I did a little mod on my VF3000 boiler like you are looking for.
I wanted to know if I had a out fire and I thought about monitoring the stack temp, but the stack temp is pretty low when the boiler is idling.
So I thought it might be better to install a snap switch on the outside of the boiler on the stoker assembly.
the stack switch is mounted on stoker wall where on the other side of the wall are the hot coals.
I used a L140-20
here is a link to the specshttp://www.drillspot.com/products/44690 ... Thermostat
I have the snap switch contacts connected to a flood monitor, so when it gets below 120* the contacts close and sound the flood monitor.
here is a link to the flood monitor.http://www.homedepot.com/webapp/wcs/sto ... 038838-_-x
this has worked well for about a year now.
I will post pictures below.