In some of the photos it looks like three pipes run into the back of the aquastat, but they don't, only one does. Two of them are the cold and hot running into the DHW coil. The one that does connect the aquastat is like a 4" long 3/4 piece of pipe, not sure if there's water in it, or if it's just a mounting bracket. Two wires come off the aquastat, one runs down to the burner, one runs up to a small gray box overhead which has all the thermostat wires (from around the house) run into it as well as wires to the circulator pumps. So i'm guessing that is some kind of control box or something. Anyways, it looks like the temp readings for the aquastat must come from inside the boiler, somewhere in the vicinity of the DHW Coil - i'm assuming this since it's basically the only place the aquastat is in contact with the boiler.