Furnance and comparator BUG

Put the summary of the bug you're having here

I expact the furnance send a redstone power when the furnance are burning logs.

The logs and wooden planks desapear(in same time, the furnance window close)

Steps to Reproduce:
1. Put a furnance and a comparatore.
2. Put logs and wooden planks inside.

I create a video to show it: http://www.youtube.com/watch?v=lU5iCM3Nn3M&feature=youtu.be