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!
That’s a good one! An alternative that does the same but gives you a bit more time to correct the issue before you run out of fuel is to use a chest to buffer the belt. Set up the speaker to alert when the chest is no longer full (indicating belt is no longer saturated).