I'm going to be completely vague and say that depends. It depends on many variables and specifics to your setup. Wood type and condition Stove type and condition Flue setup Etc Etc Etc The best vague answer is probably, hot enough to exhaust the gas without excessive buildup and as cool as possible while maintaining an efficient burn.
Lots of different variables for different stoves and setups but my external flue temps cruise around 300-400. If I drop below 300 my cat stalls and I see smoke, if I go above 400 I'm wasting fuel and heat up the chimney. Most flue temp thermometers have a optimum range printed on them, my Condar says about 250-450 is a good operating range.
Just out of curiosity I just looked at ours. Stove top 430; flue 290. We have no problems with dirty chimney either.
My favorite subject (you guys from hearth know what I am talking about). When you talk flue temps it's best to say surface or probe as it can get kind of confusing.
Is that surface temp Dennis, if so do you run yours a little hotter then some with cat stoves? 300 (surface temp) seems to be where my Drolet likes to settle at.
The lower the exhaust gas temp., the less heat is escaping the building through the flue. Sort of.... it also depends on how much air is passing through the stove. My flue exhaust temps. are usually between 280F and 380F, toward the lower end of that range most often. I measure the actual flue gas temperature, the stove pipe is about 1/2 that temp. usually and most people measure the stove pipe temp. The lowest possible exhaust temperature would be desirable although if the flue gas is below the boiling temp. of water, 212F, the vapor in the flue gasses will condense inside the chimney which can be 'a bad thing'. This company does a really slick thing: they adjust their boilers so the exhaust temp. at the top of the chimney is at 300F guaranteeing high efficiency without any danger of the liquid in the exhaust gas condensing in the chimney: http://www.herlt-holzheizung.de/engl/holzE.html Brian
Yes, I was wondering what the necessary flue temps needed to be to prevent condensation. I can't believe I never thought of this question ages ago.
"The best way to control creosote is to prevent its buildup by maintaining a briskly burning fire with dry, well-seasoned wood. Maintain a flue temperature exceeding 250 degrees Farenheit to prevent creosote condensation. Some new, more efficient stoves deliver more heat to the room than an open stove or fireplace. This reduces the amount of heat escaping up the chimney and lowers the flue temperature. Make sure creosote is not building up as a result of improper operation of these stoves. Additional inspections and clean-outs may be needed."
Every time I see one of these threads come up I cringe....... Unless you post whether the flue temps are being measured by surface thermometer or an internal flue probe this topic can be very misleading for someone who doesn't know how to interpret the differences and can cause a newbie to get frustrated trying to obtain a temp that is not possible with their setup.
Fresh reload on overnight coals surface temps measured with IR gun after getting the load settled in Stove top 650 Single wall pipe 18" from the top of the stove 330 Pipe where it goes into the chimney 4.5' run including a 90, 240 Nothing but a little white smoke at the top of the 33' cement block with clay liner interior chimney
just checked mine. stove top 425*. 6" from back exit in horizontal flue 850*. flue on a internal probe.
+1 +1 There is a big difference between a surface thermometer or an internal flue probe, if that information is left out when stating what temp you run pipe at , It's just a guessing game.
That is surface temperature and will naturally read a bit high because it is on the horizontal rather than vertical. It does get higher than that especially when we get a new load started. We usually will engage the cat somewhere between 400-500.
On my cat stove with a probe meter the flue is very steady at 400 when cruising. My old non-cat heritage would run about twice as hot on the same hearth doing the same job. The cat stove is passing only a very small flow of air through the almost closed intake flapper but that non-cat, as almost all non-cats, was running full throttle all the time at double the temperature. That's why noncats are so inefficient, they are dumping vast quantities of very hot exhaust in order to burn the emissions.
On my condar chimney probe the target temp (internal temp) range begins at 400°F. I always try to stay in the low end of that range and have had good results in burn times & lack of creosote. I think it's very helpful to have surface & flue temps.
Yep the Cats are more efficient but the Non-cats are pretty close in efficiency just not as long a burn time and you cant turn the heat down as low. The reason the non-cats are pretty good as you build the heat up in the firebox the input air can then be turn down so low that there is very little air flow thru the fire box and the residual time of the heat in the fire box lets it get radiated out into the room.
