What are you trying to do exactly? Are you planning to use the aquastat and relay to turn on your furnace blower and still use your gas for a backup? If so then I have a much simpler solution. All you need is a Honeywell 6006 (or equivalent SPDT) aquastat. Place ther aquastat somewhere in the line coming from the boiler before it goes through any of your heat exchangers (this will give you a more accurate temperature reading). First, connect the "W" line from your thermostat to the Common or "R" terminal. Next, connect the "W" line from the furnace to the "B" contact. Then, connect the "B" terminal to the Blower terminal on your furnace (Note: do not remove the existing connection from the thermostat to the furnace for the blower or you will not be able to manually turn on the blower). The idea is that when the water from the boiler is above the preset temperature (most people use 100F) the "W" or heat signal from the thermostat will get routed to the blower terminal on the furnace, and when the water is below 100F it will get routed so your furnace will use the gas burner for heat. This way you do not need separate thermostats. I attached a schematic to help clarify. Hope this helps