A fire tube heater assembly, sometimes referred to as boilers and/or water heaters, and method of accommodating elongation of the fire tubes associated with such heating devices. The fire tube heater assembly includes a plurality of fire tubes that are configured and oriented to effectuate efficient thermal exchange between the heating fluid, commonly a gas combustion product, and the fluid being heated. At least one end of the plurality of tubes are supported by a tube support. The tube support includes a bellows or other deformable structure that accommodates changes in the longitudinal length associated with thermal expansion and contraction of the fire tubes during operation of the first tube heater assembly and in a manner that maintains segregation between the heating and heated fluid flows.