Stack Temperature and Burning Efficiency

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Mushroom Man, Jan 15, 2012.

  1. Mushroom Man

    Mushroom Man
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    Is there an ideal stack temperature to achieve efficiency? If so, then at what temperature in the boiler?

    What is the flue gas temperature versus the exhaust pipe temperature (as I am measuring with an IR thermometer)?
     
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  2. Singed Eyebrows

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    The internal flue gas temps at pipe center are about twice what you will see on the IR. About 380 to 400 internal(center) is ideal for most boilers. Randy
     
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  3. goosegunner

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    Where should it be measured, at boiler connection out or a distance from the boiler?

    gg
     
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  4. Singed Eyebrows

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    My boiler is out aways, about 16". It is also to be measured after the 2nd load has burned about halfway & going well. Randy BTW, if a person wants to be picky the measurement is vacuum pressure & the tube needs to be in the center of the pipe with a right angle bend with the opening facing toward the boiler, at least thats what Magnehelic said.
     
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  5. maple1

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    I think there are 2 aspects to overall efficiency - combustion & heat recovery.

    If you've got good combustion but poor heat recovery, you'll have higher stack temps.

    Poor combustion with good recovery, you'll have low temps. But good combustion with poor recovery, you'll have higher ones.

    So it is likely hard to get a good read on efficiency by just measuring stack temps.
     
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  6. twitch

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    I have my thermometer about two feet up from the flue output and have the probe type. When my heat exchange tubes are clean it runs 400-500. As time goes on and the tubes get fly ash build up it will get over 600. Once I start seeing 550-600 I clean out the tubes.
     
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  7. heaterman

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    If you can shoot for an internal flue temp of 275-340 with a nice clean burn you have achieved about all you can get with a wood fired boiler.
     
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  8. JrCRXHF

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    Mine seem much higher but i think that is because of the old style boiler i am running (Harman SF260)

    most of the time i see 400F with my IR on the back of the boiler exhaust tube. If i lower the temp the smoke comes up but the wood use goes down so i don't know where i should be at this point.
     
  9. maple1

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    Sounds familiar. If I have all of my chimney draft have its way with my furnace, it burns great. And fast. And sucks most of the heat up the chimney before it has a chance to transfer to the water. If I choke it down a bit to try to keep heat in the firebox & transfer to the water, it kinda smokes, and creates coaling & creosote like crazy. Although I haven't committed yet to what I'm replacing it with yet, I can't wait for the day to replace it.
     
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  10. JrCRXHF

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    right now i am burning it just when i get home and at night i don't fill it and i try to keep the wood at the front to give it as much time as possible to heat transfer the only other thing i could think to do is put in a plate in the back to help pull the smoke down and burn that too before it goes out but think i think it is going to be a really smokey reload.

    It might just be how it is no real good heat transfer on the old school stuff.
     
  11. Gasifier

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    I am in the early stag of learning about burning wood through my gassification boiler. It can be a piece of cake to get heat out of it. Put wood in and walk away. But I also have noticed how many different things can effect your burning. With a thicker bed of coals my stack temperature goes down, quite a bit. Good gassification, better efficiency. Different types of wood effect that coal bed and therefor the stack temperature. How much the damper is open also has effects. Lots to learn. I also need a good stack temperature thermometer that measures on the inside of the pipe. What do most of you use? How much do they cost? Are they easy to install? Do they all need a seperate read-out for them? Or can you get one that has a read-out all in one? Can you give me a link to the one you use?
     
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  12. goosegunner

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    Where do you typically measure the temp?

    I moved my thermocouple to 15 inches above the center of boiler flue outlet. I am getting in the 440 range but I also have the barometric damper in a T bellow the flue outlet. I know it has to be cooling the exhaust stream some.

    If I hold the damper closed the flue temp rises quickly but the draft also climbs to .08 instead of the .02-.05 that Econoburn recommends.

    Gg
     
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  13. Singed Eyebrows

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    Gasifier, I use a Noshok that I got off Ebay. There are others on there. This is a high quality immersion thermometer with about a 4" stem, list is probably $125.00 to $150.00, I paid $27.00 + ship. I drilled(filed) about a 3/4" hole in the pipe & just threaded the stainless nipple in. No electronics to go bad, works every time, Randy You want one that goes to about 1000f
     
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  14. Gasifier

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    Gasifier, I use a Noshok that I got off Ebay. There are others on there. This is a high quality immersion thermometer with about a 4†stem, list is probably $125.00 to $150.00, I paid $27.00 + ship. I drilled(filed) about a 3/4†hole in the pipe & just threaded the stainless nipple in. No electronics to go bad, works every time, Randy You want one that goes to about 1000f

    Hey Randy. Thanks for the info. I am looking at a few right now. Let me ask you this. You say yours has a 4" stem. Did you get that size for an eight inch pipe? My pipe is 6 inch. So I am guessing I should try to find something with a 3" stem. Is that correct? Thanks for your time.
     
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  15. heaterman

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    Accurate measurements must be taken before the barometric. If you're at 440 after the BD you probably need to either clean your flue tubes or back off on the combustion air.....maybe both.
     
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  16. Gasifier

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    Gasifier, I use a Noshok that I got off Ebay. There are others on there. This is a high quality immersion thermometer with about a 4†stem, list is probably $125.00 to $150.00, I paid $27.00 + ship. I drilled(filed) about a 3/4†hole in the pipe & just threaded the stainless nipple in. No electronics to go bad, works every time, Randy You want one that goes to about 1000f


    I can only find one on Ebay that comes close to that temp. 150-750 Deg. F = $131.50 Ouch! :wow: That's Noshok therm..

    Here is one, not sure on quality. Any of you guys use one of these? Or have an opinion on it.

    http://www.ebay.com/itm/5-Bimetal-Thermometer-200-1000-F-C-1-2-NPT-Adjustable-/250797154562?pt=LH_DefaultDomain_0&hash=item3a64ace102
     
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  17. Singed Eyebrows

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    That looks like a copy of the Noshok that I use. There was a Noshok on there for $32.00, says the item ended, I didn't see a winning bidder though. Thats probably the one that got jacked up 4 times its original price. I don't know about the quality of the other one, its specs seem the same as mine, Randy
     

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  18. bpirger

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    Heaterman (Steve):

    Do you have any good numbers for a Garn comparing the GArn flue sensor temp with the temp at the exit of the last HX pipe? If I measure 500 at the Garn sensor (on the front), would i typically see 100 degree drop by the back exit of the last tube?
     
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  19. goosegunner

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    Maybe I have my damper in the wrong place, I have a T directly on the back of the boiler. The pipe goes up straight from there. The barometric damper is in anther T attached to the bottom of the T on the back of the boiler. Is that ok?

    I did thorough cleaning last week. If I read temps right in the flue collar leaving the boiler I get temps as high as 640. I have moved the plate on the fan to 1/2" closed from full open. Did not change much.

    Gg
     
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  20. leaddog

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    Not to keep hijacking the thread but You can use a PID controler with a k type probe. You can sometimes pick up used ones cheap on ebay or you can get the china ones under $50bucks and get a k probe for under $20. It's fun to watch the temp change by the tenth of a degree. depending on what PID you get you might even be able to use some of the other fuctions to control pump, fans etc. just use your imagination.
    leaddog
     
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  21. heaterman

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    For that kind of money you might as well go buy a Fluke 116 HVAC meter and be able to measure pipe temps for your supply and return as well as flue temps up to 750* (if you exceed that you know you're in trouble anyhow so who cares if it will go to 1000) plus have a nice multimeter to boot. I think you can get one from Grainger for around $150 or so if you watch for sales. Lot's of HVAC supply houses will carry that meter also. It's an extremely useful instrument for working around HVAC stuff of all kinds.

    http://www.fluke.com/fluke/usen/Digital-Multimeters/Fluke-116.htm?PID=55994&trck=116
     
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  22. heaterman

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    Average flue gas temp drop in the last pass of the heat exchanger is between 60-90* from what I have seen. Just got back from my kids place with my analyzer and when it was reading around 320 at the outlet the thermometer in the Garn flue was reading 370-380. I must say that his flues have not been cleaned since I put his Garn in 3-1/2 seasons ago so alittle more drop than that would be normal in a clean boiler.
    At the 500* on the Garn thermometer you would probably drop over 100* in the last pass because you have more heat available than if you were reading in the 350 range. The greater the difference the more transfer capability you have.
     
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  23. heaterman

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    Typically, one would install the BD out from the boiler flue collar and in maybe the second section of stovepipe. That's not always possible though and it can be closer. I don't think I have ever seen the configuration you describe but it should work about the same as far as admitting air into the chimney to reduce draft. 640 sounds a little high but I have never had my hands on a 200 either. I can't say for sure what the factory would consider best temp to be.
    If it were me I'd probably tweak the shutter closed a little bit more at a time until I started to see undesirable results like smoking, poor gasification flame or creosote buildup then back it open a bit. No way you could really tell without a flue gas analyzer.
     
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  24. hobbyheater

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    [quote author="Gasifier" date="1326854673" I also need a good stack temperature thermometer that measures on the inside of the pipe. What do most of you use? How much do they cost? Can you give me a link to the one you use?[/quote]

    Just a BBQ temp gauge - easy to install. Just keep playing with the drill bit sizes until the gauge threads in.

    Cheap! http://www.ebay.com/itm/NEW-50-700-BBQ-GRILL-SMOKER-THERMOMETER-OVEN-TEMP-GAUGE-2-3-DIAL-2-5-Stem-/150724294796?pt=LH_DefaultDomain_0&hash=item2317de388c
     

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  25. goosegunner

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    Thanks I will post a picture of damper tomorrow if I can.

    Econoburn website says 280-400 but Dale said to disregard that number as 280 would cause condensation.

    He said check it about 1' from collar. The problem I have is the damper letting in cooler air so it will be tough to get a accurate reading away from the flue collar on the boiler. But would it be significantly less 1' from the boiler?


    Gg
     
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