Do You Need a Flue Pipe Thermometer?
We don't think so.
We don't recommend the use of flue pipe thermometers on modern wood stoves that have glass doors with airwash (so they stay fairly clear). Your best indication of how the fire is doing is by looking at it.
There are cases in which stove thermometers may be needed. A thermometer is needed to indicate what the fire is doing in old stoves that don't have glass doors. Manufacturers of catalytic stoves normally use the readings of a surface thermometer to indicate when it is time to engage the catalyst. But other manufacturers of non-catalytic stoves attempt to use flue pipe or surface thermometer readings as part of their operating instructions. From what we hear from users, this is a misguided approach to helping people to use their new stoves properly.
It turns out that there is no correct operating temperature for wood stoves because their output is modulated to provide enough heat for the conditions. So, in cold winter weather the temperature will be higher than in the fall when the heating load is lower.
Also, each new load of wood should be fired wide open until the firebox is full of flame and the wood is charred and the edges are glowing. That will produce a high flue gas temperature. Then you might set the air control back for an extended burn, so the temperature will fall. And, as the wood load is consumed the temperature gradually falls until it is time to reload.
For most of the time a wood stove is operating, its flue gas temperature is either rising or falling. Anyone who says that you should aim for a particular flue gas temperature or even a range in temperature is setting you up for failure because steady-state burning is almost impossible to achieve.
The correspondence we see from visitors to woodheat.org convinces us that thermometers cause more confusion than clarity. People try to make their stove operation conform to the markings on the dial of a cheap thermometer or to the recommendations of someone who really doesn't know much about wood heating.
How to avoid smoldering
The fuel must be flaming brightly until it is reduced to charcoal, regardless of measured temperature. Bright flames mean the wood is never allowed to smolder. During the moderately cool weather of spring and fall you can turn down the air until the flames look slow and lazy, but never so much that the flames disappear.
That looks after the low end of the temperature range.
How to avoid over-firing
At the high end, once a fire has reached full intensity after loading, with the firebox full of flames, avoid letting it rage with the air control fully open for more than a few minutes. Reduce the combustion air setting until the flames begin to slow down. In very cold weather, you can run the stove hot, at say 2/3rds of max without doing any damage. You can tell when you've reached about 2/3rds of maximum as you turn down because the fire stops roaring and the flames slow down somewhat. That looks after the high end.
And that's all you need to know. Trying to make your system conform to someone else's idea of a suitable temperature range only tends to confuse the issue.