On 10/01/2018 21:02, Roger Mills wrote:
On 09/01/2018 17:38, Chris B wrote:


Well, it certainly isn't a *conventional* S-Plan+ system. That, as you
have noted, would have a single pump and one zone valve for each zone
(3-in total). It would be wired so that the appropriate zone valve would
open whenever its zone was calling for heat, and so that the boiler
would fire and the pump would run whenever one or more zones were
calling for heat.

To achieve the same thing with a pump for each zone, but no valves,
requires some relay logic to run the boiler whenever one or more pumps
are running. Presumably that's what the control box - with its
assortment of diodes and relays - sets out to achieve, but I can't quite
get my head around how it does it. [One place where it differs from
S-Plan is that it appears to prevent the DHW from heating whenever
either or both of the heating zones are calling for heat. If you wanted
to convert to S-Plan+ but keep that feature, you'd have to make it
non-standard.]

The only application I've come across for null-flow valves is to prevent
gravity circulation in a pumped system. My first CH system was gravity
HW and pumped CH - and the upstairs radiators tended to get warm in the
summer when the boiler was just on for the HW and the pump wasn't
running. So I inserted what we affectionately called a "foo-foo valve"
in the upstairs circuit. This had a flap which could be opened by pump
pressure but not by gravity circulation pressure - thus preventing
gravity circulation.

As others have said, your problem is probably either with the pumps or
with silting up of the system. The pumps can be checked fairly easily.
It's worth checking that you've got a water path to each side of each
radiator by bleeding it with each valve closed in turn until you get a
flow of water through the bled hole.

A couple more thoughts .Â* .Â* .

It's presumably a vented system with a fill & expansion tank in the
attic? Could that have run dry (if its ball valve stuck shut) allowing
air to enter the heating circuits? Air locks would certainly prevent any
decent flow.

Yes its certainly this type - I have not checked the header tank but
will do, along with removing the little silver disk at the end of the
pumps to check for rotation as someone else suggested.



Are there any manual valves - e.g. for balancing the flow in the 3
circuits, or for maintenance purposes?
Oh yes - several

Gate valves in particular are
notorious for jamming shut - even when the spindle appears to turn.

Its unlikely they will have jammed shut as they will not have been
touched for several years. Silted up maybe - but I don't think they
will be jammed shut.


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Chris B (News)