In an earlier contribution to this discussion,
Christian McArdle wrote:

I'm sure that Christian is technically right - but I would feel
uneasy if I didn't have a clear path from the boiler to the open
vent - even though the boiler has a manual cutout. Belt and braces,
maybe?

You may have misunderstood. There still must be a clear path from the
boiler to the vent. The manual cutout removes the need for a clear
independent path from the cistern feed outlet to the boiler. This
path is then allowed to either be valved or shared with the vent path
(but not both, obviously as the vent path would then become valved).

The solution I suggested actually involved an additional pump, rather
than using the CH one. The CH circuit can stay just the same. The new
pump just pumps the hot water circuit. The new valve just controls
the hot water circuit.

You are correct in saying that you may now need an additional bypass,
although the effect of not having one may not be predictable and it
might not be required. With this type of system with two independent
circuits, there should be a bypass on each circuit. The CH circuit
probably has a bathroom radiator doing this task, whilst the hot
water circuit should have an automatic bypass valve that is simply
plumbed in parallel with the zone valve set to around 0.3 bar.

Christian.

Thanks for the clarification. In an earlier post, you appeared to be saying
that if the boiler had an over-heat cutout which had to be reset manually,
it was no longer necessary to have a clear path to the vent.

Using 2 pumps sounds a bit complicated to me. In a conventional S-Plan (with
a single pump) each thermostat switches the appropriate zone valve - and the
volt-free contacts on the zone valves switch the boiler and pump on whenever
one or more circuits are calling for heat. In cases where the boiler has a
pump over-run stat, the pump is controlled by the boiler rather than
directly by the zone valves.

If you have two pumps, I assume that each stat would have to switch one pump
and one zone valve - with the volt-free contacts switching just the boiler.
This would mean:
A) the pumps would start before the zone valves were open
B) the pumps could not be controlled by the boiler, so there could be no
pump over-run

My feeling is that the OP has sufficient scope to be able to convert either
to a C-Plan or an S-Plan, without the added complication of two pumps.

--
Cheers,
Set Square
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