The invention disclosed is methods, apparatus, and systems of a horizontal modular heater. Whereas the prior art, being heater of a single zone and singular structure, may not be able to provide the desired temperature profile nor efficient downtime during parts breakage is accordingly insufficient for the needs of the industry. The present invention then may be a furnace used in semiconductor manufacturing, of which may be a horizontal heater with multiple modules, of which may be connected in an ingenious method or structure to limit leakage and of which improves efficiency over previous heater designs heaters in the art.