Ed Sirett writes:

Keith Refson - real email address in signature wrote:

I have posted before about the wonderful Servowarm gas CH system I
inherited with the "new" house, but now as I am preparing to upgrade
the controls and pipework to a more modern configuration I have just
noticed a new twist.

There is apparently no vent pipe on the primary!


This is standard fair for Servowarm. You may also find that the header
tank is a manual top up device i.e. not with an auto top-up float valve.
You just have to keep aware of when the sytsem is getting low . 8-(

But is it actually dangerous?


The boiler may be able to take a sealed primary system if the
manufactuers say so and there is at least one manually (= non-automatic)
overheat cutout. They usually interrupt the thermocouple circuit on this
age of boiler.

There is another cutout which appeas as you describe. (It tripped once
and needed resetting manually before i could relight the pilot.)

However there are so many other difficiencies in the control and
efficiency of the system it might be cheaper to decide do a major
overhaul (likely involving a new boiler).

I'm actually in the middle of a medium-level overhaul including the
replacement of the manual diverter and conversion to a proper
electronically controlled S-plan system, adding some thermostatic
radiator valves and a pressure bypass valve. So if I am going to add a
vent, now would be a good time!

The trouble with this is that I have spotted the other defect in the
system which is that the expansion tank feed is on the wrong side of
the pump, and a likely culprit for the air ingress problems I have been
seeing.

If I were to move the expansion pipe and add the vent to the boiler
output these would be on opposite sides of the pump, and I'd be asking
for pump-over. I could put the vent and the expansion pipe close
together on the boiler output but would not then cure the air ingress.

The pump is integral to the boiler housing and well separated from the
area I am working on by the cylinder, so it would be a larger
operation than I am happy with to move the pump at this stage.

Funnily enough the boiler itself appears to be performing well and I
don't think it needs changing at this stage. I was rather hoping to
upgrade the rest of the system so a drop-in replacement would be
sensible when it becomes necessary.

Keith Refson

--
Keith Refson 01865 435302 (Home)
23 Napier Road 07773 074233 (Mobile)
Oxford OX4 3HZ K.Refson AT Ntlworld.com