sndevereux.uk wrote

Can anybody help here-We run a wood burning stove with a back boiler
for hot water.

Neat systems. I've used them at camping cabins well in the woods and off
all electrical grids.

After being on for half an hour we get a loud knocking
and have to turn it down. Is this air in the back boiler- and how does
it stay their- cold feed at bottom/hot at top.

Hot water making steam no doubt? Actually if the water is hot enough for
use at that 1/2 hour stage, this is when you are supposed to turn it down.
If not, you may blow a pipe fitting if it's like the one I used. There was
a bleeder valve on it that would 'whistle' like a teapot almost and we'd
lift a little plug to let pressure escape (much like bleeding off a pressure
cooker).

Can I cure this-
chemicals added to feeder tank? or some sort of air outlet value????

Air outlet valve would be the trick *if* it's like the one I used.

22mm pipe I think Any Advise appreciated.

You may find the following link useful. One fellow in Wales has one with a
back burner like you may have.

http://www.off-grid.net/forum/topic/101