Replacing cast iron waste pipe

I live in a bungalow which has a two-pipe cast iron waste system from the bathroom. I want to replace the sink/bath waste pipe with plastic.
Currently, this is a large affair with a vent above the gutter line, like the w.c. waste. Do I have to replace the large pipe with the vent, or can I just run the waste pipes from the bath and sink separately straight into the drain?
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Steve Loft

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Steve Loft wrote:

Do you mean you have two vents from approximately the same part of the drain? If so you should only need the one.
However it seems unlikley that this has been done for no reason. But I am at a loss as to why its like it is, or who would be competent to give an opinion.
Notmally the bath and basin are tee'ed into the same waste pipe as the bog, or perhaps they are in different parts of the house? In which case a separate vent or an air admittance valve may be required.
Do I have to replace the large pipe with the vent,

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The Natural Philosopher wrote:

It's two completely separate waste pipes, one for the wc and one for the basin and bath. Both waste pipes are long things going up above the roof line, each vented at the top. I understand this was a common way of doing things pre-60's.
Basically, I need to know if the basin and bath actually need to be vented like this, or whether they can run straight into the drain, bearing in mind they're on the ground floor.
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Steve Loft

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Steve Loft wrote:

They should be able to share a common downpipe. Its NOT a common way of doing things tho. - twin stacks - unless they are physically very separate.
The stack pipe is there to relase noxious gases from the drain, and prevent the rush of a flushed loo from sucking the water out of the bath and basin traps. You must have at least one such stack vent somewhere.
Howver, it may be best in your case to replace all the iron work above ground with a modern plastic stack, and get the bath and loo installation checked over by a competent architect, who will be familiar with modern regulations.
Whn in doubt, rip it out. Tradesmen are far better at installing new bits than tampering with older ones. My guess is the job will work out cheaper as well. Work out waht you want as far as layout goes, get an architect to check over it for falls etc etc and any potential structural problems, get a quote from decent builder, get rid of the iron and replac with a modern plastic system.
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Hi
I wonder if the 2 pipes go different places, its quite possible. I'm told that can be done to allow use of a smaller septic tank. If so they'll need to be kept on the 2 separate flows.
Usually, on Vic houses, the grey water pipe venting is done at ground level, courtesy of an open drain and open pipe ends.
Your question: I dont know. Certainly if you used an open drain you wouldnt need a vent pipe.
Regards, NT
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N. Thornton wrote:

Ah - good point. The house used to have a septic tank, but now it's on the mains, so it could well have been designed like that.

Well, it's an open drain, so it looks like I'll be fine doing away with the vent pipe. Thanks.
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Steve Loft

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Steve - under the Building Regs (and local Byelaws before that) there is a maximum permitted length for an unventilated waste pipe - the intention is to prevent the waste water flowing down the pipe from sucking or siphoning out the water in the trap. The max length varies according to diameter and fall, but for a 32mm dia basin waste the max length is 1.7m and for a 40mm bath waste it is 3m. Any pipes longer than this need to be ventilated, like your system. The fact that they are proves to me that your system was a quality installation at the time. It is a fairly common arrangement on large older buildings.
These days there are various other ways of overcoming the problem without needing a vent pipe, but I presume your wastes discharge into a drain gully outside which limits your choice. The two simplest methods for you are to either: a) increase the size of the waste pipes after the traps, or b) use anti-siphon or "anti vac" traps
If IMM were here he would be saying to use a special system he knows about :o)
Regards Peter
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Ahhh. It is also possible, and much quicker, to convert your non-anti-vac trap into an antivac, with the aid of a lil bit of paltic tube (sawn off felt tip) and 2 wire ties. Look in any antivac trap to see where to stick the tube. Far quicker than replacing it.
Regards, NT
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