Low flue Temps

[Aranyic]

I'm having issues keeping my flue Temps as high as I think they should be. I'm assuming it's a temperature issue maybe but wanted to confirm.

It's a out 45f and clear out here tonight. I put a small kindling load in log cabin style and burned it. Chimney went up to about 350 with stove about the same or less as it was warming up. Left the primary air wide open until the small stuff burned down stove we about 400 and chimney at 375 or so.

Loaded up 4 splits about 3 inches square each and let it all catch and get burning. Good secondaries and turned down the primary air to totally closed. Great secondaries stove top about 575 to 600 and holding. Chimney started dropping though clear to 260ish 16 inches about the stove.

I open the primary some and the chimney temp starts to rise. Is it just. OT quite cold enough yet and it's not drafting strong enough to pull the heat up? Or will I need to watch closing the air that far.

All the Temps are ir thermometer. Also just a terminology question so I know is flue and chimney temp interchangeable or am I using one incorrectly?

 

[webby3650]

So you are using an IR thermometer on the single wall stovepipe? If so then the flue temperature would be higher than the surface of the pipe, 1/3 higher internally if I remember correctly.

 

[Corey]

Truthfully, I wouldn't even worry about it. This is fire in a steel box, not the space shuttle. Typically, a hot stove / cool flue is the sign of an efficient burn...wood is burning, secondaries are burning heat is in the stove - and presumably being taken out into the room and the flue is cooler as a result. You could open the primary air back up some, cool the stove - and the room, and flush more waste heat up the flue... but why do that? So I say leave it.

Also, if your flue is stainless steel - your IR gun will read low. Typical guns are calibrated for an emissivity of around .95 - which corresponds to rusty steel, quite a few paints, coatings, fairly close to black stove paint, etc. Silver stainless steel could be around .44 - or roughly half the emissivity of iron, so your gun would read about half the temperature. If the stainless is polished, emissivity could drop into the teens and your reading could be 5-6x lower than it should be. If you do have stainless, you could spray a blotch of black stove paint on it and aim the IR gun there for a truer reading.

 

[Highbeam]

Your temps sound fine. Internal flue temps are typically double the surface temps which is what you are reading with an IR gun. You need a proper surface as indicated above for accurate IR gun temps.

Max flue temp is 1000 so you bumping up near 500 is the top of the range anyhow. After the initial burn off, the internal flue temps are typically the same as the stove top temp which as you note would be 300 or less measured on the surface. You're fine.

Remember that you heat with the stove. The flue is not the heat source. You only want the internal flue temps to stay up above the condensation point of water so that you don't get creosote formation. I aim for 200-250 degree minimum surface temps on single wall to accomplish this.

 

[begreen]

I'm not disagreeing with the assessment that the OP could be ok given that the 260F temp was a surface reading. However that would depend on the flue system. If the pipe takes a 90 turn, has a 3 ft horiz run (lots of heat loss) and heads outdoors and up a 25ft chimney, then the exiting flue gases could be below 250F. This is where an IR thermometer or a probe reading would be helpful if the surface thermometer is not all that accurate. 260F with a 600F stove top sounds super efficient by the 1/3 multiplier and ok by the 100% multiplier.

FWIW, our probe thermometer (double-wall connector) reads about 100F below the stove top usually when the stove is at cruising temp with robust secondary combustion. So a 600F stovetop will see about 500F on the probe.

 

[Highbeam]

So a 600F stovetop will see about 500F on the probe.

Which corresponds to 250 on the surface. Same as the OP.

 

[begreen]

Could be, but without proper instrumentation it is conjecture at this point. I will have to dig back to some testing Pen and I did with probe vs surface readings on his stove and my probe thermometer. IIRC results showed closer to a 50% higher temp in the flue pipe than on the surface.

 

[Aranyic]

I'm a straight approx 15ft chimney (maybe a little more I have not run the soot eater up to measure yet). I know I've got about 5.5 ft of black stove pipe inside; 2 or a little more once it changes to class A in the attic to the roof and another 4 pieces of class A once it exits the roof to get it above the peak of the house. Sweep cleaned it the first time I figure in a month or so I'll use the soot eater and mark/measure exactly how much I have to the cap.

Thanks for the info on temps I'll just keep an eye on it. I know once I get it going there's nothing dark coming out of the chimney just white steam that dissipates pretty quickly when I can see it. Since the time change I don't get to see it too much, most of the burning is in the evening after it's dark.

 

[Aranyic]

I'd like to resurrect this thread real quick with a couple pictures. I was looking at the stack today and I have some runs on it where moisture is condensing. The top is also getting blackish and I honestly don't remember if it was like that a month ago. Is this normal or are the gasses too cool at the top?
The sun was setting so there is some shadow from the cap at the top, let me know if a picture tomorrow morning would be more helpful.

 

[ddahlgren]

An internal probe of some decent quality is the real deal the rest just hacking around.

 

[Highbeam]

Looks like normal black soot near the cap that rain has washed down. I don't think that those streaks are from condensing smoke.

 

[klxrelic]

Quick question:Is the black stove pipe double wall?

 

[Aranyic]

Quick question:Is the black stove pipe double wall?

No I am just basic black single wall in the house just a little over 5 feet stove to cieling.

 

[klxrelic]

Okay, I had to ask. Surface temps of dual wall pipe would be a lot lower.

 

[Diabel]

Okay, I had to ask. Surface temps of dual wall pipe would be a lot lower.

Measuring such temperature would be meaningless.

 

[klxrelic]

Eggs-actly.... That's why I had to ask.... You never know!

 

[Diabel]

Eggs-actly.... That's why I had to ask.... You never know!

With double pipe, flue probe is the only and best way to measure flue gasses.

 

[klxrelic]

Also, if your single wall pipe has been in use for a while, the crud that builds up inside can affect the surface temperature of the pipe as well.

 

[Diabel]

Also, if your single wall pipe has been in use for a while, the crud that builds up inside can affect the surface temperature of the pipe as well.

If that occurs, it is time for cleaning!!

 

[klxrelic]

Yup, that's for sure. But it is something to consider if you have lower than normal flue temps.