I've set it to 1.0 speed and 5 max, but tested it and found the furnace stops working if 5 chimneys are placed on it.

It's got to have 4 or less to work. Kind of weird.

did some testing, this is because setting the speed to 1.0 will cause the burn time of items to equate to 0, meaning they wont be cooked. i'll change the max speed value to 0.99.

heres some pseudocode on how the math works:
to calculate chimney speed: (chimneys on furnace / maximum chimneys) * speed
to calculate an items burntime: burntime - burntime * chimney speed

19a1601

Ah ok, hope it gets fixed in the next update then.