In article , Flat Eric wrote:
What I can't fathom is where this is flowing to as the other
end of the circuit should be blocked by the flowshare valve, so this
shouldn't happen. It just doesn't make sense.

Here's one I did earlier ...

Easy mistake to make - thinking of the return pipe as being like an electric
neutral that you can tap into at any convenient point. When the water is (in
theory) flowing round the HW only there will be a pressure gradient along the
return from the cylinder to the boiler. If heating returns connect to it at
two points (A & B) the pressures at these points will not be the same, so
water starts to flow from the return at A through a rad into the flow, then
into another rads flow through the rad rejoining the return at B.

reverse circulation
---------+-------------+---------- CH flow (off)- || --+
RAD RAD |
reverse^circulation V |
| | Pump |
+--------A-------------B------------------Boiler --X----MV
| |
CYL |
| |
+---------------------------------------O---HW flow ----+

The quick and dirty fix is to put a non-return valve at A, but this may
restrict the flow and at some later stage may stick. The better answer is to
ensure that all the heating returns are joined to one another before the
combined return is joined to that from the cylinder. The same effect can be
achieved by running a new return from the cylinder and teeing it into the
return near the boiler.


--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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