Strange happenings with a tub drain

but i would guess to say pressure difference? I notice back home (third world country) when the gas station attendant would put oil on your car's engine, they would always poke two holes, the vent always being bigger.

(assuming that the holes are being poked into the lid af a cylindrical oil can) They've got it backward: oil has a higher viscosity, so should be flowing through the larger hole. Air has a far lower viscosity, and will flow through a small vent just fine. I used to make a small triangular hole with an opener, maybe 1/4" (6mm) on a side, and the drain hole was easily three times as wide (9 times the area), and the cans drained fine, without any gurgling or signs of restriction due to vent size. "Used to" - everyone went to screw-top plastic bottles for oil years ago here in the USA.

Stock tub, interesting. I would think that the outlet of the vent would have to be above the level of the water in the tub and connected to the same pipe. One could find a drain kit for a tub at any plumbing fixture store or one of those "big box" home improvement outlets.

What did you do? Did you run a 4" drain pipe from the tub to the sewerline? Does it neck down to a smaller diameter pipe along the way?

Another thought... 4" is rather large. Do you own any cats? Are any missing? Any fluid that drains, such as into a funnel that is tightly connected to the mouth of a bottle, must displace air. The air must go somewhere, so a vent is employed to allow air in the system to vent outside the building, typically. Does the tub guggle so as to suggest there is trapped air that does not escape?

Right now I am sticking to my cat theory, so maybe a better question is, does your tub meow?

I used a regular tub drain, no problem hooking it up. No difference in this and a regular tub other than than the fact that it is large. I have noticed that sometimes it drains extremely fast and other times it drains extremely slow. The oil can analogy doesn't hold for a drain vent since the top of the tub is open to atmospheric pressure. Essentially it is a container with a hole in it and a pipe attached to the hole. I really don't see where a vent would influence the flow rate. I was hoping someone could either verify my thinking or direct me towards something I'm missing.

I think I got carried away and didn't read carefully (sorry). I don't hav any cats, but I might ask around and see if the neighbors are missing (pun intended). The drain is a standard 1 1/4" tub drain, which empties into a 2" pipe which empties into a 3" pipe, which goes into the septic tank (the entire run is probably 10'). The septic tank is open at this point since I am still installing and testing. I also have a vent pipe installed on the tank side of all of my fixtures. So trapped air should all have a place to go. No guggling, rather, a very slow drain or a violent sucking (very fast drain) as to indicate a syphon. These two phenomena seem to alternate. I am truly baffled.

I am assuming that you have a trap in the tub drain line since you connected it like any other tub. Your tub drain needs to have an individual vent. It needs to be connected between the trap and the tub line connection to the branch line leading to the main. From it's connection at the tub drain line the vent needs to be connected to the building's sanitary vent system or out the roof which ever is more convenient. Also tub drains are usually 1-1/2 in. minimum. Double check the size, it's easy to mistake one for the other since they are so close in size.

It may be 1 1/2" I will double check. Can you explain the interaction of the vent with draining process?

What is the weather like there in Texas when the drain seems to run slowest? Fastest? Can you associate the sluggishness with rain? With when the soil is more saturated and might impede outflow in the septic tank drain field? We have a septic tank also, and a similar phenomonon arises during the winter months (the rainy season here instead of summer as in Texas). When the soil is saturated the drain can slow down. Also, a failing septic tank or drain field can cause the same effect. Check to see that the water level in the tank is just below the outlet pipe(s)--that the pipes are visible just above the water level in the tank. If the tanks outflow pipes are partially submerged, it is an indication that the drain field is compromised.If the septic tank can't drain readily, then neither can the house drains. It just is more noticable when trying to drain a large tub. Another cause of sluggish drain can occur if the tub sewer line tees into another waste line. Something from another line could be hanging at the juncture and impeding flow. Then the obstruction gets flushed through (in the other waste line) and your tub drain runs fast...until another obstruction is placed temporarily in its path. Then it slows again.

Just like the tub in your house the overflow drain provides a vacuum break because it is located above the waterline.You must install the same. Any plumbing supply will sell you an overflow kit. It consists of abs fittings and you cut the pipe for your application. Hope this helps.

Assuming that the drain eventually connects to a horizontal drain line with a 2% slope (1/4" per foot) the flow velocity will be on the order of 2-3 feet per second. This is the velocity needed to keep solids from settling out of the waste water. At that velocity, a 1-1/4" pipe will carry around 10-15 gallons per minute. That is about 1.5-2 cubic feet per minute. A 4 foot deep stock tank will probably have a volume around 40 to 50 cubic feet, so my guess is it would take about 1/2 hour to drain. (unless I forgot something from the fluid dynamics course I took a long time ago!)

Look up the life work of the Engineering Genius C.Y.O'Connor, who trained in Ireland but was the power behind many late 19th century infrastructure projects in Australia and New Zealand. He designed the Perth to Kalgoorlie water pipeline among many other things. This was his final project as the nay sayers so ridiculed him that he committed suicide during its commissioning. In 1895 no one, including Mr O'Connor understood that air had mass, viscousity and momentum. The final ridicule that destroyed the great man came (from the journalists) because the water was 2 days late arriving in Kalgoorlie (at least 1500km). Shortly after the waste of a great man, the water gushed forth. Go and be unconventional the world needs more people like C.Y.O'Connor. No I'm not related.

seriousely, yer father-in-law may be correct. All vertical risers should be vented for the same reason that an ejector pump works. You might also want to check the main house trap and ensure that the vent for that is unobstructed as well. Not enough space to explain in detail, however more often than not, if that main trap vent cloggs up you will have the results that you indicated. The fast vs. slow confirms that you are "stuffing" your pipes somewhere. Vent ALL vertical drops and vent all fixtures from the first vertical drop after the trap, and the height only matters so that you dont smell anything, remember, 1 1/4" to 2" to 3" will only flow as fast as the smallest restriction will allow. They should all vent to the top of your building, or roof of your house. Try this for an experiment when the water is running slow; submerge a plunger under the water so that all of the air escapes the cup and give the drain one good shot down, then remove the plunger BEFORE you pull up on it, now your water goes out the drain fast...................

You aren't by any chance living in a place that gets a lot of snow are you? With fast drain times on warm days, and slow drain times on cold days? Your vents may simply be plugged with snow and ice...

No, I'm down here in Texas. No snow or ice yet. I'm beginning to think the guy with the cat theory is on to something!

you may be venting too far from the trap if this is the case you will trap bubbles in the pipe that will restrict the water flow. does the drain ever bubble when the water level gets low and then start draining quickly? Mike Bush

No bubbles, but my other traps don't have water in them yet. Perhaps I should fill them and see if I get bubbles. It's looking like the situation can be remedied with and overflow drain.