It's a little unclear how to use it properly, so I may be using it wrong:
For example, a constant combinator with 'Variable 1 Item' and 'Variable 1 Furnace Result'
Setting 'Item' to Iron Ore should replace 'Furnace Result' with Iron Plate. However it doesn't substitute with anything

I am playing with Space Exploration which may change Furnace rules, but those recipes remain the same

Thanks! I'm traveling and hope to get to this in the coming week

Fixed in 0.1.3

It was failing when there were multiple smelting outputs. Additionally, it was finding the molten iron recipe, so I switched it to select the first recipe it finds where the ingredients contain only the entity variable and no other ingredients.

This may be a problem when you get into overhauls with crazy recipes.