Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Looks to me like perhaps your heater surface is a bit too narrow and your flame a bit too high. The outer thermic syphons are receiving no flame at all, while the ones on either side of the central thermic syphon appear to be "funneling" the oversized flame onto the central syphon. I believe best practice with the ceramic heating elements is to have very little flame above the ceramic surface, while the actual heating is done by the radiant heat, "orange glow" of the ceramic material itself.