray. I have however by passed the ASV valve and routed my water inlet line for the BTF directly to the spay line in the black tank.
 

LBR

Well-known member
With an empty, relatively clean black tank, when I leave the dump valve closed and run water into the flush port, it takes 2 or 3 minutes for the black tank to fill to the 1/3rd level. During this time, when I shine a flashlight down through the held-open toilet valve, I can NOT see any water spray or flow, although after a while I see the level start rising into view across an apparently sloped bottom or pipe connecting to the toilet location.

I would have thought that the flush nozzle would create a spray like a regular garden hose nozzle, and that this spray would be directed at the offensive location directly under the toilet. But this seems to not be the case. That is, the flush seems to be nothing more than a refiller, with no spray intent upon cleaning the most likely dirty part of the tank.

What's up with that?
If it's like our plumbing, there is a 45° elbow a couple feet below the toliet that goes into the tank....so naturally you can't see the water level until the sprayers fill it up and water rises above the 45° part.
 
So, @LBR, the implication there is that the design does NOT include a spray at the point of excrement initial contact? I can see not wanting to drill a hole in a brittle elbow, but that's how I would have designed it -- with the sprayer on the inside (upper side) of the elbow, pointing right at the landing point. Sure, there might be complications with sealing a penetration on a curved surface, of course.

Meanwhile, it's my intent to immediately refill black tank to 1/3 level, which puts a little water at that landing point, and then have that 1/3 black tank coupled with 1/3 fresh water tank as my standard tow configuration (with gray tanks empty). I'm tight on resultant tongue weight and tow vehicle payload.

@sengli , what's "UDC"? Also, so you have a direct line from outside your RV to the black tank spray? That's not a healthy design and if I understand your setup, it violates health ethics for the community. Sure, you might never intend to flush while the black tank level is above the nozzle -- but you might not know that actual nozzle height and you never know if somebody else, today or after you resell your RV, might do things differently. Sure, you might only use the flush at home or only at the dump station -- but the dump station water is probably common with the campsite utility, and so backflow into the dump station water does creep toward the campsites. It's just NOT a good idea. I'll tell a little story... I was in Delaware at a very nice camp. But the campsite dump pipe was in a 12" square concrete pad that also had the fresh water pipe stand in the same place. As I walked down the camp road, I looked at RV after RV. Half the RV owners had carefully gone through pains to keep their fresh water pipe from laying on the concrete pad or the semi-permanent dump pipe, in order to prevent any exterior contact between the fresh water pipe and the whole dump experience. The other half of the RV owners let their fresh water pipe simply lay on top of the dump pipe and the presumably dirty concreate pad. So half the people "got it" and the other half "didn't get it". Sure, it's small probabilities and percentages, but you don't sh*t where you drink. Even my dog knows that -- he specifically goes off the walking path into the weeds when it's time to dump. In reality, those other half of RV'ers touch their fresh water pipe and spread around bacteria, and drag that spread to all kinds of other places. It may never add up to much, but it's just not the way to do it. Realistically, maybe one in a 1000 of them have a full sick night once in their RV carrier. The other RV'ers never have that sick night. If I misunderstand your setup, then sorry for the complaining...

I'll dump this load from my mind and move on to more important things... (Hey, I never said I was a good comedian.)
 

NYSUPstater

Well-known member
UDC = Universal Docking Center

The flush does spray into black tank, but maybe not at same force as you'd see out of a garden hose sprayer. After the initial 1st dumping of black tank, I close the valve, turn on flush and let it run for2-4 min. Open valve w/ flush still going and keep repeating until most if not all water coming out is clear. Best to have a clear elbow at RV fitting or where it goes into CG fitting to see. Tho the mfr says to keep the valve open while flushing, I like to get water into the tank to help push things out better. Just remember that water is your black tanks friend.
 

david-steph2018

Well-known member
So, @LBR, the implication there is that the design does NOT include a spray at the point of excrement initial contact? I can see not wanting to drill a hole in a brittle elbow, but that's how I would have designed it -- with the sprayer on the inside (upper side) of the elbow, pointing right at the landing point. Sure, there might be complications with sealing a penetration on a curved surface, of course.

Meanwhile, it's my intent to immediately refill black tank to 1/3 level, which puts a little water at that landing point, and then have that 1/3 black tank coupled with 1/3 fresh water tank as my standard tow configuration (with gray tanks empty). I'm tight on resultant tongue weight and tow vehicle payload.

@sengli , what's "UDC"? Also, so you have a direct line from outside your RV to the black tank spray? That's not a healthy design and if I understand your setup, it violates health ethics for the community. Sure, you might never intend to flush while the black tank level is above the nozzle -- but you might not know that actual nozzle height and you never know if somebody else, today or after you resell your RV, might do things differently. Sure, you might only use the flush at home or only at the dump station -- but the dump station water is probably common with the campsite utility, and so backflow into the dump station water does creep toward the campsites. It's just NOT a good idea. I'll tell a little story... I was in Delaware at a very nice camp. But the campsite dump pipe was in a 12" square concrete pad that also had the fresh water pipe stand in the same place. As I walked down the camp road, I looked at RV after RV. Half the RV owners had carefully gone through pains to keep their fresh water pipe from laying on the concrete pad or the semi-permanent dump pipe, in order to prevent any exterior contact between the fresh water pipe and the whole dump experience. The other half of the RV owners let their fresh water pipe simply lay on top of the dump pipe and the presumably dirty concreate pad. So half the people "got it" and the other half "didn't get it". Sure, it's small probabilities and percentages, but you don't sh*t where you drink. Even my dog knows that -- he specifically goes off the walking path into the weeds when it's time to dump. In reality, those other half of RV'ers touch their fresh water pipe and spread around bacteria, and drag that spread to all kinds of other places. It may never add up to much, but it's just not the way to do it. Realistically, maybe one in a 1000 of them have a full sick night once in their RV carrier. The other RV'ers never have that sick night. If I misunderstand your setup, then sorry for the complaining...

I'll dump this load from my mind and move on to more important things... (Hey, I never said I was a good comedian.)

Hope this help explains the flush system on the factory installed black tank flush setup.
 
I very appreciate the input, sincerely, @david-steph2018 , but that install manual doesn't [directly] address my complaint. The manual doesn't show the plumbing between toilet and the tank. I believe that were I the designer of this, I would have put the nozzle in whatever pipe connects the toilet to the tank, so as to spray right where the solids fall. Of course, the manual does indirectly speak to my question, because it shows a nozzle location and such location is obviously NOT where I would have liked to have it.

Well, I just need to get over it and forget about complaining and move on.

So, @UDC, I was thinking of doing something similar. It takes 2-3 minutes of "flush" to refill the tank to 1/3, which coincides to seeing a wedge of water directly under the toilet. In essence, I maintain the black tank at a level never below 1/3 except for a few minutes between dump and refill. That way, there will never be a "dry fall", but only a fall into water. This along with future things to learn should keep my black tank more happy and thus me and my wife more happy. (Yes, I read the manufacturer warranty and intend (LOL) to never forget. I use a timer reminder on my phone, just in case. But for 2-3 minutes, I might just stand or sit on the floor next to the toilet and wait, perhaps praying to the toilet gods a little in the interim, periodically peering down the abyss with a flashlight, perhaps more reliable that watching the level detector increase to 1/3.) (Yes, I realize you, @UDC, are talking about refilling in order to dump again immediately. I may do both your refill to dump again, as well as finalizing with a refill that I leave in there. That would be a good time, right at the end of whole dump process, to add a treatment to the black tank. I used Commando to clean. But any recommendations out there for daily/every-dump treatment?)

I do have both a clear straight between dump valve (fullway valve) and dump pipe, and then a clear elbow where it goes into sewer. So I can see all the brown water and solids I [don't] want to see!

Thanks to everybody for the help, regardless of where I was or was not already in my process.
 

NYSUPstater

Well-known member
How big is the black tank's capacity? One way to know how fast the flow rate into the UDC is to time time filling a 5 gallon jug or (2 gallon) of water so as to knowhow fast it maybe going into the black tank. tho it will be slower into the tank than the jug. Are you also using a pressure reducer of any kind?
 
I haven't really worried too much about the black tank size. I know many ways I could measure it. It takes about 2.5 minutes to fill to 1/3, using my home garden hose on the flush in-port. No pressure reducer. The flow rate is roughly the same with the vacuum breaker temporarily removed. (The only reason I would care knowing how large the black tank is, is in order to tow with it other than at 1/3. I don't plan to, so I don't need to know. I don't need to know in order to predict when it will get full. I'll learn that over time, and I can't easily change it. So I really don't need to know! Sure, I'm an engineer and would LIKE to know, but now I'm lazy and semi-retired, so I know better than to worry about it. INVEST IN REAL ESTATE!!!!)
 
I apologize for being a little bit rude in my last post, regarding black tank capacity. I don't know what got into me. In fact, the one place I can envision needing to at least have a rough estimate is in adding treatments to the tank that are volume adjusted.

BTW, I do still stick by the advice I gave... If you're not already doing it, research investing in real estate. It made ALL the difference in my ability to get to my current financially stable state.
 

david-steph2018

Well-known member
Another reason to know the size is while back flushing you can estimate how much water you put in there. There have been times when people are back flushing, they become distracted. Next thing they know, it is running out/off the top of the RV. Because with the drain closed, toilet closed, it has no place to go but up the vent pipe onto the roof.
I seen the results of a couple who had hooked up the water line to the black tank flush, turned on the water, then they went out to eat supper. They had the contents of the black tank all over the rig, on the ground. They left the campground and trailed the contents of the black tank all the way out the campground, about a 1/2 mile. The campground ended up power washing, scrubbing down the site, roadway all the way to the entrance.
 
I've settled on the following with regard to the black tank.

Yes, I closed the dump valve and ran the flush. I was not on city water pressure and I had the pump turned off so there was no flush water. I held the toilet foot flush valve down and looked down the hole with a flashlight. I let the flush fill the black tank until I could see water at the bottom below the toiled opening. The bottom is a flat slant. It doesn't look like an elbow. I could see the water rising as it went across the slant. As soon as the bottom that I could see was totally covered, I stopped the flush. With my home water pressure, this took 2.5 minutes. Note that I had a small valve "Y" between the home garden hose and travelling flush hose (different from fresh water hose), and so I had that on the ground immediately outside the trailer door, in case I had to run! Didn't have to run. If there's per-site sewer, I intend to use that same "Y" valve at the water source, for one fresh water hose and one flush hose. I won't open both at the same time. Writing this, I'm thinking I may only use the "Y" if I'm staying a long time and expect to dump and flush multiple times. Otherwise, if I only ever dump the morning before leaving, and there is per-site sewer, it's not much more work to remove the fresh water pipe and add/use/remove the flush pipe on my way out...

So, the level sensors work. The procedure above leaves me just a little bit higher than when the 1/3 led started to flicker. So I figure I have roughly 1/3 of the black tank full already. My intent is to tow at this level. Therefore, my intent is to ALWAYS be at 1/3 or higher in the black tank. Some reading this morning makes me think I'll add a small amount of Dawn Ultra dishwashing liquid, which is septic safe, and I may add a little fabric softener too (dunno what that's supposed to do or yet if it's septic safe). I'll drive with this in case either of us want to use the bathroom on the road, rather than going inside somewhere, which we do as little as possible post Mar2020. I did a deep clean on that black tank using Commando, which seemed to do a good job. I may use that once/quarter as the directions suggest. Otherwise, with the addition of a little Dawn and driving, this may keep doing some residual cleaning while driving.

We intend to stop for many one-nighters, with a few two-nighters, and then rarely week-long or more. The USA is HUGE, and domestic travel is our aim! I'll keep the black dump valve closed even when connected to per-site sewer (but gray valves open in this case). I may or may not dump at the end of each one-nighter. Maybe I will. Maybe it will depend on whether or not there's an easy per-site sewer or only a central dump site. I'll watch the led tank level. Whenever I get above 2/3, I'll consider dumping, because I don't want to find it full on the road while wanting an interim bathroom break. I'll flush as the last step of the dump, and then refill again and add more Dawn. So it seems that I'll only use the middle 1/3 capacity of the tank -- maybe I really do want to know how many gallons that is, but then again I could use less flush water, having started at 1/3...

Anyway, that's the strategy I'll try. Any suggestions, @david-steph2018 or others?
 

taskswap

Active Member
That's good practice anyway, keeping the blank dump closed unless you're actually dumping. Otherwise the liquids run out leaving the solids behind to dry out and become poop-bergs. :D We always batch-dump even on full hookups, and we always treat with a tank treatment as well. I find I get better dumps that way when I do go to dump.

If I know we're hopping from one campground to another (instead of heading home) I will commonly dump the grey but NOT the black. The rocking motion helps scrub solids off the tank sides so I get a more complete dump the next time. I've heard of folks doing this with ice cubes but that seems like an unnecessary expense to me especially for those of us who do have flush ports, no matter how well they're plumbed.
 

david-steph2018

Well-known member
I've settled on the following with regard to the black tank.

So, the level sensors work. The procedure above leaves me just a little bit higher than when the 1/3 led started to flicker. So I figure I have roughly 1/3 of the black tank full already. My intent is to tow at this level. Therefore, my intent is to ALWAYS be at 1/3 or higher in the black tank. Some reading this morning makes me think I'll add a small amount of Dawn Ultra dishwashing liquid, which is septic safe, and I may add a little fabric softener too (dunno what that's supposed to do or yet if it's septic safe). I'll drive with this in case either of us want to use the bathroom on the road, rather than going inside somewhere, which we do as little as possible post Mar2020. I did a deep clean on that black tank using Commando, which seemed to do a good job. I may use that once/quarter as the directions suggest. Otherwise, with the addition of a little Dawn and driving, this may keep doing some residual cleaning while driving.

We intend to stop for many one-nighters, with a few two-nighters, and then rarely week-long or more. The USA is HUGE, and domestic travel is our aim! I'll keep the black dump valve closed even when connected to per-site sewer (but gray valves open in this case). I may or may not dump at the end of each one-nighter. Maybe I will. Maybe it will depend on whether or not there's an easy per-site sewer or only a central dump site. I'll watch the led tank level. Whenever I get above 2/3, I'll consider dumping, because I don't want to find it full on the road while wanting an interim bathroom break. I'll flush as the last step of the dump, and then refill again and add more Dawn. So it seems that I'll only use the middle 1/3 capacity of the tank -- maybe I really do want to know how many gallons that is, but then again I could use less flush water, having started at 1/3...

Anyway, that's the strategy I'll try. Any suggestions, @david-steph2018 or others?
You will find eventually the sensors will malfunction, no matter how often you "clean" the tank. I listen for the toilet to "burp" the air bubble when flushed. When this happens, I know it is ready to dump. For stopping for one-nighters, I do not dump the black tank, not enough liquid in the tank. But after sitting at a location for a while, I dump before hitting the road. Since we are full timers, I do not add any water into the tank before hitting the road. This is to help lighten the total weight of the rig.
 
@david-steph2018 , please clarify this "burp" thing in detail. Perhaps because I'm an engineer, I need to fully understand what you're talking about before I can understand it at all, LOL.

I get it about lightening the towing weight. That's why I'm thinking only 1/3 fresh, which we're unlikely to use at all unless there's a snafu and we go HipCamp or other boondocking one night. Then in my mind, the 1/3 black makes sure that, while driving down the road and stopping for a bathroom break, [sorry for the frankness...] we're never leaving a relatively dry pile of excrement in the tank, only in a shallow water bath. Meanwhile, others say they flush the toilet itself with a bunch of water. If we flush the toilet with minimal water, we'll soon be ahead (that is, lower water volume in black tank). Total black tank volume is on the order of 40 gallons, I believe -- I'll test measure at a later time -- so 14 gallons is only about 116 pounds. For 8000 pound target trailer weight, that's only an extra 1.5%. I figure it's one of the most important 1.5%-ers being towed! It might be better to leave behind my toolboxes than this black water.
 

NYSUPstater

Well-known member
When we dump all the tanks and flush the black tank, we then add 10 gallons to black tank (40 gallon capacity) along w/ chemicals. The 10 gallons aids in sloshing around to perhaps loosening anything that didn't come out when they were dumped & flushed. As for water in fresh tank (60 gallon capacity), depends on how far we are going. If say NY to FL, we add 1/3 = 20 gallons for roadside stops or perhaps an overnight someplace w/o hook-ups.

The burping is when the black tank is nearing it's fullest capacity when you flush toilet. In most cases you can dry camp or even w/ full hook ups, be able to go a week w/o a problem or before things start to burp. Basically it's telling you "I'm getting full", and will need to dump real, real soon.
 

david-steph2018

Well-known member
I tow with more fresh water than anything else. Simple reason, earlier this week the water was shutoff here where we are staying, because they hit a water line with a backhoe. Water was off for about 6 hours, this time. We were at a campground in Ohio when there was an accident on the road outside the park's entrance. Took out a fire hydrant, about 12 hours later and 3 days of boil alert, we had water back.
We have a 110 gal. fresh tank and I carry about half full now because of the parks losing water at different locations.
Some say I wait until I get to the park, then fill the tank. But I pulled into a park and their water is off for repairs, so at least I have some water on board.
 
@NYSUPstater , your process (first paragraph) sounds essentially the same as I am proposing. Thanks.

@david-steph2018 , thanks for your unreliable camp water insight. DARN, I wish there was a fast path from the fresh water tank to the black tank. Maybe I should carry a hose that fits the [to be replaced] bathroom faucet. Then, we could accomplish nearly the same thing by towing with 2/3 fresh water, and transfer 1/3 to black water tank only on #2 demand, not #1.

I still don't understand the burp. If I experience it, I'll recognize it. But I'm seeking a mechanical understanding in advance. Perhaps... is the top of the tank often not level, or else above the top of the entrance elbow, if one exists? Then as more water goes in, it might displace trapped air? The point for me is that it seems like it MIGHT be that only SOME systems do this, and not all. That would make it a less reliable indicator.
 

LBR

Well-known member
[QUOTE="Dr.Lightning, post: 658459, member:] I still don't understand the burp. If I experience it, I'll recognize it. But I'm seeking a mechanical understanding in advance. Perhaps... is the top of the tank often not level, or else above the top of the entrance elbow, if one exists? Then as more water goes in, it might displace trapped air? The point for me is that it seems like it MIGHT be that only SOME systems do this, and not all. That would make it a less reliable indicator.[/QUOTE]

The burp is not so much mechanical analytics.... it's better to revert back to grade/middle school science.

When the lower black tank fills up most of the way, the fluids will cover the vent tube opening, essentially blocking it 100%. Then imagine a completely full toliet bowl of water and dumping it fully at once. The now trapped air between the black tank to bottom of toliet dump valve has to go somewhere....path of least resistance dictates upward to battle 1-2 gallons of incoming flushed water vs. downward against the lower 35 gallons.

The burp starts very innocent, then rares it's ugly head with each flush. You need to purposely experience this for your RV learning curve.
 
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