Reddit mentions of Condar FlueGard Flue Gas Thermometer Probe

You want to minimize the amount of time that the fireplace is operating at too cool a temperature, because when hot flue gasses cool down on the way up the chimney, they form creosote. And that's what causes chimney fires. Get it burning hot quickly, keep it burning hot, and do not let it smolder.

How do you know if it's hot enough or too hot? Ideally, you want to measure the flue gasses directly.

https://www.amazon.com/Condar-FlueGard-Flue-Thermometer-Probe/dp/B001D1J7YA

Since you already have a relationship with a chimney sweep and he or she knows your setup, I'd call them back over and see what they say.

As for wood, you want "seasoned" (read: dry) wood. Not too dry, though. If it's been cut and split and stacked for a year or two, it'll likely have the correct moisture content. You do not want to burn anything but real firewood in there. No construction lumber (new or used).


If you're really curious, you can get a moisture meter. To get an accurate reading you'd bring in a piece of firewood in and let it warm up to room temp, split it, and then take a reading from the freshly split surface. I can recommend the Lignomat E/D or S/D. I've been using an S/D for a long time and it's been great. Support from the company has also been good. (replacement pins)

https://www.lignomatusa.com/product/pin-meters/

Hardwood vs. softwood. Hardwoods are generally more dense than soft, so you get more BTUs from hardwood than softwood when you're comparing equal volumes. Softwood tends to burn fast and hot. That's fine if you're there to tend the fire. If you want to leave it be for an hour or two or four, hard woods will work better. Do softwoods contribute to more creosote buildup? Not if you're operating the stove correctly. Properly dried softwood will burn just fine and with no more creosote than hardwood. But it you're burning wet cedar (and it'll burn when it's wet because it has high resin content) then you'll end up with a cool fire, low flue temperatures, and a lot of creosote buildup. You're less likely to burn wet hardwood because it just doesn't burn well. So again, it's the operator error and not the wood that causes problems when burning softwoods. If I were buying firewood I'd get exclusively hardwood. If I were scavenging or harvesting, I'd burn whatever I could get, with a strong preference toward hardwood. (But cottonwood and box elder I just leave for someone else.)