Abstract: Disclosed is a modular masonry heating system having a modular firebox assembly positioned within the flue chamber of an outer modular heat shell asembly. The heat shell comprises an enclosing wall and a cover each of which is formed from smaller modules and is self-supporting without these modules being secured together by a bonding material. The firebox comprises a foundation, a fire chamber enclosure and a riser each of which is formed from smaller modules and is self-supporting without these modules being secured together by a bonding material. The sets of modules for the heat shell wall and cover and the firebox riser and foundation may each have a different shape but the modules in the same set may have a substantially uniform size and shape. After assembly, the firebox and heat shell are each freestanding independently of the other.