A linear fireplace system, assemblies, modular units, and related methods that can be installed in a modular fashion at a selected installation location so as to avoid drawbacks experienced in the prior art. The system can include modular linear units, corner units, and/or end units interconnectable to form a modular linear fireplace assembly. The system can include an alignment track system with a track member that receives alignment rails on the bottom of the modular units to axially align the interconnected units. The system can include a combustion air flow passage within the fireplace that maintains a relatively low exterior temperature of the assembly and that allows combustible and non-combustible building materials to be installed against or immediately adjacent to the top and base portions of the modular units of the assembly.