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!
In brief the system was installed by servowarm about 12 years ago consisting
of an Elite 50 boiler feeding a fully pumped system and an indirect HW
cylinder. The system appears to be a prehistoric version of a Y-plan
except that the diverter valve is a manual one and for the complete
absence of any thermostats (other than the boiler stat). The primary
circuit is fed from a conventionally set-up expansion tank/cistern
arrangement in the loft which is hooked in just before the diverter
valve splits the feed from the boiler.
I believe the usual recommendation is for a 22mm or larger vent pipe
to discharge over the expansion tank, but there is no sign of this at
all. This leaves just the 15mm feed pipe to the cistern which is far
There is a vent pipe on the *secondary*
circuit discharges over the
larger cold -water cistern, but this is of course not connected to the
The only possible safety feature I can see on the primary is a device
fitted to a Tee at the highest point in the primary circuit which I
can not identify. This is a vertical brass cylinder about 5cm high
and 4cm diameter with a screw-on brass lid with an offset plastic
knurled nut just under 1cm diameter with a screw head in the middle.
There's a single 15mm pipe entry at the bottom connected to the
Can anyone help me identify this device? I'm rather hoping it's some
sort of pressure relief valve, but if not it seems that the system is
potentially dangerous in its current state.
Since I'm redoing some of the pipework anyway, it would actually be a
relatively straightforward matter to install a vent if that's
necessary. I'd very much appreciate a comment from one of the
well-informed regulars on this system and whether adding a vent is
essential or not.
Keith Refson 01865 435302 (Home)
23 Napier Road 07773 074233 (Mobile)
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