On Sat, 22 Mar 2008 00:19:44 +0000, Roger Mills wrote:

In an earlier contribution to this discussion, jgharston
wrote:


I know they're functional schematics, but, probably because I'm an
electrician, I tend to expect functional schematics to have some
relationship to the equtipment they represent. A central heating
pluming circuit is essentially a parallel lighting circuit - live feed
out, tee off to emitters in parallel, return back to source.

Indeed - and it matters not one iota whether the returns join before
going back to source, or whether they go separately - this does not
affect the *function* of either an electrical or a water circuit.

Have to disagree, but I might have mis-understood you. It is improtant
that the return from the HW coil comes before or after /all/ the
radiators returns join together.

In an electrical circuit the voltage drop (equivalent to pressure
diffence) along conductors is typically two magnitudes less than across
the 'loads' (equivalent to radiators).
In plumbing this is not true, if the return from the coil divides the
return from upstairs and downstairs (say) then this happens:
A small proportion of the return water from the coil will travel against
the usual direction of flow into the returns from one lot of radiators
and then via the point where the radiators flows are joined into the
other radiators and then via their return pipework to the main return.
It's only a small pressure difference but it can and does drive enough
primary water into radiators to cause a problem in summer.

An indication that this is happening is that the warmest of the radiators
warm up from the return end, whereas when this happens due to the zone
valve letting by or thermosyphoning the radiators heat up from the flow
side.

--
Ed Sirett - Property maintainer and registered gas fitter.
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