An apparatus for heating a product includes a storage compartment for a product to be heated and a heater module physically and thermally coupled to the storage compartment. The heater module has a housing that defines a reaction chamber. A rigid barrier is inside the reaction chamber and defines first and second portions thereof. A first reactant is inside the reaction chamber, and a flexible bag (with a second reactant) is in the first portion of the first chemical reactant. The first and second reactants react exothermically upon contact. A piercing element can pierce the flexible bag. After piercing, the a fluid path and one or more fluid channels carry the second reactant to a section of the first portion of the reaction chamber away from where the flexible bag is located.
