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Summer time burning

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by sparke, Jan 6, 2008.

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  1. wdc1160

    wdc1160 New Member

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    First both you and I agree that we are talking about a new phenomena. High return temps are necessary-- you see it in all literature
    I know your correct that mixing valves are constantly touted as necessary, but I don't understand why < 120 water creates condensation. What is the principle that corrodes in this example.

    Water+creosote= caustic.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Let me take a crack at this....

    Wood is about 15% water by weight, even when it's dry. Combustion combines any hydrogen that's in the wood with oxygen to create water vapor. Thus, flue gas has a good amount of water vapor in it. There are other trace (and not so trace) elements which are soluble in water, or which react with water, to form corrosive compounds. If the temperature of the water jacket is cold enough, water vapor will condense out of the flue gas onto the cold metal. Combined with other compounds, this can cause corrosion. I would expect this to be most severe in spots where the water is cold and where the flue gases have had a chance to cool down or where there's not much flow - the bottom back corners of my boiler for instance.

    I've seen other boilers (non-gasification) that have rusted through in similar spots.
  3. wdc1160

    wdc1160 New Member

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    Nofo-

    I agree with everything you said, but how do we get the water jacket that cold. Are we pumping in cold water from non-use or under 120F

    Cool is not 120F
    Cool is below surronding environments temp.
  4. Nofossil

    Nofossil Moderator Emeritus

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    For this discussion, cool is below the dew point of the flue gas. With wet wood, that could be well over 100 degrees.

    When a cold zone opens up, I get a pulse of cold water. I try to keep the input over 140.
  5. wdc1160

    wdc1160 New Member

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    Yah, your right I just looked it up. 131.3 is the dew point inside the boiler with 18.6 percent humidity fuel. @ sea level.
    dew point of 265 with 50 percent water wood. Jeez.
    I really don't know how dangerous A dew point above boiling could be. Any insight?
    Is this right???
  6. wdc1160

    wdc1160 New Member

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    Do you think the water is still in the flue gas late in the burn?
  7. Nofossil

    Nofossil Moderator Emeritus

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    Towards the end I think it drops sharply. Once the wood has been reduced to a coal bed, all of the moisture has been driven off. In that case, the only moisture would be combustion byproducts, and I think that the hydrogen-bearing compounds in the wood have mostly been burned by this time as well.

    The EKO keeps the fan on high long after this point - usually until the coals are dead. That seems like just blowing hot air up the chimney to me.
  8. Eric Johnson

    Eric Johnson Mod Emeritus

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    Based on the white vapor that comes out of my chimney when it's below zero, I'd say that most of the moisture is front-loaded. It gradually diminishes until it disappears altogether about halfway through the burn. Makes sense when you think about it. I'd wager that by the end of the burn, there's very little moisture left in the firebox or the stack.
  9. wdc1160

    wdc1160 New Member

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    return temp corrosion
    1 Is this phonomina more likley in heavy idlers?
    2 Is this phonomina more likely to occur during summer or winter? If it makes a difference?

    Is it still a threat if you have a return mixer of 160 or higher?
  10. Eric Johnson

    Eric Johnson Mod Emeritus

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    From what I've been able to learn (mostly by asking the pros at another website), keeping your return water temps above 120 at all times is all you need to worry about. Most recommend shooting for 140 to be on the safe side.

    On my system, I have a circulator connected to the controller, which pumps water from the supply into the return at lower water temps. I have an aquastat on the return line that shuts off the main system pumps below 160. Working in concert, they keep my return water temps above 140 and maintain the boiler water temp above 160, for optimum gasification. At least according to the guy who helped me design the piping. As a result, my boiler rarely sees return water temps below 160.
  11. Nofossil

    Nofossil Moderator Emeritus

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    I think I'll let this one wait for someone with actual knowledge.... naaaaah, who am I kidding?

    Might be more of a problem for idlers IF the circ is pulling in cold water from somewhere when there's no apparent demand. The EKO runs hotter when it's idling than when it's not. In fact, it idles only when the water jacket reaches 180. This might not be true if you were using the room temperature input on the EKO controller, but I don't know of anyone who is doing that. Other boilers may differ.

    I would expect that it would be a problem if there were prolonged periods where the inlet temp was too cold. That would be most likely if you were heating a large load that was very cold, such as a storage tank or a radiant slab. As far as I can tell, a return mixer should eliminate the risk except at startup. Startup is really short, so I don't see that as a real risk. 160 seems really high for a return temp. Cooler means more efficient / higher output, all other things being equal. I think the safe temp is no higher than 140.

    I would worry about summertime corrosion if the boiler were stored in a location with wide temperature swings and exposure to outdoors air. For example, my garage floor gets cool at night, and will get completely soaked with condensation if we have a warm and humid day. A boiler has lots of thermal mass and could see the same condensation. If there were any caustic compounds in the fly ask, that could make a nice wet poultice that might eat steel. Keep in mind that I don't actually know what I'm talking about here....
  12. Eric Johnson

    Eric Johnson Mod Emeritus

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    I should add that low temp return corrosion is, from what I've been told, only an issue when the boiler is firing. I asked the guys on The Wall about that because with my previous boiler, I was getting return water temps well below 120 at times on my idle gas-fired boiler, and almost always below 140. They said that as long as there is no combustion in the boiler, the return water temp doesn't matter. It may be a different story with a wood-fired unit.
  13. wdc1160

    wdc1160 New Member

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    Me niether, but I have not heard anyone anywhere even give good guesses. So I think we have to push forward.

    recap...
    We all agree a temp mixer must goto a long way towards stopping condensation.


    Agreed, very rare that this could be a problem.

    I am not sure what you mean here?? I was assuming circ on damper/fan off. I think that could happen correct? no mix valve.

    Eric I have heard the same thing. I think your also talking about on the water side of the boiler.

    At least NG boilers don't make as big a deal about it as their wood-fired counterparts do. Maybe it has the same effect.
  14. Eric Johnson

    Eric Johnson Mod Emeritus

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    I first heard about this issue from Craig last year when I was in the early stages of researching a gasifier. I've asked him to drop by and help us out.
  15. Nofossil

    Nofossil Moderator Emeritus

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    On my system, if there's no demand then there's no open zone valve and the circ is dead-ended - no flow. If there were a flow path that allowed cold water to reach the boiler AND the boiler controller knew that there was no demand and was therefore idling, you could have a problem - especially during prolonged idling. Wouldn't happen in my system, but let's say you have an outdoor boiler and you recirculate a small amount of water through poorly insulated underground lines when no zone is calling for heat and the boiler is idling. That could create a problem. I don't know how likely a setup that would be, but I'm often surprised by the configurations that people are running.

    I try to be cautious about ever making a blanket statement indicating 'that won't be a problem'.
  16. webbie

    webbie Seasoned Moderator Staff Member

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    I wish I had a scientific answer - my guess was that there were various issues involved....especially the tendency for the boiler to develop condensation inside the firebox. That would make the boiler rot in the usual way - from the "air" side to the water side. There was also the issue of the heat transfer...boilers are designed (like hot systems) to heat the water a certain amount on the way through (Delta-T?).

    There are many boilers which are designed for lower temp water, most of them I know of are cast-iron. Examples include Buderus and others with setback (outdoor) sensors. But being as I was mostly involved with Tarm (steel) stuff, I do know that temps below 140 were highly discouraged if you want the boiler to last for decades. As far as what happens off-season, there are two variations....those with backups (like Tarm Excel or 500) would often stay hot to provide DHW, and those without should probably have a light bulb hung in the box or something similar (dehumidifier in basement with boiler door opened, etc.)...

    As a further point of discussion, a large percentage of earlier Taylor waterstoves failed in less than five years. I suspect this was due to the same.

    There is a tendency for newer manufacturers of boilers (and this includes many!) to think they have reinvented the wheel and not do their homework on what the earlier problems were. Some of these companies (or their customers) may have a surprise coming down the pike, because engineering DOES matter! At Tarm, everything from the exact grade of the steel (German steel designed specifically for boilers) to the storage of the welding rod was controlled....very strictly. That was part of everything from their own internal QC, to the ISO and ASME standards.
  17. wdc1160

    wdc1160 New Member

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    This type of post is exactly the kind where you can't just say that will never happen.

    I am not going to bring it location of boiler or even outside wather.
    I was just think of ways you could drag water were it shouldn't be. A better way of saying it is work hard against the mix valve.
    I was thinking that a circ or 2 by the storage could create a vacuum on the boiler, if you didn't use a 4 way mix valve.

    Do 3 way valves fail this way? I figure most storage has alot of circ power surrounding it. Is this stretching it?
  18. tug hill rook

    tug hill rook New Member

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    Bummer, I wanted to heat my home and shop with a eko 60. then in summer, throw some ball valves and use the shop zone to heat my 20,000 gallon above ground pool. . will the unit run on high with one zone calling for heat 24hrs a day for a week? or will i need to find another way. Please help >HDWF THANKS THR


    (heavy duty wife factor)
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