Flue Thermometer Questions

[Cluttermagnet]

I just got a Fluegard 3-39 probe thermometer for my free- standing stove. For mounting, about the best I can do is 12 to 14in back from the exhaust flange. Further back and it gets real hard to read, being too far into the fireplace opening. Instructions say minimum 18" and if you place it closer, expect it to read slightly high. Instructions also advise that if mounted in horizontal pipe, expect readings about 6 percent lower due to lower heat transfer. Sounds like these two factors may just about cancel each other out. My stove has a rear flange so that's how I have to do it. Sounds to me like it ought to be fairly accurate anyway. Opinions?

BTW stated accuracy is +/- 5 percent of full scale which works out to +/- 85 degrees at 1700 deg FS.

 

[Cluttermagnet]

(Bump)

I'd appreciate hearing experiences from anyone who runs or has run a probe thermometer. Especially if run in horizontal pipe. I just want to learn more about them. What accuracy to expect, Some ideas about a 'good' temperature range to shoot for, any problems which might result in the unlikely event of a chimney fire, etc. Thanks!

 

[BurningIsLove]

Like yourself, I have a horizontal flue pipe connector and was not able to put it 18" back from the collar.

My connector pipe is single wall (a topic for a different thread), and I found that at 6" from the collar, the probe thermometer reads nearly exactly 2.5x what the surface magnetic thermometer reads. When I only had the magnetic, I liked to keep it around the 400-600 degree range, so that was in the 1000-1400 range for the probe. That's the range I found optimal for my stove. Lower temps would cause my downdraft stove to stall out and turn into a smoke dragon.

Keep in mind that a probe thermometer is not going to be an accurate indicator of a chimney fire, which is likely going to be much further up the stack where creosote is more likely to accumulate (at least in my setup). I use mine as a measure of optimal stove performance to prevent creosote buildup in the first place, and indirectly preventing a chimney fire (no substitute for regular chimney cleanings). It's also indicative of an over fire or if/when a gasket/seal goes bad.

 

[Cluttermagnet]

Thanks for the info. I'm also running single wall 6in pipe. I can get the thermometer about twice as far from the flange as you did. That's about it.

Yes, I believe a probe thermometer is going to indicate over fire, but I had no expectation it would indicate a chimney fire. I guess that, as a newbie, I'm worst- casing it and wondering what contribution that little quarter inch hole would make to a chimney fire if one ever got started. Let's say if the thermometer had fallen out of the eyelet somehow. Of course, I'm going to do all the maintenance to assure that one never gets started. So far as 'indications', I understand that the jet engine- like roar, and tongue of flame exiting the chimney, ought to be a pretty strong indication. ;-)

Does it make sense to also get a surface thermometer and set it on top of the stove?

 

[Heat Miser]

Mine is in a 45 degree pipe section. It provides enough accuracy to do the job intended which is to indicate the flue is hot enough for a good burn, but not so hot that I'm over firing the stove. Keep in mind you're just looking to keep the thermometer in a pretty wide range so absolute accuracy isn't really a factor and, honestly, the thermometer itself isn't a pillar of scientifically accurate calibration to begin with.

I also run a thermometer on the surface of the stove as well, but I rely on the flue thermometer the most because it provides a more instant assessment of temperature than waiting for the mass of the stove to heat up and cool down through the burn cycle.

 

[BurningIsLove]

Exactly....the probe is a real-time measure, the magnetic surface one is more of a rolling average as your stove exterior doesnt heat up/cool down very quickly in comparison w/ the firebox changes. As stated, you are shooting for an optimal temperature range on both, which is going to be unique to your stove, chimney, etc. Doesnt matter if the probe is accurate to degrees as long as you know what the offset is. You'll be able to determine that range not necessarily by what it reads, but by the emissions out of the top of the stack (clear or smoky), the surface temp of the magnetic when settled, appearance of the fire (if you have a viewing window), air intake settings, etc. It will take a little guesswork to figure out your range. Dont necessarily stick to what the green/yellow/red indicator ranges tell you, they are often based on decade old ranges that are not as applicable to modern stoves which are designed to burn hotter and cleaner.

Also wouldnt worry about the probe coming out. Mine is simply set inside, no cement or other materials. No backpuffs or other emissions come out. If you do have an over fire situation, its not going to contribute to the firebox temp as there are limited combustibles in the exhaust and a little extra draft at the point is not going to affect much. If you have a chimney fire, it would add extra oxygen, but not much. At that point, you should be extinguishing your stove anyhow and would have the door open to do so, so a pinhole isnt going to contribute.

 

[Cluttermagnet]

Thanks for the info, it's been helpful. I do see that each stove- chimney combo would be unique and you have to learn what's right for yours. Yes, a probe thermometer has to be much more accurate and near instantaneous reading compared to the stove top ones. I've already learned to recognize which wood is dry, and when I'm getting a good, hot burn. That correlates with pretty much invisible exhaust from my chimney, no visible smoke.

BTW that inexpensive wood moisture meter that has been mentioned elsewhere in the forum, the cheaper, two-pin Centech at Harbor Freight, is now available in the store fliers, and I see it's on sale this month. I'm going to get one- they are incredibly inexpensive!

Has anyone in the group ever compared a real expensive, professional model thermometer against these inexpensive consumer models for accuracy on flue temperatures?

 

[Cluttermagnet]

I finally got around to installing my probe thermometer. Pretty simple- choose your location, drill a 1/4in hole, insert eyelet and then probe with ring magnet. I was surprised just how fast a small hardwood fire had flue gasses moving up into the 200-300 degree range. In a few minutes more, I was closing in on 400. A few more minutes and I was up around 550-600+, where the stove seems to spend most of its time with a small hardwood fire. As commented above, the thermometer is pretty fast responding. I'm seeing confirmation that I was indeed building hot, clean- burning fires last winter.

 

[BurningIsLove]

Nice....you'll see well into the 4 digit range when you get around to building a normal, winter fire. Keep in mind that though that 300-400 at the probe means a whole lot less at the top of the stack depending on your setup (masonry, ClassA, internal vs. external, etc etc). I have masonry w/ clay liner, so even for a new fire reading 800 when the setup is cold, I'm still building up creosote at the top of the (cold) stack until it warms up. Once warmed up, it provides delightful radiant heat for the garage/shop, but it does take about 90 minutes on average.

 

[bokehman]

Consider an IR thermometer. You can measure everything with it, stove pipe, stove exterior, stove interior, stove glass, house walls, etc. You can even use it to make sure the oil in the chip pan is the correct temperature.

 

[Cluttermagnet]

Consider an IR thermometer. You can measure everything with it, stove pipe, stove exterior, stove interior, stove glass, house walls, etc. You can even use it to make sure the oil in the chip pan is the correct temperature.

That is really tempting. I think the good ones are at least a hundred dollars and up, however. Not in my budget right now, but sure would be a very useful instrument. I'm wondering if there are possible savings on the cost of one of these by either buying a cheap, Asian made unit, or by purchasing only the sensor 'front end', the transducer part which would work with digital multimeters I already own, for the display? If the sensor was cheap enough, I wouldn't mind doing the conversion in my head- from millivolts DC to degrees F. I did things like that all the time, for years, when I worked as an engineer and also electronics hobbyist.

Compared to those IR instruments, my probe thermometer is only a crude 'indicator', but I do believe it is measuring the flue gas temp fairly accurately, FWIW.

 

[Cluttermagnet]

Nice....you'll see well into the 4 digit range when you get around to building a normal, winter fire. Keep in mind that though that 300-400 at the probe means a whole lot less at the top of the stack depending on your setup (masonry, ClassA, internal vs. external, etc etc). I have masonry w/ clay liner, so even for a new fire reading 800 when the setup is cold, I'm still building up creosote at the top of the (cold) stack until it warms up. Once warmed up, it provides delightful radiant heat for the garage/shop, but it does take about 90 minutes on average.

Yeah, I bet there is a heck of a thermal lag with the masonry. A friend of mine estimates upwards of a week for the entire stack of an internal masonry chimney to equilibrate, based on his experience over the years. He said he doesn't think the top end ever really gets all that warm. Liners are of course way better, and in a perfect world, that's what everyone would have. But OTOH they do make a lot of different brush sizes for those flue tiles. ;-)

 

[bokehman]

That is really tempting. I think the good ones are at least a hundred dollars and up, however. Not in my budget right now, but sure would be a very useful instrument. I'm wondering if there are possible savings on the cost of one of these by either buying a cheap, Asian made unit, or by purchasing only the sensor 'front end', the transducer part which would work with digital multimeters I already own, for the display? If the sensor was cheap enough, I wouldn't mind doing the conversion in my head- from millivolts DC to degrees F.

There are plenty of cheap ones out there. Doing a conversion would take the fun out of it and slow you down. With mine there is a laser circle to tell you where you are pointing. If you get one make sure it at least goes to 1000 degrees. With my insert I take the glass to 750 degrees before closing the damper. Then it spikes a bit before starting to come down.