I admit that a back boiler isnt supposed to function without the fire connected,but the question was will it be safe to use,and the answer to that is yes as long as ventilation is still available and easy access can be gained for maintenance.One has to ask why you would want to remove the fire.If the reason is to make your living room into say a bedroom then that is a DEFINITE no no as Conventionally flued boilers are not allowed in sleeping quarters and it becomes an immediately dangerous situation so dont go there.
Tony S, February 2009