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Blaze King thermostat faulty

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by RustyShackleford, Mar 2, 2010.

  1. RustyShackleford

    RustyShackleford Minister of Fire

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    Well folks, I had a suspicion I had a problem - ever since I got my new
    BK Princess Ultra (freestanding) last autumn, and it looks like I was right.

    I finally removed the cover on the thermostat. I tried to do it awhile
    back, and after I removed the two square-drive screws, it was still
    really stuck in the back (up against the stove) and I was afraid to beat
    on it too hard, given the dire warnings against removing said cover (on
    the sticker). But the tech guy at BK said it was fine to remove it, once
    he realized I wasn't an idiot. And with the stove hot, it came off
    quite easily. Removing the cover does not affect the airflow, although
    of course you want to replace it when done trouble-shooting. One
    thing he said (IMPORTANT): turn the control knob down so that the
    flapper is close to fully-closed before re-installing the cover, lest the
    edge of the butterfly flapper wedge against the cover.

    So here is what I found. I set the thermostat control at about 2 and
    left it there throughout this experiment. I measured the width of
    the valve opening - between edge of flapper and the opening (where
    is unimportant, as long as you're consistent). With the stove almost
    completely cold, it was about 1-1/8" open. I built a good fire and after
    an hour or so the opening was close to 2". I let the fire burn down
    overnight and by morning the stove was pretty cold and the opening
    was down to 1" or so. Clearly this is wrong - for a given control knob
    setting, the valve should close as the stove heats up and open as it cools
    down.

    I confirmed this with BK and the dealer is sending me a new thermostat
    assembly with prepaid return label for the old one. It appears to be held
    in place by two screws and some silicon sealant (presumably some high
    temperature variety, hopefully available locally). So stay tuned ...

    The kicker is ... even with this gross malfunction, this is still, by far, the
    best woodstove I've ever had, easily keeping a fire going for 12 hours !

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  2. Patapsco Mike

    Patapsco Mike Feeling the Heat

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    I am vaguely suspicious that mine doesn't really do what it's advertised to do either, and that is really acting more or less as a normal air control. But I do not care, I have no desire to poke around inside it, and I'm not sure I would swap it for one that "works."
  3. RustyShackleford

    RustyShackleford Minister of Fire

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    Well, I'm an engineer familiar with the basics of feedback control loops and it's
    clear to me that mine is giving positive feedback, rather than the desired negative.
    (You want negative feedback in control loops).
  4. Patapsco Mike

    Patapsco Mike Feeling the Heat

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    It makes sense to me that closing the air intake while the stove is cooling will increase the burn time. You are getting 12 hour burn times, as far as I know the longest burn time on the market. This is in my view the most desirable feature of the stove. I understand what you are saying about keeping temperatures even, but as an engineer you know that sometimes results in the real world are counter-intuitive. Maybe it works opposite of what you would think you want to get an even burn, but in reality it is giving you what you want by extending the end of the burn by a few hours...

    I will be very interested to read your results, please post what you find when you get the new unit. We got our stoves just a few weeks apart, so if your unit starts working better I'll be opening mine up as well.
  5. SolarAndWood

    SolarAndWood Minister of Fire

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    Mike, try the experiment of getting a good burn going with the fans on high and then pull the plug on the blowers. The tstat almost immediately shuts the air down to get back to the original temp.

    The thing the tstat doesn't seem to do is open the air all the way up at what I would consider the end of the useful burn. I guess I am ok with that as I would hate for it to ever fail that way earlier in the burn and it leaves a coal be for a long time to reload on.
  6. Highbeam

    Highbeam Minister of Fire

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    The woodstock guys seem to think that their stoves get hotter as they close the draft, to the point where they snuff the fire. Could it be that that 1" open setting is the max heat output setting as determined by BK? Opening it further would just blow air and heat through the stove and up the stack?

    I expect a thermostat to act as a thermostat which by definition means it regulates stove temperature. The assumption that I am seeing made is that the thermostat should open the air intake when the box is cold and close the valve when hot. I propose that this is not necessarily desired since an open air valve may cause a cooler stove.
  7. SolarAndWood

    SolarAndWood Minister of Fire

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    I think that depends on the phase of the burn and only to a certain point. On the King, I think the max output in the early stages of the burn is about 3.25 with the blowers on high. Any more than that in the early stage of the burn and I agree that it is likely just going up the pipe. Once you get 2/3 of the way through the burn, opening it all the way is the way to get the btus out of the firebox the fastest and be ready for a reload.

    Now maximum efficiency is another discussion. My goal is generally producing as much heat as required to keep everyone happy as opposed to squeezing every last BTU out of every stick of wood. If I had to guess about max efficiency, I'd say it is probably is somewhere in the middle of the normal range which is 2 on the King. That doesn't cut it in my house when it is under 20 and/or blowing.
  8. Todd

    Todd Minister of Fire

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    I'm confused now and wonder if the BK thermostat is more like the PE EBT where it just opens up at the early stages to give more air for complete combustion and pass the EPA test?
  9. SolarAndWood

    SolarAndWood Minister of Fire

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    I don't know anything about EBT, but the tstat definitely lets you set a heat output and is very repeatable load to load. This is demonstrated by running similar loads with and without the blowers on. With the blowers on, it is fairly repeatable that you cut a third of the burn time.
  10. RustyShackleford

    RustyShackleford Minister of Fire

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    Maybe, but I don't think so. That would be pretty sophisticated behavior, more in line
    with a microprocessor controlling the burn than a simple bimetallic coil.

    And the BK tech guy seemed pretty clear that it was behaving wrongly.
    Will do. I will be seriously chagrined if the new one behaves the same way.

    I wonder if there're other stoves out there with this malfunction. I imagine it's not so
    easy to access the thermostat in an insert.
  11. ecocavalier02

    ecocavalier02 Minister of Fire

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    So if you dont run the fans you can get a longer burn time? so you saying that it uses more air when the fans are on to make up for the fans being on? because the fans cool the stove off so it gets hotter?
  12. SolarAndWood

    SolarAndWood Minister of Fire

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    Yes. For a given setting on the thermostat, I have found that having the blowers on cuts burn time by about a third. As I recall, N60 had similar results with the Princess.
  13. ecocavalier02

    ecocavalier02 Minister of Fire

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    Does this mean i need to actually pull the plug or can i just switch the fan to off
  14. SolarAndWood

    SolarAndWood Minister of Fire

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    Just switch the fan off, the thermostat is purely mechanical. You will also notice a difference in burn time between low and high speed on the fans.
  15. ecocavalier02

    ecocavalier02 Minister of Fire

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    thats pretty cool. learn something new on this beast everyday. temps have pretty warm here during the day so ive been gettin between 16 and 19 hours a day before loading. curious to see how much further i can stretch it. This stove just continues to amaze me. when i got it on jan 25th i had just bout a cord that i wanted to use for the rest of the year. so far i have used half of that cord in over a month. i think my wood savings are going to be just about in half. also climbed up on the roof to take a peek down the chimney not even a flake on the cap or down the pipe. by this time with the other stove the cap wood have a bunch of fluffy crap in there and some flakes in the pipe
  16. RustyShackleford

    RustyShackleford Minister of Fire

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    Ok folks ... I received the new thermostat (along with a prepaid
    UPS mailer for sending the old one back).

    I did the hair dryer test - which supposedly they do with testing
    each thermostat, but obviously some slipped through the cracks.
    Holding the thermostat in your hand, you simply turn the control
    knob so that the valve is about halfway open. Then blow directly
    on the bi-metallic coil with a hair dryer. Within a few seconds, the
    valve should start to close a little; that's exactly what the new one
    did. On the old one, sure enough, the valve actually opened a
    little - the wrong direction, positive feedback (a hot fire gets
    even hotter). Apparently the problem is that the bi-metallic
    coil is "wound backwards". The coil is mounted the same way
    in the old/bad and good/new thermostats; they appear identical.
    But in one case the coil unwinds as it gets hot, in the other it
    winds tighter. Apparently the coil is wound with the wrong
    metal (of the two metals that make a bi-metallic coil) on the
    inside (of the curve).

    The is some question about what sealant to use to install the new
    thermostat. I got some "high-temp RTV silicon gasket maker" at
    the car parts store, which says it's good to "650-degrees intermittent".
    Talking to the guys at BK, sounds like similar spec to what they use.

    I have initially just set the thermostat into place (see pictures) and
    not even put in sealant or the two screws that secure it. I built a
    fire last last night and I was amazed. Much more even/controlled
    burn; I had no idea what I was missing, and already loved this stove.

    Below are some pictures of the removed thermostat. I think they are
    mostly self-explanatory. The first one shows the coil. The control rod
    rotates a gear which meshes with another larger dark-colored gear at
    right angles. The big gear has a tab sticking out (on the lower-left)
    which engages a hook on the outer end of the bimetallic coil. The
    inside of the coil is attached to the shaft which rotates the butterfly
    valve which actually controls the airflow.

    I'd like to include a video of the hair-dryer test, but I don't think an
    attachment that big is allowed.

    Attached Files:

  17. RustyShackleford

    RustyShackleford Minister of Fire

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    I urge all BK owners to check their thermostat. Who knows how many
    of these bad ones are out there. A simple initial test is as follows. With
    a cold stove, turn the control knob "down" (cooler) and hopefully at some
    point you will hear a distinct metallic "clink" sound when the valve is
    fully-closed. Note what o'clock the control knob is at. Build a fire,
    using the control knob normally, and when things are good and hot,
    note what o'clock the fully-closed "clink" occurs. With the hot fire,
    it should close at a "higher o'clock" if the thermostat is working right;
    with a bad one, the opposite will occur (this is assuming clockwise is
    a higher/hotter control knob setting, as it is on my freestanding Princess -
    I have no idea how it works on an insert or a King).

    If this test is inconclusive (you can't reliably hear the "clink") or seems
    to indicate a faulty thermostat, remove the cover and re-test (as I describe
    in the first post of this thread).
  18. Patapsco Mike

    Patapsco Mike Feeling the Heat

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    Thanks for following up. The next cold weekend I will be checking mine!
  19. Todd

    Todd Minister of Fire

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    Good to hear you got it fixed. I'm surprised it worked so well when it was wound backwards.

    Curious question for you, what is the measurement from the floor to the bottom of the door on the Princess? Do you have to squat down to load or just bend over a little?
  20. SolarAndWood

    SolarAndWood Minister of Fire

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    I thought I would miss the top load of the VC, but loading NS in the BK is pretty easy.
  21. RustyShackleford

    RustyShackleford Minister of Fire

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    It looks like it would clear an 18"-high obstruction no problem, possibly 19".
    bend over works fine. It's tax season, so I'm in-shape for doing that.
  22. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Good catch on that thermostat.
  23. Highbeam

    Highbeam Minister of Fire

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    Note the hole in the butterfly. You can't shut this stove down.
  24. RustyShackleford

    RustyShackleford Minister of Fire

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    Well, you can shut it down low enough to get 20 hours, that's good enough for me !!

    The BK tech guy said EPA *required* them to put the hole there; I am not quite sure
    of their rationale. Obviously a person could partially or totally seal the hole if
    they were so inclined. You probably wouldn't want to do it with a bolt, I would
    think adding significant weight to the butterfly could affect its operation (since
    it's attached to a fairly flimsy spring - the bimetallic coil). Maybe some kind of
    tape that can withstand the high temps, silicon or something.
  25. Todd

    Todd Minister of Fire

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    aluminum tape? It would be interesting to see what she will do with that hole plugged.

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