"Danny" wrote in message
om...
Hi,

I'm having a problem with my central heating boiler, the safety cutoff
keeps 'popping' out when ever the room thermostat is *not* calling for
heat. Looking at my system I can see the flow feed from the boiler
passing through the pump (high speed setting), through two 'T'
piece's, and then the motorised valve (and onto the rads). The two
'T' pieces are for (I think) a) a loop circuit and b) to feed the hot
water storage tank. There are also two gate valves on these two
pipes.

Can anyone give me ideas on why my system keeps cutting out like this?

How open should the two valves be? Should the loop valve be more
closed than the others?

Thanks in advance

Danny

PS: I've just flushed the system completely, used cleanser for a
week, flushed twice, re-filled and treated with inhibiter.
Boiler does not cut out when the room thermostat is calling for heat

When everything *stops* calling for heat, and the boiler burner shuts down,
the water needs to continue circulating for a little while. If this does not
happen, the residual heat in the metal parts of the water get transferred to
the static water inside - causing the temperature to rise to a point where
the safety cutout operates.

In order to prevent this, two conditions must be satisfied:
1. The pump must be controlled by the boiler - so that it goes on pumping
until the boiler has cooled down sufficiently
2. The water must have somewhere to go! If you have individual motorised
valves for central heating and hot water, there *must* be a by-pass circuit
to allow the water to circulate when both of these are closed. [If you have
a single 3-port valve, this is not usually a problem - since it can never
close both of its outlets at the same time].

The first thing to check is that the pump is wired correctly, so as to be
controlled by boiler, as described above.

Then - if you have separate motorised valves for CH and HW - identify the
by-pass circuit, which should short-circuit the boiler flow and return pipes
without going through any motorised valves or through the CH or HW circuits.
The by-pass may well have a gate valve to stop *all* the flow from going
straight back to the boiler without going through the CH or HW circuits. If
this gate valve is fully closed, you ain't got a working by-pass!

Start by opening it fully, and check that it cures the problem. Then close
it progressively in order to find the position where it is open as little as
possible without causing the boiler to trip.

HTH,
Roger