I've came up to this idea by looking at the default skin of scaled furnace where red gems are not deeply embedded into the furnace "body" and instead located in the outer sockets (which in turn resemble ornate pedestals).
In addition to current 2 red gem set, 2 more sets would exist based on combination of red and blue gems. Gems can only be replaced by hovering over the furnace (as if you were gonna cook the gems) so the sockets can't be empty. Both new variations would retain small light radius.
1 red gem, 1 blue gem - no heat output, can still be used for cooking.
2 blue gems - negative heat output - same as fully fueled endo pit, can't be used for cooking anymore.
This would be a great change for boat bases where scaled furnace isn't a good option due to overheating in summer. It would also be a welcome change for regular bases as you won't need to have separate spots for furnace and endo pit which are large structures and require a lot (3.2 wall units) of empty space around them.