"Steve Loft" wrote in message
...
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?
--
Steve Loft

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 )

Regards
Peter