Flue ducting for atmospheric boilers

A flue ducting comprising concentric inner and outer pipes defining an annular air space therebetween extending a length of the ducting, corresponding helical grooves being formed in opposed surfaces of the inner and outer pipes to extend the length of the pipes, and a helically coiled spring being located in said grooves to react between the inner and outer pipes, said helically coiled spring having individual coils which are of undulating shape defining alternating peaks and troughs therearound with the peaks therein engaging in the helical groove in the outer pipe and the troughs therein engaging in the helical groove in the inner pipe to determine and maintain the space between the inner and outer pipes, a flexibility of the ducting so formed enabling the length and a longitudinal configuration thereof to be varied, one end of the annular space between the pipes being closed.