On Thu, 27 Aug 2009 08:14:35 -0700 (PDT), Andy Dingley
wrote:

There are two sorts of diverter: manual flap valves, and automatic
spill weirs. Don't get a manual, you'll never bother to switch it.

I was thinking of the weir type. I have seen manual diverters but I
hadn't thought of them for rain water, I thought they were more about
choosing whether to recycle or reject bath water.

Rain in a drainpipe mostly runs down the inner surface of the pipe
(capillary effects), rather than cascading through the middle. The
simplest diverter (which Wickes sell for less than the effort of
making it) is a smaller pipe placed inside a piece of downspout,
constructed to make a circular dam inside this surface. A side outlet
takes water away from this dam and into the butt. If the butt is full,
water in the pipe backs up and the weir of the dam overflows down the
middle to the drainpipe's normal outlet. Similarly if the rain is
torrential and too much for the narrow butt pipe.

That last sentence "if the rain is torrential and too much for the
narrow pipe" is what the advertisers used to sell their (still
unknown) diverter with the 32mm waste pipe. Does this mean there is an
advantage to having a larger pipe to the water butt?

Is the pipe to the butt connected below the weir or aligned with it? I
have a diverter for square pipe and would like to buy one that fits
round pipe (I know I could get adaptors but there's no need to make it
look more ugly than it already does!). On this particular diverter,
the tee is aligned with the weir so the pipe could never get more than
half full. This seems to defeat the object of using any diameter of
pipe if half of it never gets used to carry water.