"Aidan" wrote in message
oups.com...

Dave Baker wrote:

In the event of a control failure, it is a safety device; it releases
steam, prevents the system becoming over-pressurized and bursting. In
your system, such a failure would probably now cause the contents of
the heating system to be discharged into the loft and the boiler would
dry-fire to destruction.

Nonsense. The system isn't pressurised other than by the head of water
from
the loft tank. The contents of the system can't be discharged into the
loft
other than by the same pipe from the bottom of the loft tank which is
continuously filling it up anyway.

You and Drivel have misunderstood what I have written. It is not
nonsense, it is common sense.

I never mosunderstodd at all and I agree with you. I was looking into what
he had. If the boiler is a sealed compatible boiler (which it "appears" to
be) then there is no immediate chance of explosion, and as I said get it
sorted ASAP.

If the boiler thermostat fails in the ON position, the temperature of
the water will quite likely exceed 100 degC. The water in the boiler
will boil, producing steam. This happens. Your heating system will THEN
be pressurized by the steam and the steam pressure will become greater
than the static pressure provided by the head of water in the F&E tank.

If a sealed compatible boiler then the high limit cuts in. Another
protection level.

The water in the heating system will be pushed up the cold feed pipe
into the F&E tank. It will be pushed out rapidly, probably faster than
the F&E overflow pipe can remove it. There will be no water in the
boiler and it will continue firing.

This assumes that the cold feed pipe is clear and that there is not a
stopcock (acting as a non return valve) fitted to it. The cold feed on
my heating system was totally blocked with limescale when I bought the
house. This is a common fault. I had to cut out the tee and replace it.
There had been a longstanding leak from the heating system through the
coil in the indirect cylinder.

That is why on combined feed & expansion pipes it has to be 22mm minimum.
The one pipe has system water move up and down the pipe, so water with
inhibitor will be passing the tee reducing the liklihood of only fresh water
being there leaving behind scale. In very hard water areas it is best to
fit a 28mm tee and a small length of 28mm pipe. An aerator can be fitted
instead of a tee. Have compression joints for disconnection purposes and
the unit can have the odd clear out in case. The aerators have a greater
volume inside, so les lilely to scaleup, and are recommended by the likes of
Baxi and Potterton on combined feed and expansion pipes.

With a non-fubarred system, such a failure would cause steam to be
blown out of the open vent, but cold make-up water would enter the
system through the cold feed. This could continue indefinitely.

You can't get 2-way flow (steam up & water down) in one pipe, which is
probably what Faber & Kell were referring to. Their book (20 years
since I had a copy) deals with commercial installations on which such a
combined CF &OV would be inadvisable.

Yep. You can also get water circulation in one pipe. I have come across
that before.