Hmmm, you sound like a newcomer to the home water collection arena. But I
assume that you know that if you have a municipal source of potable water,
you need to disable the cistern supply to eliminate an inadvertent
connection.
Might want to start with this site for an overview and partial answer to
your question.
http://www.flotrue.com/page/page/516203.htm
Even though they don't mention it, excluding the initial water tends to cut
down on nitrogenous waste material (read 'bird****') that would otherwise
get into the collection device. And having an automatic diverter allows you
to collect while you're not at home.
In my experience, you also want to have a float sensor in the cistern to
stop collection at a predetermined level, or simply a discharge pipe with a
trap.

Zeb Kagloonpop wrote in message
om...
I've got an unusual plumbing question:

I've got a house that is setup of to have an optional a rainwater
collection system. The runoff from a metal roof is collected from the
gutters and run thru 3" cast iron drain pipe to the basement. From
there I need a valve to divert the water to either 1) a cistern (or
preprocessing) in the basement or 2) a 3" cast iron pipe running to an
outdoor drain. All gravity fed. What would be the best type of manual
valve to use? I looked in the Grainger catalog but couldn't seem to
find anything.

(The cistern and/or preprocessing doesn't exist yet. I want to put in
the valve now to make it easier to install a complete system later.)

--zeb