I've a wood burnring stove, with a backboiler- simple gravity feed to
the tank. When its been on a while it 'bangs and hissses' and then I
hear water rushing around. Assume this is trapped air ?? If so can I ;
a) Undo the top hot pipe and let the air escape??
b) Fit something near the top hot outlet to let the air out maunally
c) buy something to add to the feeder tank to stop it- do these work ?


Any help/advice appreciated.

beginners don''t play with your boiler when it's hot. you're playing
with steam or hot water under pressure. don't add water to a hot
boiler, you can crack it.
is there an air expansion tank? or an overflow? is this hot water
radiator system or steam? is this a one room house or a multiple
radiator system?
ifthe boiler heats the house it needs water in all the radiators and in
a closed loop hot water system you will find bleeder valves at the top
of each radiator to let the air out when the system is under pressure.
in my 3 story hot water radiator system before heating season begins
you run the circulating pump adding fresh water at the basement water
pressure limiting valve. the fresh water has dissolved air that rises
to the 3rd floor so you let it out there at the radiator tops as you
run the system if it hasn't caught it in the system's basement ceiling
expansion tank that we keep half air half water. your system is going
to be different. maybe like a 1922 era system i formerly ran it's just
a manual water valve to add water in a sight level. maybe the boiler is
all on one level with the radiators and there's an expansion tank
higher in the attic with an overflow pipe to the sewer.