"ITM" wrote in message
...
On Sun, 25 Jul 2004 22:52:38 +0100, "Set Square"
wrote:

I've done a Google search on "Fuel Guardian" and "Fuel Economy Systems
South" to no avail! Have you looked inside the control box? My guess is
that
there are some relays inside, and that it attempts to provide a poor
man's
boiler interlock - to turn the boiler off when the demands are satisfied
(although it can't totally achieve that without valves). If a relay had
failed it would probably prevent the heating from working. Worth
investigating, anyway!

I've looked inside, and there are a couple of relays in there. The
trouble is that I can't find a supplier for this unit (I also tried
Google to no avail!)

If all else fails, you could throw away this box and install a
conventional
programmer - but this won't use the cylinder stat unless you also install
a
motorised valve. If you *do* include a valve, you can have a C-Plan
system
with full boiler interlock. [See
http://content.honeywell.com/uk/homes/systems.htm ]

I do have a conventional programmer - it's connected to the Fuel
Guardian box. That's why I can't work out the purpose of this
mysterious box!

Just a guess - but a major issue with a gravity system is that the hot water
heats to the temperature set on the boiler thermostat. It could be that (on
gravity HW only) your cylinder thermostat and the 'fuel guardian' system
will stop the boiler firing once the cylinder thermostat hits the correct
hot water temperature (although that still won't work if the CH is running).
At least it would stop the boiler cycling on and off repeatedly once the HW
was up to temperature (which wastes fuel).

If this is the case then a faulty cylinder thermostat could possibly cause
the symptoms you are experiencing.

You could try turning the thermostat up and/or moving it away from the side
of the cylinder - just in case it is partially working but shutting off at
too low a temperature.

HTH
Dave R