bad smell from the bath!

[Kat74]

Recently, when i let the water out of the bath there's a really bad smell comes from the plug hole. The bathroom sink's drainage links into the pipe coming out of the bath and then this pipe runs under the batheroom and bedroom foor to the outside wall. I've tried one of those wire unblocker thingies but can't get it through the bends in the bath pipes to get to any blockage. Short of getting under the bath and undoing the pipes (very restricted access) is there anything else i can do see if there is a blockage?
Someone suggested i try the wire unblocker down the toilet - does the toilet soil pipe connect to the bath/sink outlet? I hope not - sometimes if there's any bubble bath in the bath Iget a few bubbles coming back up through the sink when the bath empties!
I've tried household cleaning products - they seem to remove the smell for a few days, but don't seem to shift the blockage.
Can anyone suggest what else I can try?

 

[ijc]

Does the bath have a waste trap under the plughole? Either a U bend or a bottle trap? If not, that would allow bad smells up from the waste pipes.

If you do have a trap is possible that the water seal it is supposed to form is being drawn off, again allowing bad smells up. This can happen when the waste pipes run quite steeply or where they are shared with other appliances. The force of the escaping water simply sucks the seal from the trap...

You can buy anti-syphon waste traps which hold a little water in reserve in case the seal is drawn away. eg.

http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=54772&id=19709

Hope that helps

 

[Kat74]

It's a u-bend under the bath. Thanks for the link, I'll have another look tonight to see how this trap would fit on to the existing pipes.

 

[ijc]

No probs. I had a similar problem in my bathroom whereby there was no trap whatsoever. It's not pleasant!

If you follow the bath's waste pipe out the house, you might find it joins to the main soil stack (where the toilet waste leaves the house). So all the bad air can come up the soil stack, through your waste pipe and out the bath plughole.

A good trap should solve it - maybe put one on the basin as well. If the pipe was blocked you should have noticed by how long it takes to drain the bath, so hopefully you shouldn't anything else to worry about.

(apart from the bill!)

 

[kevplumb]

check the overflow

 

[dave2]

I put a non return valve in the waste pipe between the bath and whb, to prevent syphoning from the bathe when the whb empties.

 

[ChrisR]

You only need one AAV on the waste system. So it would be easier to put it on the basin than the bath, usually. Unless, that is, the pipe run from the bath waste to the juction with the basin waste is long, say more than 2m. High is better so it doesn't spend a lot of its life under water.

AAV = Air Admittance Valve. There are various designs. Some traps have a small valve on like

The appendage on top is the AAV. Some traps do a similar job by letting air in without so much water going that the trap doesn't re-seal when the sucking stops. Those Screwfix ones look like that. They do the job but make a glug glug noise as the air is let in, or out.
You can also get 32mm or 40mm AAV's which you can put in the branch of a T fitting, which are silent.
A neat solution is to use one of these on the basin

Called a Hep v O waste, they let air in, but won't let anything back up.

 

[Kat74]

Thanks chaps! lots of info here. I've had a look and there is a u-bend under the bath and the whb. The waste pipe from the whb runs under the length of the bath and then join the bath's waste pipe just under the bath u-bend.
I'm going to call the plumber in to do this - I normally small jobs a go myself but because all the pipes are in quite restricted places I'd be happier getting it done properly. Now I can sound intelligent though, which helps a lot.