I’m trying to figure out why an airtight box (almost a vacuum) would slow the flow of water. See this diagram here:
Where I live, we have an inactive greywater system. Part of the system involves a pre-filter unit under the kitchen sink that’s normally used to catch solid debris. However, because the system is turned off, the pre-filter box is empty. Right now, it’s just a hollow box with a pipe at the top and bottom and a door in front. There’s a silicone seal around the edge such that when the box is closed, it’s pretty much airtight (except for the pipes, of course).
So here’s the phenomenon: Whenever the pre-filter door is closed, the kitchen sink almost always clogs. When it’s even slightly ajar, the sink drains with incredible force, almost like the water is being sucked through, and almost never clogs.
But the water only ever flows top-to-bottom, not out the door. All that happens when the door is open is that air can come into the box. So why does this happen?
The OPENINGS at the top and bottom of the box look to be almost the exact same size.
The top PIPE seems to be just slightly wider in diameter than the bottom one, but again, the actual openings drilled into the box that water flows through both seem the same size.
If it matters, I just noticed that the bottom of the box is slightly sloped, like a drain, towards the hole in the center.
I’m not sure how you can have the door open without water going everywhere, but that’s irrelevant.
I suspect the short answer is, when the door is open, it’s acting as a vent and introducing air into the system which allows the water to go down faster.
Two questions:
1)When the door is closed does the water always slow down or only when you’re sending a lot of water down at once? That is if you fill the sink up with water and send it all down at the same time (like if you pull the plug out).
2a)Where is the trap? Is it between the sink and the box or between the box and the rest of the sewer system?
2b)If the trap is before the box, is the vent before or after the box?
Inside the box, the water basically doesn’t fill up enough to ever leak out the front. The bottom part is almost like a bathtub.
Why would a vent help water flow faster down another direction?
With the door closed, the water is always slower. The extent of the slowness depends on the amount of food caught in the trap in the sink; even a little bit will slow it down significantly such that it feels like a complete clog. But the second I vent the filter box even a little, all that water goes whooshing down despite the food still in the food trap.
What’s a “trap” in this context? It’s not the little net that catches food debris, is it? (That’s in the sink’s drain hole and removable by simply lifting)
2b) I don’t understand this question. The only deliberate “vent” is the box itself with its front door slightly ajar.
Drain pipes, when not in actual use, contain mostly air but when waste water enters these pipes, it compresses the air until it slows or blocks the progress of water moving through the pipe unless the air is allowed to escape or vent to the outside.
An unscientific example of this involves a drinking straw. If you vertically immerse the straw into a pail of water, the straw will fill with water very quickly as the air escapes from the top of the straw. Now repeat, but close off the bottom of the straw and it fills with water more slowly. Closing off only the top of the straw prevents the majority of water from entering the straw because the water compresses the air in the straw and it has nowhere to go.
I’m not sure your drain beneath this box system is vented to the out side but suspect it may not be since opening the door allows the water to drain. If it is vented, the vent may be clogged .
A “trap” is a small “U” shaped pipe located at the beginning of a drain and traps a small amount of water presenting an airtight barrier that prevents sewer gas from entering a building.
Hmm, the air compression makes sense in a closed environment like a water pail, but in the case of a sewer line, why wouldn’t the water just keep pushing the compressed air through the pipes and into the sewer system? Why does the air/water mixture get stuck in the box?
…could it be that as the fluid leaves the box it takes more air with it than can be swiftly replaced, creating a lower pressure in the box, and acting as a sort of ‘air brake’?
Opening the door facilitates the entrance of air into the box, increasing the flow. The longer pipe exiting the box would produce a greater force out of the box than the shorter pipe entering it from above can successfully provide (due to gravity acting on a longer column of water in the longer pipe). As air is compressible/ expandable and water isn’t, then the air itself should act as a brake. Once the pressure in the box reaches a low enough point, it will then be drawing as much air down into the box as is exiting it via the base, the system will equilibrate, and a slower but constant flow rate should be observed.
At least that makes sense in my head, but i’m not a plumber, or offering a citation, it just seems right. Is it a correct explanation…?
Idea’s right, direction wrong.
“primary vents” let air into drain pipes to eliminate that “straw” analogy , the in-rushing air lets the sudden water flow freely along the pipe wall, (theoretically not more than 1/3 full)
without that air input, a sort of “stopper” is formed by the water itself, pulling any air or depression it can get, slowing down (stopping) flow in the pipe, exactly as if you had a ball stuck there. When it does flow through, it usually sucks the water out of your siphon, letting sewerage odors up.
Your “box” probably aggravates the whole process, accumulating a certain quantity of water, then unleashing it by siphon effect, all at once, thus overcharging the pipes, etc, opening the trap stops the siphon and lets the water flow steadily.
You should probably get rid of the whole rig, otherwise a secondary vent can be installed (to simulate the open trap)
good luck
with a greywater system you would need no sewer gas water trap (P or S trap is common) because the water drains into a filter or dispersion bed. you have a bed in the box for a particulate filter before on to the rest of the system. because you are draining into an open system you need no plumbing vent.
with a sewer or septic system you need a sewer gas trap. you also need a plumbing vent system to allow water flow in such a draining system.
the system has been altered so that you need a sewer gas trap and a draining system vent and you don’t have either.
Thank you all for the answers! I never realized air could play a role in a system like this, but it makes sense now. If we end up keeping the system, we may just have to add a vent to it.
Hopefully there’s already piping for a vent from before the greywater system was installed; if I’m lucky, it’d just be a matter of connecting it right. If not, well…
Where does it drain now? Does it just connect to your house plumbing? If it does you might be able to tell if it’s vented by looking at the pipes behind the wall. If the house is one story and you see a pipe going down (to the basement) and a pipe going up, that’s the vent. Of course, if there’s a window above the sink, the vent won’t be right there it’ll be over a few feet buried in the drywall.
BTW, what’s that black tube above the box. Next to the latch on the right side. Sorta looks like someone else already tried to add a little vent to it.
Otherwise, you could probably get away with just drilling a couple of holes in the top. But if you do end up using it as a filter, I’d imagine you’ll want to empty it daily or I’d think it would smell pretty bad.
locating vents and traps correctly are an important part of your waste (drainage) plumbing. in the wrong places it might not work correctly, you want no standing water except a small amount in sewer gas traps and you want free flow of liquids and air through it. it is important for health and preventing damage.
Right now it just drains to the house plumbing and then the city sewer. There’s a switch outside that toggles it between the inactive greywater marsh and the city pipes.
That black tube… I’ll be darned, none of us ever noticed that! I had to closely examine the picture to even see what you were talking about. I don’t know what the tube does. Apparently nothing? It’s not connected to anything else at the other end, and if was meant to be a vent, it certainly wasn’t working very well.
We’ll definitely look into this if we end up keeping the pre-filter box. It seems unlikely at this point in time, due to some unrelated factors.
It looks way to small to do anything useful. As long as splashing isn’t an issue, and it doesn’t appear to be if you can leave the door open, I’d just pop a couple of one inch holes in the top. Actually, If leaving the door open has been working, you could probably just do that. If it ain’t broke…
No, shotgunning a beer (or any other can) involves opening an air vent at the top of the reservoir; this box is below the reservoir; the only mechanism I can imagine is that the hole in the bottom is creating some turbulence leading to positive pressure, which is relieved by the open door. My original thought was a sort of Venturi effect, but I don’t think that works when the bottom hole is smaller.
