Flue temp measurements, how hot is too hot?

[FNG47130]

First, thanks to this community for being such an informative resource.

Had a PE Neo1.6 insert professionally installed recently and am using a thermocouple screwed to the single walled pipe to measure flue temp. The mfg recommends it be installed 18 inches above the appliance, however the flexible liner is pretty thin walled and we weren’t confident in attaching the thermocouple into it via the self tapping screw. Instead it is installed onto the collar connecting the pipe to the appliance. Probably about 4 inches above the top of the insert.

Any guesses as to the actual flue gas temp inside the pipe in relation to the external reading I’m getting where this probe is attached? Gemini says 2x but I don’t trust it given it’s frequently wrong about other things. How hot is too hot? I was seeing temps peak at 480 externally on a small load with the damper as low as it goes during the first burn to cure the paint.

Laser thermometer turned out to be dead so I’m unable to measure the front of the appliance until it’s replacement arrives in a day or two.

Lastly, any recommendations where to set the alarm threshold?

Visually the fire looks pretty good but I’m pretty inexperienced and value your collective input.

 

[Highbeam]

We can tell you that class A chimney max sustained temperature rating is 1000 degrees F. We can tell you that condar sells a thermometer with internal temperature ranges shown below.

But your problem is that you are putting your probe right on the stove so close that even flames could touch it so you're going to have a hard time getting useful information to compare. At that location you could use the stove's temperature ratings, usually 800 max surface temp which would correspond with 1600! inside the firebox.

Are you wanting to know for safety or for efficiency reasons?

I'd start with following the flugard ranges in the attached photo. Realizing that your temps could be a little higher than the ranges because you are closer to the stove.

 

[FNG47130]

Highbeam,

Thank you for your helpful response. My interest is safety. I’ll proceed as you suggest using the temperature ranges in the image you provided.

 

[fao1989]

Highbeam,

Thank you for your helpful response. My interest is safety. I’ll proceed as you suggest using the temperature ranges in the image you provided.

Hi FNG47130 and Hearth.com enthusiasts!

I have had a similar question that im still puzzled about. My BK A25 insert has a dealer installed stainless steel flue liner which I understand to be "reasonable" grade stainless steel - as in, good in normal operation to 1000F and a bit higher for short 'surge' periods.

Like you, I was hesitant to pierce the thinnish flue liner 18 inches above the collar which would be the usually recommended location to insert a temp probe. So like you, I attached an [Auber] thermocouple on the exterior screw where the liner attaches to the stove collar. But, consequently, it measures the OUTSIDE temp near the collar, not inside temperatures 18 inches up.

Making use of the wisdom expressed on this forum, I have conservatively assumed the inside flue temps to be *about twice* what I see on my externally attached thermocouple near the insert collar.

Consequently, I think I might have remained OVERLY cautious. I find that the temp at that external thermo couple will hit 490F, with the BK A25 thermostat knob near only 10-20% of max open. So I never run it WOT, out of concern that IF the inside temp is indeed double the outside temp measurement, I might exceed that 1000F.

But this thread, and Highbeams comment have me rethinking this. He noted that, "problem is that you are putting your probe right on the stove so close that even flames could touch it so you're going to have a hard time getting useful information to compare. At that location you could use the stove's temperature ratings, usually 800 max surface temp which would correspond with 1600! inside the firebox."

So I've always wondered, WHY the recommendation to insert a probe 18 inches ABOVE the stove in the pipe? If the stainless pipe or liner attaches to the collar, doesnt it stand to reason that the pipe or liner connection point AT THE COLLAR, would routinely exceed 1000F, if a probe 18 inches higher reads, say, 750F/800F?

In any event, I've often wondered if I am being overly cautious, by interpreting an external thermocouple temp of 490ishF at the collar as near 1000F inside liner temp? Is that a false interpretation? And what does it mean that a stainless steel liner rated to 1000F would regularly be installed to connect to a firebox nominally running at +1000F near the collar?

Regards,
FAO1989