I have been in the house for about a year and the toilet has always had poor drainage. It doesn't overflow but there's hardly any suction at all when you flush it. The tub next to the toilet drains very slowly and will gradually fill up when using the faucet but just running the shower head it drains fast enough to keep up. Both are tied to the same drain line and I figure maybe there's a blockage somewhere. Btw, I replaced the toilet a few weeks ago and it made no difference. (reasons other than just drainage)
I had a plumber come out and they think that the venting was done improperly. The pipe on the left coming off the T is the vent and goes straight up through the roof. To the right, tying in just in front of the vent, goes to the bathtub.
I was quoted at $1200 plus materials to rearrange this. I'd prefer to keep that money in my pocket to fix other things and have no problem crawling in there myself to do the work. I'm just not sure exactly what their plan was to correct the placement of the pipes.
Here's what they wrote on the invoice:
Remove a section of the four inch main drain line into the crawl space area and tie in the drain and
the vent line separately so that it is no longer wet vented and. This conclusion was found due to having the customer flush
the toilet while I was on the roof to see what/ if any air was pushed through and it was minimal.
Best Answer
It's a common misconception, but blocked vents do not cause slow drains. If you Google it, you'll find lots of incorrect information that completely ignore the physical realities of how a vent operates. The purpose of a vent is to prevent the P-trap from being siphoned, it is not intended to help water go down the drain any faster - in fact, a properly vented fixture will actually drain somewhat slower than a fixture with no vent.
People often talk about a vent in the context of an inverted soda bottle analogy, likening the vent to putting a hole in the opposite side of the bottle so the liquid doesn't slowly "glug" out of the mouth of the bottle. That analogy is nonsense, because there is no such thing as a plumbing fixture with only one entrance/exit like a soda bottle - water going down your pipes does not need to "switch places" with the air ahead of it. Every plumbing fixture ever made has a separate inlet and outlet, and is already open to the air at the inlet side, allowing the air to follow the water down the pipes. Adding or removing a vent does not change that at all - it is not analogous to adding a second hole to a soda bottle, as it's actually adding a third opening to the already two-ended plumbing system. A blocked vent could potentially contribute to slow drainage if the inlet side is also blocked and cannot let air in, but in that case, it's really the blockage in the drainage pipe that's the issue, which is only exacerbated by the blocked vent. I can't imagine a situation where water can go down the drain, but air cannot.
As an experiment, you can pour water into a sloped pipe and watch it drain as fast as gravity will carry it. Now add a T and a vent somewhere along the pipe, pour the water in again, and see that the vent does not slow the water at all. In fact, plumbing without a vent will drain slightly faster, as water accelerating through the pipes "pulls" the water behind it. Although a vent slowing the drain may seem counterintuitive, it makes sense when you consider that the purpose of the vent in the first place to to prevent the pipes from emptying completely! You can watch a video of such an experiment here - in a realistic setup with a drain, trap, and vent, blocking the vent actually makes the basin drain faster.
Any air that needs to be pulled into the pipe can be pulled in from the inlet - a vent would only help in draining a toilet, for example, if the lid forms an airtight seal around the bowl. The vent is just there to prevent a suction effect that would siphon the P-trap. Your venting may be done improperly, and it may cause different problems with your plumbing, but it is not the primary culprit of a slow drain. If anything, a blocked vent would result in a drain that is faster and smellier (due to the siphoned P-trap). Look for a blockage in the drain pipe, not the vent.