In an earlier contribution to this discussion,
mike ring wrote:


But one pump is fed not only from the boiler but from the other
circuit, unless there's something to prevent backward flow, as the
are common at the input of both pumps, and at the boiler return which
would be at a higher pressure.

The boiler input and output will be at virtually the same pressure. The
output of a pump will be at a higher pressure than the input - and this
pressure will be dissipated by the resistance of the particular circuit. If
only one pump is running, all points on the other circuit are going to be at
the same pressure (ignoring static pressure differencers due to height
differences) - so there's nothing to induce any flow.


The gravity bit is the other thing; in the absence of any forced flow,
would the hot water cylinder cause a flow through the rads and cool
itself down?

What I was referring to here was gravity (sometimes called thermo-syphon or
convection) flow. Some (mainly older) systems were designed to have the HW
circuit operate by gravity flow without needing a pump - and used a pump
only for the CH side. In this dual pump setup, if the HW circuit took it
upon itself to start circulating by gravity when the boiler was on for the
benefit of the CH but when the HW pump wasn't running because the HW demand
was satisfied, there's nothing inherent in the design of this system to stop
it from doing so. A system using zone valves would physically stop the flow
by closing the valve under these circumstances. There's no danger of heat
being transferred from the hot water to the radiators. There *may* be a
danger of the hot water getting too hot when the CH is on, or of the
radiators getting warm in the summer when the HW is being heated.
--
Cheers,
Set Square
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