# Measuring flue temperature



## centennial60 (Jan 21, 2010)

How far away from the stove are you measuring your flue temperature? What are you using to measure the temp? Right now I am just using a cheap magnetic thermometer and everyone I have talked to can't really give me an answer on how far down the flue pipe to place it. I've also considered using a thermocouple to measure the flue gasses.


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## gravely_todd (Jan 21, 2010)

I use one of those infrared thermometers that you hold and aim the laser dot at. i measure it about 8" off the back of my wood furnace.

Todd


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## CrappieKeith (Jan 21, 2010)

Anywhere between the furnace and your thimble is fine for reading flue gas tems. If you have a barometric draft regulator you should take the reading before the BDR.
To take exterior temps do not mean much. To get an accurate reading you must do it in the flue and dead center of the flue.


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## centennial60 (Jan 21, 2010)

So if I set up a thermocouple to measure the flue gasses at the center of the pipe what temperatures would be best to run at?


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## vwboomer (Jan 21, 2010)

I have a thermocouple/PID controller setup with the 6" probe inserted halfway into the flue pipe before entering the masonry chimney. It's about 17" from the collar on the stove.

I have to run pretty high temps if I want to get significant heat coming out of my ductwork.


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## CrappieKeith (Jan 21, 2010)

centennial60 said:


> So if I set up a thermocouple to measure the flue gasses at the center of the pipe what temperatures would be best to run at?



That's all gonna depend on how the furnace or stove was designed to operate.
Ballparking 400-500 are in a good range without throwing way to much heat out the flue.
I know some will say they need to get their units hotter to get any (good) heat from them.In that case I'd advise to look for another unit that has more heat exchange surface area,more thermal mass and maybe a thermostatic controlled burner with an afterburn in a gasier or cad type.


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## centennial60 (Jan 21, 2010)

Wow! There is a huge difference. I installed a thermocouple in the flue pipe and with the furnace crankin pretty good I was showing 500 deg. on the magnetic thermometer and 990 deg. on the t/c reader! I guess I'm running the furnace a lot hotter that I thought. But some times I have to run it this hot to keep the house warm. Because even with the furnace cranked up I'm lucky to get 110-120 deg. air out the top of it.


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## coppermouse (Jan 21, 2010)

18" up from the collar is what the instructions mine came with said


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## Highbeam (Jan 21, 2010)

This is way easier than you're making it. Go buy a condar probe flue thermometer. You just drill a hole and shove the probe in the hole. The directions require either a 15 or 18" distance from the appliance, I can't remember. A probe meter is the only way to measure flue gas temps. 

As to what temps are proper well, below 400 you'll be making creosote too fast since you need the flue gas to be hot enough at the top of the chimney to have not condensed the liquid out of it yet. You can run up to and hold 1000 degrees forever as that is the continuous rating of class A pipe. I make a habit out of bumping 1000 at least once a day to keep things burned out.

I cruise in at 500-800 in the flue above my hearthstone heritage woodstove.


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## laynes69 (Jan 21, 2010)

Rule of thumb is double your external temps to get a rough idea of the inside flue temps. 500 external would be considered overfiring. Your problem is your sending too much heat out the chimney. I'd imagine your using alot of wood at those temps. Do you have a baro? Our flue temps are half of yours and we get 150+ at the plenum of the furnace. As far as where your flue temps need to be, thats impossible to answer. When a units dampers are open it will operate much higher than a normal operation with the dampers closed. So your home and climate will determine where you need it to run to stay comfortable.


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## centennial60 (Jan 21, 2010)

I've tried running different damper settings and different settings on my induction fan to achive different flue temps and the most i can get out of my furnace is 120 degrees at the plenum and that was probably over firing the stove. i'm begining to think that my furnace is just a poor design, 150 degrees at the plenum would sure be nice.


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## slofr8 (Jan 21, 2010)

Highbeam said:


> This is way easier than you're making it. Go buy a condar probe flue thermometer. You just drill a hole and shove the probe in the hole. The directions require either a 15 or 18" distance from the appliance, I can't remember. A probe meter is the only way to measure flue gas temps.
> 
> As to what temps are proper well, below 400 you'll be making creosote too fast since you need the flue gas to be hot enough at the top of the chimney to have not condensed the liquid out of it yet. You can run up to and hold 1000 degrees forever as that is the continuous rating of class A pipe. I make a habit out of bumping 1000 at least once a day to keep things burned out.
> 
> I cruise in at 500-800 in the flue above my hearthstone heritage woodstove.


 That's what I do. Exactly. Same stove too!


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## vwboomer (Jan 22, 2010)

Highbeam said:


> This is way easier than you're making it. Go buy a condar probe flue thermometer. You just drill a hole and shove the probe in the hole. The directions require either a 15 or 18" distance from the appliance, I can't remember. A probe meter is the only way to measure flue gas temps.




While it's true that there are easier ways to monitor than a thermocouple/PID controller setup, you can't read your dial thermometer from the living room when the furnace is in the basement 
Plus the geek factor of the digital readout in the living room of course.


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## Merc1100sc (Dec 25, 2012)

Highbeam said:


> This is way easier than you're making it. Go buy a condar probe flue thermometer. You just drill a hole and shove the probe in the hole. The directions require either a 15 or 18" distance from the appliance, I can't remember. A probe meter is the only way to measure flue gas temps.
> 
> As to what temps are proper well, below 400 you'll be making creosote too fast since you need the flue gas to be hot enough at the top of the chimney to have not condensed the liquid out of it yet. You can run up to and hold 1000 degrees forever as that is the continuous rating of class A pipe. I make a habit out of bumping 1000 at least once a day to keep things burned out.
> 
> I cruise in at 500-800 in the flue above my hearthstone heritage woodstove.




Hello all. I just came across this thread via an Internet search. I have a Lopi wood stove with an open/close type damper. It has a Condar flue temp thermostat mounted about 18" above the top of the stove. The thermostats reads up to 900F. It has a "too cool" range up to about 220. I try to run it around 400 and get it up to around 500 or so at least once during a days use. Anything around 400 or so it throws a ton of heat with the damper in the closed position. After reading this thread I'm wondering if I'm running it too cool? Or do I just have a different type of stove than mentioned above. As I said the thermo oh reads up to 900. I've had it to 750 or so a couple times and it generates a lot of heat. I was scared of overturning it. Any thoughts anyone?

Thanks


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## Mac88 (Dec 26, 2012)

Welcome to AS. 

We have a circa 1988 Vermont Castings airtight. The owner's manual states:

*Use these temperature ranges as general indicators of heat output:

Low to medium 350-500°F (177-260°C)
Medium 500-600°F (260-318°C)
High 600-750°F (318-399°C)

Measured at the stove griddle.*

We use a digital handheld IR thermometer (instantaneous read). We also use a magnetic chimney thermometer mounted about 16" above the stove, for reference. The chimney thermometer does not react quickly to temperature changes.

There are as many opinions as there are stove owners. I always go by what the manufacturer states.


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## fun175 (Dec 26, 2012)

*low plenum temp*

you may be moving air thru the plenum to fast......
not picking up enough heat from surface of firebox. Just a thought.
to many cfm from blower????


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## pilot1 (Dec 31, 2012)

I drilled a hole in stove pipe about 18" from furnace and installed an old grill thermometer. This works pretty well for me.


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