On Oct 5, 11:48*am, The Natural Philosopher
wrote:
disconnect back boiler and fill with sand.

Seal the piping too, because otherwise that can "plumb" CO to
undesired locations.

Hot water from a back boiler is such an insignificant factor its not
worth it unless you have a 24x7 fire like an Aga.

Back boiler piping sat in cold chimneys can burst during extremely
cold weather, one contributory reason for their demise when combined
with expensive wooden floors.