[Greenbuilding] flue gas thermometers, was firewood moisture content

Reuben Deumling 9watts at gmail.com
Sun Dec 18 11:06:33 PST 2011


I've corresponded with John Gulland and discovered his antipathy toward
flue gas thermometers. To each his own. I love mine; wouldn't want to be
without it for anything.

Then I came across this passage on his site:
"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.*"

I found this surprising, because the way I heat (which, admittedly, is
toward the active end of the spectrum) I am able to keep the flue gas
temperature in a fairly tight band around 600F once it has come up to
temperature, so for an hour and a half to two hours. Firewood split small
helps this immensely. Adding a piece at just the right time, pulling the
pieces inside the firebox the front ends of which have burned,  and
adjusting the vent allows me to keep that temperature steady.

I'd even go one step further and argue that it is the presence of the flue
gas thermometer which allowed me to achieve the 'almost impossible.' It
seems pretty clear that John Gulland doesn't use such a thermometer, and he
obviously has discovered ways to heat that yield the results he is
satisfied with, and for all I know they are as good or better than mine.
But my point is that having the thermometer on hand, reading it a lot,
teaches you what combination of firewood size, loading frequency, and vent
adjustment will yield this steady state burn condition.

To me the flue gas thermometer is akin to the tachometer in a car. Sure you
can and most of us probably have driven without one, but once you have one
and pay attention to is you can learn how to drive in a band close to the
desired rpm.
Gulland's statement above suggests to me that he drives by listening to the
engine. Fine. But don't discourage others from buying and installing a
tachometer by arguing that it is almost impossible to hold steady at those
rpms....

As for the relationship between flue gas temperature and the conditions in
the firebox... well, I'm all ears.
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