Adding the following furnace recipe doesn't work; removing recipes works fine.

# Remove grout -> seared brick
furnace.remove(<TConstruct:materials:2>);
# Add grout -> nether seared brick
furnace.addRecipe(<TConstruct:CraftedSoil:1>, <TConstruct:materials:37>);

the support for the vanilla furnace is from Minetweaker itself.
http://minetweaker3.powerofbytes.com/wiki/Tutorial:Furnace

Apart from that you always need to supply the error log.