Jimmy, just for grins, try running the circulator continously. You may not get enough heat transfer to your snap-disc and the water is really hot in the loop above the fire, but can't transfer to the snapdisc. Once the snap disc brings on the circulator, the water cools the pipe and disc shutting down the circulator. So you get an on/off, wait on/off
If you circulate the water continously it will be constantly picking up some heat from the 1200* fire, and even a little over a period of several hours will net a lot of hot water.
Let us know if it works.