I used to run out of coal for my furnaces from time to time in the early game of my playthroughs. But recently I realized how to alleviate that: the siren.

The siren can be connected to a belt with a single green or red cable, and this is all that is needed. Take a siren, and connect it to a belt that takes coal to the furnaces. Then set up the belt to continuously send the item counts, and the siren to go off whenever there is less than four coal on the belt. Set the siren to global, and you will always be notified early when coal runs low in the furnaces.

Cheers!