A powered external heater is supposed to enable an oven to smelt items without needing fuel when placed next to it and optionally increase smelting speed a bit. However, when PiTweaks is installed with the fast furnace option enabled, the oven block keeps oscillating between active and inactive and no smelting progress occurs, effectively rendering the external heater useless.

(Tested with PiTweaks 1.1.1 and IE 0.12-98)

Not sure how the external heater works, but it probably messes with the furnace's tick time, which is what PiTweaks does.
I'm not working on 1.12 anymore. Do you know if this repros in the 1.16 version?

Workaround is to turn off fast furnace tweak in pitweaks.cfg. Then break and replace your furnace blocks, and the heater should start working again.