I use an L6006A aquastat for outfire detection. Mine has a SPDT switch which has 2 sets of contacts. 1 set opens on a temp drop and the other set closes n a temp drop. I use the contacts that close on a temp drop to supply AC power to my oil boiler. This works as an automatic switchover to oil as well as shutting down the stoker. Seems to work OK. You will need to wire bypass switches around the aquastat so you can bypass it when you are building a fire or want to supply AC to your boiler for testing/maintenance. I don't have a picture of my setup but I bought a single metal outlet box and a dual switch that was the size of an AC single recepticle. I used a nipple to mount the box on top of my aquastat and then ran my conduit out of the top of the box. Wire one of the 2 switches across one of the contacts on the aquastat and the other acros the other set of contacts. When the sitches are "off" or open the aquastat automatically controls the AC and when they are "on" or closed then it is in manual overide mode. I labeld my switches as Oil/Coal and Automatic/Manual for each switch. I hope I didn't confuse you too much.
I haven't tested the automatic fallback to oil yet as my coal fire hasn't gone out since I lit it 2 weeks ago. It did however work as planned to automatically turn the AC off to the oil boiler when the water temp in the coal boiler reached it's preset. I assume the reverse will work on an outfire.
Questions or comments are welcome.