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Flue temeratures probe vs. surface test

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by ddahlgren, Mar 31, 2013.

  1. ddahlgren

    ddahlgren Feeling the Heat

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    Finally took the time to install a probe I have had for a month to compare it to surface temp. I have never been able to get a Rutland magnetic thermometer to go much over 220 degrees with a stove temp of around 500. So some testing was done.

    The Rutland is off by 40 degrees at room temp.
    Checked Rutland in boiling water by using tongs so it would not touch the pan and reads 212.
    Installed probe and sealed with high temp RTV at 20 inches high
    installed Rutland at 18 inches high above flue collar so just below the probe.
    Below is a list of the test starting with a kindling fire and wood added along with time kindling was kiln dried pine and fire building scraps with no paint etc. time in minutes and temp in degrees F

    Time Probe Surface Changes
    1 230 25
    3 320 40 Cut air to 75%
    5 375 50
    7 430 100 Cut air to 50%
    10 410 100 Add a few very small slits and air 100%
    15 350 100
    17 450 125 Add a few more small splits
    20 425 125 Cut air to 50%
    25 480 160
    30 520 180
    35 540 175
    40 560 200
    45 570 210 Add 8 inch split 40% full fire box 3 year oak open air to 75%
    47 Split fully burning cut air to 25%
    50 600 210 Cut air to 10% partial secondaries
    60 580 190 End of recording info

    Transfer Rutland to stove top reads 475

    Problems I think with the Rutland thermometer.
    On a round surface the bi-metal is about an 1/8 inch from the pipe and air can freely circulate around it but on a flat surface the air has a much harder time getting to the bi-metal. I know the surface temperature is well above 210F as if water is sprayed on it the water instantly boils and is gone. Touch it with a wet rag the same thing. To me the Rutland is pretty much useless for any real world use as it would take over 1000 degrees in the flue to get into the burn zone and while safe a large waste of wood and very short burn times.

    Probe data
    Industrial SS probe factory calibrated to under 0.5% error
    Inserted and sealed to mid point of straight 6 inch pipe 20 inches from top of stove.

    Problems with test
    Fairly low draft as it was above 45 outside with a total stack height of only 13 feet from stove top.
    Door gasket looks like end of life with little compliance though easily passes 'dollar bill test'. Personally I think this test is somewhat useless other than to find gross errors as on high spot will jam the bill while leaking all around high spot. My other clue is hard to get secondaries lit and light sooting by door hinges and latch along with the latch instantly going from loose to will not move so does not draw the door closed but goes to what must be metal to metal right away.

    Hope this might add a data point for this stove and test conditions.

    Any and all comments appreciated other than ones about wood quality as it was a test of instruments rather than wood. The next question is with everything I have read creosote forms below 250F so at what point in the test am out out of the creosote problems. The chimney was cleaned 3 months into the season and a small amount of soot but no creosote. Stack is 7 ft of close to vertical single wall inside and 5 feet of Dura-Vent+ Through ceiling and outside with chimney cap.

    Dave
    Joful likes this.

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  2. rideau

    rideau Minister of Fire

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    Dave,

    I think for anyone who deson't have a flue probe thermometer, the magnetic thermometer placed on the flue about 18 inches to 24 inches above the stove is the best way to regulate the fire.

    Bet you have double wall pipe?

    I decided long ago, based on my onservation, that the magnetic thermometer registered just about 1/3 the internal flue temp. Decided this because my cat always lit off if I engaged the bypass when the magnetic thermometer read 170 (which was way before the stovetop read 250, the guide provided), which meant the internal stove temp had to be 500.

    Look at your readings once you are 15 minutes into the burn:
    15 min: 350 probe; magnetic 100 x 3 = 300
    17 450 125 x 3 =375
    20 425 125 x 3 =375
    25 480 160 x 3 = 480
    ----------------------------We are near the temp where the cat can be engaged:
    30 520 180 x 3 = 540:
    ---------------------------Both thermometers indicate cat can be engaged: End point for me. Since I load all
    my wood at the same time at the beginning, I can now just take a few minutes to adjust
    the air for the type of burn I want, and leave the stove for 12 hours.
    35 540 175 x 3 = 525
    40 560 200 x 3 = 600
    45 570 210 3 = 630
    50 600 210 x 3 = 630
    60 580 190 x 3 = 570


    I don't think you need any more accurate readings than those that the magnetic thermometer gives, to get your stove functioning to optimum performance. However, until I had the probe, I firmly believed I was right about my interpretation and manipulation of magnetic thermometer exterior flue temps, but i didn't know. Interestingly, ICC had never teted and couldn't give me any guidance there!

    I firmly believe that in the absence of a probe thermometer, magnetic flue readings are the way to go to run your stove. with double wall (at least ICC double wall, and whatever brand your double wall is --you might post that for other's information) simply multiply the surface temp by three. Many post that you can multiply single wall by 2 as effectively.
  3. maple1

    maple1 Minister of Fire

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    I think there is a real danger in magnetic thermometers & people blindly relying on them.

    They are quite far out to lunch.

    If you know this, and know how much out to lunch they are - that's OK. But you're likely way in the minority.
  4. ddahlgren

    ddahlgren Feeling the Heat

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    The interior stack is single wall where the magnetic and probe are both mounted. 5 ft of duravent plus going through the ceiling and outside. Stove is an 1997 Avalon Pendleton with single secondary air in front near the glass so no cat to engage. I will modify my sig line to indicate that for anyone not familiar with the stove as they seem to be found very regionally from what I have found. Maybe not all that common all over the USA.

    From what you have said is the probe temps are ok if I understand correctly. If I misunderstand please do correct me.

    The probe I am sure is reading well within 1% of what is going on with the flue gases as made for industrial process monitoring as opposed to being sold in a stove store. Not that there is anything wrong with a stove store but something that would make a QC engineer happy. My son is a QC engineer and sent him the specs and his comment was it is a wood stove not a reactor..LOL.. I also showed him the Rutland thermometer and it generated a belly laugh so big I thought his sip of beer was going to come out his nose.. Then came the ok dad this is why it is junk.
    Dave
  5. rideau

    rideau Minister of Fire

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    There goes the rule re magnetic temps on single wall being 1/2 actual temps.....

    So, I now only feel comfortable saying that with ICC ultrablack stove pipe, the temperature as read on a Woodstock supplied magnetic thermometer (I have two, and they are about 10-20 degrees different in their readings, close enough for our purposes) is about 1/3 the actual flue temperature.

    I like having a flue probe thermometer to use as the guide for achieving efficient burns. I recommend them for everyone, but especially for those with cats.

    Seeing your readings on single wall pipe makes me wonder what the actual interior flue temps are with some of the burners here who let their single wall pipe get to 700, 800, even 900 without a great deal of concern. They are very concerned about keeping their wood dry enough to keep the chimney clean, but not so concerned about burning within the parameters that the pipes are designed to withstand on a regular basis.
    Joful likes this.
  6. ddahlgren

    ddahlgren Feeling the Heat

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    All I can say is I tested 1 Rutland thermometer vs. 1 industrial probe on 1 particular stove and stack on a 38F night. My sons first comment was you can not measure surface temperature if you do not contact the surface and this does not. So it is affected by the stack material color and paint along with the air flow in the room that changes convected heat. He looked at the probe read the specs again and suggested if it reads 600F the in should be between 594 and 606 way better than it needs to be. The had me do things to change the stack temp and time in seconds for a change before comparing to response specs and more than fast enough and in seconds rather than minutes. Open or close air and temp changes withing seconds to a new number.

    I am burning what I feel is marginal wood so the reason for the test and post is am I far enough from creosote to be in fairly good shape? With my results at 800F with a Rutland or at least mine you would need a chimney fire to generate that temp. I found a place that has miles of 7/8 door gasket I think I need and will change that and see if I can get full secondaries with a perfectly clean glass. Next week for that as today Easter dinner to be finished and work and travel until Wednesday late..
    Dave
  7. oldspark

    oldspark Guest

    "There goes the rule re magnetic temps on single wall being 1/2 actual temps....."
    Not a rule, it was through some testing by members from this site with some good equipment.
    I check my surface thermometer with a IR testor and its not that far off but gets worse as the temps go up.
  8. oldspark

    oldspark Guest

    "Seeing your readings on single wall pipe makes me wonder what the actual interior flue temps are with some of the burners here who let their single wall pipe get to 700, 800, even 900 without a great deal of concern. They are very concerned about keeping their wood dry enough to keep the chimney clean, but not so concerned about burning within the parameters that the pipes are designed to withstand on a regular basis"
    If you have ever run a IR tester up a length of stove pipe you would see how fast the temps drop as you go up the pipe just a short distance, not a problem unless you have had bad burning practices in the past and you light off the creosote.
  9. ddahlgren

    ddahlgren Feeling the Heat

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    Maybe next week end I should see how close I can place the probe to the ceiling, make sense?
    Dave
  10. oldspark

    oldspark Guest

  11. ddahlgren

    ddahlgren Feeling the Heat

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    Well my goal is to offer the flue enough heat to stay away from creosote but not give too much heat away to waste wood so makes a lot of sense to me. My idea of the ideal probe would be at the chimney cap to make sure it is over 250F on a 0F night. The more I send up the flue the shorter the burn and the more wood i go through and the more I have to play reload the stove. Where is the upside in that? I put in my chimney at minimum length so that it would be just long enough to draft ok but not so long that it cools down the flue gases enough to make creosote, Every foot more I add to the chimney is just flue gas cooling pipe in my mind and a liability to clean or cause a chimney fire. I may be new at EPA wood burning as the last stove from 20 years ago was as much incinerator as stove but did keep us warm and burnmost anything hot and fast and actually made a bunch more heat that the toy i have now. I burned mostly pallets and scrap lumber scrap furnature from the dump so the ultimate recycle. LOL.. Was it safe no but chimney fire could never happen as it burned so hot and fast there was no chance getting below 700 let alone 250F.

    Dave
  12. ArsenalDon

    ArsenalDon Minister of Fire

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    I used to freak out about temps. Now granted I have double wall pipe, not single wall, but when my IR shows stove top 500-650 I feel confident the creosote is not building up excessively and the clean-outs back that up
    Trilifter7 likes this.
  13. oldspark

    oldspark Guest

    Well some have reported high stove top temps with fairly low flue temps, not sure what causes that but ignoring flue temps is naive.
  14. rideau

    rideau Minister of Fire

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    Cat stove, once the cat engages the flue temp drops, unless you have really excessive draft and lose a ton of heat up the chimney. No problem because you've got no visable emissions.
  15. ArsenalDon

    ArsenalDon Minister of Fire

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    I do not ignore them, but without an internal probe and with a double wall flue I just have learned to use the probe to judge both.
  16. rideau

    rideau Minister of Fire

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    ?
  17. Trilifter7

    Trilifter7 Feeling the Heat

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    I just installed a flue temp probe on my double wall pipe and I've found that the flue temp almost always matches the stove top temp within 75-100 deg. My chimney is only 15' from stove top to cap so I shouldn't have an issue with creosote. I'm sure yours is the same Don.
  18. Treacherous

    Treacherous Minister of Fire

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    I usually bring my flue up to around 750-825 degrees on a reload and then slowly close the air as long as the surface temps on the stove continue to rise and secondary combustion continues to occur. I then reach a point where I can completely close the air. The digital probe really helps to see the real-time changes. My flue temps have always been higher than my stovetop temps.
  19. ddahlgren

    ddahlgren Feeling the Heat

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    I am seeing the same as this morning an a unusually cold one for the season. low 30's today. Did a reload from last night with a bit of kindling and ball of newspaper to get a draft and some marginal oak and maple. Flue 675 stove 475 house 76 the number that matters the most. No secondaries as the wood is not good enough so a brisk fire burning clean but the house is 76 and no smoke and should be no creosote either at 650 to 675 in the flue.
  20. oldspark

    oldspark Guest

    My flue temps are higher then the stove top until the stove settles in after reducing the primary air, the flue temps with my Summit are higher than my old non EPA stove. The new chimney has reduced my flue temps some, the smaller diameter of the chimney might have something to do with that.
  21. ddahlgren

    ddahlgren Feeling the Heat

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    Did a bit more accidental testing today. All I had ready to burn as in the house and the middle of the night was a bunch of small splits on a kindling fire knowing they would not last but would take the chill off. Well at 2 am loaded and lit it and decided to just finish sleeping on the couch in the same room as the stove. Woke up at 6 am and a big bed of coals glowing bright red and a 400 degree stove and 440 degree flue. Stove room 76 and house 1'st floor 72. 8 am headed to the shop room still 72 stove still 350 and flue 375.

    What i think I learned is while coaling is not desired most of the time but if it happens for whatever condition there are a ton of btus in those coals and not to be discounted in any way. Seems to work well actually when you just need a little heat like it is in CT at this time of the year. I am thinking I am actually better off forcing it to coal and then let those cruise for a few hours to keep the chill off. It could make good use of small splits and not waste the treasured at least for me very large well seasoned oak ones. Also a might be a good time to burn some of the more marginal wood as they seem to coal easily for me.

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