On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:


What I would go for is a standard pump + an automatic bypass (which is
cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.

OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.

Does it matter? The combined system has a pump driving the head and
the flow. Even if the bypass valve has a very sharp response the pump
doesn't. The sharp or soft response of a bypass combines with the
pump curve. Or, if you like, a sharp response bypass will keep the
maximum head at say 3m (if is it set to 3m), whereas a soft response
one will keep the maximum head at say 2.5m to 3.5m depending on how
much it has to open. In either case the bypass will open enough to
let the flow stay constant-ish.

BTW a bypass is just a spring loaded valve. A knob on the top (or
similar) allows you to set the maximum head. When that is reached the
valve opens, just enough so that the head stays at that level. The
increase in flow then allows the pump head to decrease and the system
settles to a stable value.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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