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Old 01-27-2017, 02:04 PM   #1
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Lightbulb DIY Tankless Toilet Mod?

Theory: Mod a toilet for tankless operation (and maximum whoosh!)

I know toilets occupy the minds of mobile dwellers, be they in skoolies, RV’s, Tiny Homes, or otherwise to an unusual extent, so I figured I’d throw this brainstorm out there and see if anyone has any ideas about using an electric bilge pump to supercharge a toilet.

Yes, I know composting toilets are awesome… but, for various reasons, no. I have also done the “toilet seat on a bucket” thing… and no. I’ve seen some really nicely done versions of these on the forum, don’t get me wrong, but my family is not going to cooperate with something that manual.

We’ve used regular RV toilets, but my experience is that they’re junky, breakable, and don’t flush cleanly even when “working.” Skid marks need to stay on the pavement where they belong. Enough said, eh?

So, here is my thought process…

A typical toilet has a tank which holds about 6 liters (1.6 gallons) when full. The flush last about five seconds. That means the tank, which is gravity fed to the base/bowl, drops water at a rate of 4,320 liters per hour (1,140 gallons per hour).

Some commercial toilets are able to be tankless due to their ability to have 1” high-volume supply lines that can supply the entire volume needed to flush a toilet without a water accumulation tank.

There are 12v bilge pumps available for around $25-50 that are rated anywhere from 1-2k gph.

Idea: Could we attach a 1,200 gph rated bilge pump to the onboard water reservoir, plumb it to a toilet base via 1” line, and thus create one of those super whooshy industrial strength toilets?


Additional notes…

Potable water interface - Ok, bilge pumps are not intended for potable water use, so you couldn’t actually attach it directly to the reservoir used for drinking water. You could tap into the pressurized plumbing by creating a secondary reservoir. It could just be an unpressurized container that holds a couple gallons, even a two or three gallon bucket might work. It would be filled by plumbing in a traditional toilet float valve stem (already equipped with anti-backflow and good stuff like that). Then, you put a submersible bilge pump inside this tiny reservoir and plumb it out to the toilet, possibly via the same kind of flexible 1” drain line that is used for washing machines.

This small reservoir could be located in the immediate vicinity of the toilet, though easily concealed in a cupboard, wall, floor, etc. to save space.

Bilge pump activation & runtime – There would need to be a button or some other activation mechanism located near the toilet. Pressing it would turn on the bilge pump. A simple Normally Open button wired to the pump would work, but only if you wanted to count on users pressing it for an appropriate flush interval and could be sure some child or irresponsible adult was not going to just hold it down so long that the secondary reservoir ran dry, thus allowing the bilge pump to burn itself out trying to pump faster than the tank could resupply it. A more robust implementation would place the pump on a relay and control it via a simple mechanism which activated the relay for five seconds per press but could only be triggered when the reservoir float valve was in the “full” position. A small timing circuit could accomplish this function.

And… I just went to a large amount of effort to figure out how to build a tank. For a tankless toilet. Hmm. Still, some of those “tankless” toilets are actually just toilets with tanks hidden in the walls anyway. This might provide a way to adapt such a hidden-tank toilet to a setup where the tank has to be in the floor or a closet simply because the wall is not thick enough to hide a water tank in.

As for advantages, for one, those fast flushing toilets stay super clean. That seems to be a good way to score points with the spouse (at least with mine). Also, maybe being able to use one of those toilets that mount directly back against a wall could help with layout issues. Trying to install a toilet with a holding tank behind it could use up just a few inches too many, depending on how your bathroom is set up. There would definitely be none of the traditional odor that typically emerges from the straight-drop style RV toilets.

Would it be water friendly? Well, it obviously uses more water than a sawdust toilet or even a traditional straight-drop RV toilet. To compensate a little, you could always plumb the system so the mini-reservoir which supplies flush water sits slightly beneath the lowest level of the greywater tank. That way, the flush reservoir would passively fill with previously used greywater first, but if the grey tank is empty, it would still be able to fill via the fresh water system. That would at least get two uses from the water before it becomes blackwater.

Did anyone actually read this far? Any ideas or reflections? I may attempt something similar in an RV I'm refitting in the near future, so feel free to throw out any ideas. If I have completely missed something obvious, or if there is an RV toilet out there that flushes cleanly, suppresses odor, doesn't break constantly, and is of normal toilet height/size, then please let me know.

And yeah, I know marine setups often use macerator toilets to achieve low-odor, high flush rate solutions. My concern is how quickly someone would throw something dumb down it and break it.
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Old 01-29-2017, 01:57 PM   #2
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On further consideration, it would probably make a lot more sense to just build a custom 1.6 gallon tank which could fit in the thin wall of the bathroom. Thin in comparison to the normal 2x4 or 2x6 framing that in-wall toilet tanks are designed for use with, I mean.

It could probably be done simply enough with some FRP or ABS sheets put together as a waterproof liner and then sealed up into the wall. Though... you might want to make a note of which wall you concealed a very wide, thin water tank in. I could see someone deciding to hang a picture in the bedroom and suddenly finding that they have stuck a screw into the toilet tank. Surprise!
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Old 04-03-2017, 09:08 PM   #3
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Have you had a look at marine toilets? It's a little more complex, but an interesting idea. If nothing else your idea made me think of some toilets on sailboats and other marine vessels.

How Does a Marine Toilet Work? | Our Pastimes
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Old 04-04-2017, 06:06 AM   #4
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if you have a problem with a little streak in your crapper maybe a condo in fl is more your style.
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Old 04-04-2017, 06:51 PM   #5
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Originally Posted by superdave View Post
if you have a problem with a little streak in your crapper maybe a condo in fl is more your style.
I agree if skid marks are a problem then you might not want to go this route, but Lucasd has a point the RV toilets are "fragile"

Might be an idea worth looking at. Can't wait to see the DIY Lucusd.
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Old 04-05-2017, 11:09 AM   #6
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if you have a problem with a little streak in your crapper maybe a condo in fl is more your style.
Har har. It still gets old when you're the only person who ever actually bothers to clean those streaks. Anyway, the point was to discuss technical merits of a possible plumbing implementation, not make condescending remarks.

Seriously, a straight drop rv style toilet is the simplest option that looks like a normal-ish household toilet, but I've seen them wear out quickly. As soon as that blade seal in the base starts leaking and lets the bowl water empty on its own, there is no water seal to keep the sewage gasses in the black water tank from leaking up into the bathroom.

Yet, there are other issues with non-rv toilets, as one of the other toilet related posts on here pointed out. They aren't designed to keep the bowl water in against the motion of a vehicle. It's always a trade off somewhere.
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Old 04-05-2017, 11:27 AM   #7
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Read up on how they do it in airliners.
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Old 04-05-2017, 11:28 AM   #8
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Read up on how they do it in airliners.
Oh, good thought. I will do that. Those things have some kind of vacuum effect.
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Old 04-05-2017, 11:48 AM   #9
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I understand what your looking for in a crapper but what your suggesting is creating a bigger mess than what your trying to prevent . yes i was being a smart ess . n o . Power flushing. Or regular house toliets create alot of poop soup. You have to store and dump it. Wax rings are not made to keep things from coming up, also the vibrations would ruin the seal. Ever have the hose burst while dumping the black tank leaving a fest in live oak Florida with 30 pissed off hung over people wanting to get home. Not fun! Safety wise it wouldn't take much of a jolt to snap the mounting ears off a toliet and then have a porcelain god flying thru the air at 50 mph. If you must have a flusher go with a rv model with the spray wand, you can get them cheap from a. Scrapyard .
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Old 04-05-2017, 12:39 PM   #10
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Isn't that why it's good to get a toilet and sink inside a shower stall. You can just spray your streaks away with the shower wand. I always felt like I needed a shower after emptying the black tank.
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