Jumping in late, but interested, as I have been burning a Resolute (wood) for 30 years with this type of thermostat and recently added a Vigilant Multi-fuel (coal for me, finally!) with this thermostat. This to me sounds like a bimetal spring that is jammed or not responding. To my knowledge they do not go bad, but would like to hear from others. In my case, also a fireplace installation, the thermostat operates as others have described: set it, be patient; observe result! When at temp, damper closed, and flap very close to closed, the little flap sometimes claps, I suspect due to draft variation at the chimney top (25' of 6" stainless inside a flue tile lined masonry chimney). I do not have a baro, or room for it in the fireplace, and by some accounts should not need one on a thermostatically controlled stove. It would, I suspect, stop the clapping by absorbing the puffs, but I'm ok without one. At 450* on the griddle I'm seeing about 190* on the tee just behind the stove (rear exit). Any thoughts about keeping more heat in the stove and/or stopping the clapping would be welcome. BTW, I am burning nut, as when I burn pea I have to flow more air through the inlet, which I believe is taking more heat up and out. Thanks in advance for any feedback...I'm just 3 months into burning coal, but LOVING IT!