The Bellows is an add-on block to improve the performance of the firepit, the bloomery, and the forge. Right-clicking the bellows after it has been placed adjacent to one of these structures will increase the heat output for a short time. Clicking and holding the right mouse button, or repeatedly right-clicking, will increase the heat further.
However, two conditions must be met:
1. It must have sufficient fuel to run. For the bloomery, it must have at least as much coal as ore.
2. It can't be running directly in the rain. Placing a fencepost over the forge/bloomery will prevent rain from entering.
There are also certain maximum temperatures that cannot be exceeded. The forge can have the highest heat output, followed by the bloomery, and then the firepit.