yep, we bought two to use in-series, partly for redundancy ... worry
is, even a trickle - we didn't measure it, but looked to be something
like 1cc per minute - might be 0.1cc per minute, with two in-series -
adds-up over time ... ie: 1.0 or 0.1 x 60minutes x 24hours x (say)
3days = 4.3 or 0.43 litres = either-way, not much, even if multiply by
three in-case my estimate was conservative ! OK, should have done
the sum before ...

Chris


On Mon, Jul 6, 2009 at 6:31 AM, Tim Hawes<timsyahoo@...> wrote:
>
>
> Sounds about right!
> Even the industrial ones are never considered to be 100% leak tight
> and the conservative approach is to put in two of different types and
> then you can assume that the backflow is only 10% of the rated forward
> flow.
>
> That said, in many domestic situations you can probably tolerate a
> liitle bit of back-flow (trickle?) as the bulk of the flow should be
> stopped.
>
> HTH,
>
> Tim.
> (ps McAlpine was the name i was thinking of, have just binned one of
> their catalogues along with a load of other stuff)
>
> On Fri, Jul 3, 2009 at 11:00 PM, Chris Hunter
> (home)<chris.cjcmhunter@...> wrote:
>> we could do with one, too ... we bought a couple of (IIRC) McAlpine
>> (Sp?) ones, and found they let a trickle through when closed, via the
>> hinge arrangement  ...
>>
>> Chris
>
>