Well you guys made me second guess myself so I took a look and yup . . while I usually let the flue temp on my probe thermometer run up to 450 or 500 before I start closing it, I have and my wife have started closing things down at the 400 degree mark . . . and things are good . . . sustained secondaries, long burns, plenty of heat and most important of all -- a clean chimney. Probe thermometers are very useful in my opinion . . . but like most other inexpensive stove thermometers I take what they say with a grain of salt and realize the temps can be off a bit . . . one of the reasons I tend to run the probe temp up to 450 or 500 before I start to close things down. They are also very simple to install . . . pretty much you drill a hole and stick the probe in . . . so easy even a caveman . . . I mean a firefighter could do it.