Wetroom shower draining slowly

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Hi all, I fitted an en-suite wetroom shower a couple of years ago, and for the last 9 months or so it's not been draining too well. I don't remember there being an issue before.

Initially I thought there could be a blockage so used a solution and an unblocker tool, but that didn't work.

Then I suspected it could be due to pressure, so I removed the trap from the sink in the en-suite so as to allow air to enter the system freely, which worked very well.

So I installed an anti-syphon bottle trap (with air admittance valve) on the sink hoping it would fix the issue, but there doesn't seem to have enough negative pressure to open the valve. If I open the valve manually then the water in the shower drains away within 20 seconds or so. I can't keep the valve open though because then it leaks when using the sink.

Is this a common problem? Have I made a school-boy error on the install somewhere?

I've attached a couple of sketches of what's going on.

Thanks in advance.
 

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I's still suspect a partial blockage in the pipework, preventing the water from getting away freely. Removing the sink trap will allow air out, thus allowing the pipework to the basin, (sinks are in kitchens ;)), to fill with water, temporarily increasing discharge capacity. The trap will allow air in, not positive pressure out, which is why it drains more quickly when you manually hold the valve open.

I dont rate chemicals, if the pipe is only partially choked, they will have limited effect, running past any detritus stuck above the level of flow, and any mechanical means needs to clean the whole bore of the pipe, along its entire length.
 
OK thanks Hugh, will try cleaning with a brush pipe cleaner.

The trap will allow air in, not positive pressure out, which is why it drains more quickly when you manually hold the valve open.

So what other options do I have to allow more air in, in case cleaning still doesn't work?
 
You dont need to let air in, the problem is possibly due to the pipe is filling with water and it cannot get away quickly enough. By removing the trap/allowing air out of the leg going to the basin, it allows this leg to fill with water, thus giving the impression it is draining away quicker from the shower, when in fact it is probably running backwards and filling this section of pipework, before slowly draining away to the stack once the shower is finished. One test would be to remove the basin trap and leave the shower running, see if the problem eventually reoccurs.

If you cant get the pipe clean enough to avoid the problem, another solution would be to upgrade the pipework to 50mm at least from where the basin connects.
 
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By removing the trap/allowing air out of the leg going to the basin, it allows this leg to fill with water, thus giving the impression it is draining away quicker from the shower, when in fact it is probably running backwards and filling this section of pipework

Really? The shower waste pipe is below floor level, the basin is at waist height, and they only meet 2m+ down the run towards the soil stack. In any case, i'm pretty sure the waste pipe from the basin is empty, as it would have flowed away already. The only water in that run would be in the trap. Unless i've misunderstood what you're saying.

One test would be to remove the basin trap and leave the shower running, see if the problem eventually reoccurs.

That's what I tried first - see original post - removing the trap from the basin allows the water from the shower to drain away perfectly. Whether that's because of air getting in, or air getting out, i don't know, but that's what suggested to me there is a vacuum/pressure issue.

Thanks again.
 
For what it's worth, this morning I tried another test - while the shower was running (and filling up) I turned on the basin tap (just enough so that it wouldn't decrease the pressure of the shower too much) - the water collecting in the shower at best stayed the same, if not worsened, and the basin started to fill up. The basin does start to fill up sometimes anyway, but it seemed to fill up quicker today with the shower on.
 
Afraid that just reinforces the theory that the pipe is full of water, and back pressure is building. Its definitely a pressure issue in my mind, water will always take the path of least resistance, and whilst the leg to the basin wont fill with water above the level of the shower spillover point, it could still take some water from the shower in the horizontal section, and 'store' it until the shower has finished, before this 'stored' water can drain away naturally again.
 
I'm going to upgrade the pipework this weekend - do you think that upgrading the straight section out the back of the house (which leads to the stack) to 50mm will solve the problem?
 

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