Finally working PCB!

At the moment it is working as expected with no issues but with a reduction in functionality vs what was originally intended.  The temperature monitoring is not implemented or hooked up. Also the SSR used to trigger the furnace fan is not being used. Fundamentally, the problem is solved but I'd like to take it a step further.

Optos were replaced with a K844P quad opto. With the extra board space I reduced the complexity of the installation by having this device simply sit between the thermostat and the furnace. Adjacent to each connected screw terminal on the left side in the picture is a terminal connected via a direct trace. This prevents having to jam multiple wires into a single screw terminal. Just makes it clean and easy.

Based on the arduino serial print, the boost fan speed is being ready correctly.  I've got the board running on a sparkx arduino clone but will move to a FreeTronics EtherTen shortly once I have a place to monitor the data.