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5 phases of a wood fire

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Nofossil, Mar 5, 2008.

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  1. Nofossil

    Nofossil Moderator Emeritus

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    I've been doing some thinking about what goes on over the course of a wood fire, and I've started developing a control strategy based on this idea. I've broken the life of a fire into 5 phases, with different strategies for each.

    Phase 1 - Cold start. Boiler is cold, bypass is open, flue temperature is starting to rise.
    Turn on tank circulator and wood boiler circulator to flush cold water from the system and get boiler close to storage tank temperature (probably around 120 degrees).

    Phase 2 - Ramp up to operating temperature. Bypass is closed, boiler is not yet up to temperature.
    Tank circulator is off. Wood boiler recirc valve is open, and wood circulator is off or low speed. Optional heat loads (hot tub, DHW, storage tank) are off.

    Phase 3 - Primary heating. Boiler is in operating temperature range. Baseboards, hot tub, and / or domestic hot water need heat.
    Modulate wood circulator pump speed and optional heat loads to keep outlet temp within target range - near 180 degrees - to maximize baseboard effectiveness.

    Phase 4 - Storage tank charging. Primary heat loads are satisfied, boiler is still in operating range.
    Since all boiler output is going to storage tank, modulate pump speed to keep outlet temp just above tank temp (or inlet above 140, as needed). Lower outlet temps result in lower stack temps and increased efficiency.

    Phase 5 - Scavenging. Fire is out, outlet temp is below storage temp, but boiler still has usable heat.
    Don't worry about inlet temperature protection. Shut off circulator and wait for combination of demand and usable outlet temperature (140 degrees). Run circulator for five minutes. Repeat until outlet fails to reach 140 after 30 minute wait.

    Does any of this make sense? Does this represent a dangerous degree of obsession? Any other ideas? I have all of this implemented in code, and I've already tested most of it. I'd be more than glad to try any other ideas. As soon as I run another fire, I'll post a graph.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    I'm going to make you Lead Researcher for the Boiler Room, nofossil.
  3. Sting

    Sting Feeling the Heat

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    When will the program be available for retail?
  4. Nofossil

    Nofossil Moderator Emeritus

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    When there's a large enough market of wood boilers with the necessary sensors ;-)

    Seriously, I've had a lot of inquiries about instrumentation and control systems. I'm not interested in selling controllers - too much liability and support issues. I might see if I can get some simple circuit boards together for a reasonable price for folks who want to add thermocouples. Other details of my system are on my site free for anyone with the inclination and resources for the kind of tinkering involved.
    For any C programmers, the
    control code is here.
  5. MarcM

    MarcM New Member

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    I don't have a boiler (although I hope to within a couple years)... so just one question.... why does a lower boiler oulet temp result in a lower stack temp? I'm thinking that the more heat you remove from the final combustion gas, the lower the stack temperature (unless I'm getting mixed up somewhere?)
  6. Nofossil

    Nofossil Moderator Emeritus

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    Good observation. There are plenty of things that would cause lower outlet temps and higher flue temps - HX crud buildup, for instance.

    However, if you consider the same boiler with the same fire, it's a different story.

    All other things being equal, if you push more / colder water through the boiler, that will mean that the HX tubes are colder. That will allow more heat to be extracted, resulting in cooler flue gas. That's what I'm doing.
  7. JustWood

    JustWood Minister of Fire

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    BOILER MESSIAH indeed !!!!!!!!!!
  8. Sting

    Sting Feeling the Heat

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    I can open with notepad but I must be cyber stupid or need the correct programing software to take advantage of your hard work - and then - I most likely am not able to understand that language so there is my need to ask. I know if I have to ask how much -I cannot afford the preprogrammed controller so I best stay with my manual technology and "bang bang" system operation.
  9. MarcM

    MarcM New Member

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    Gotcha. That makes sense.

    Also curious after looking quickly at your very interesting website... have you ever tried supply your thermistors with +5V via Cat 5 from a PC power supply? I'm thinking in my house I might try it since I'll have Cat 5 running all over the place anyway... haven't decided on RTD's or thermistors yet... RTD's are a bit more expensive, but the cheap thermistors I find are only good for about +/- 5%... do you run into problems with yours approaching temps of 90C?
  10. Nofossil

    Nofossil Moderator Emeritus

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    Yeah, if you aren't a fairly hard-core programmer, the link above does no good at all. Part of the reason that I went the wau I did is because I've been doing that kind of programming for a lot of years.

    The computer itself is pretty cheap - under $200, but it's not trivial to get it configured and working.

    I'm hoping some of the things I figure out can be implemented in a simpler way.

    I bought glass encapsulated thermistors and calibrated them individually. I actually use Cat 5 for almost all my wiring, including the thermistors. I use a precision +5v reference that's run by the 12v supply that powers my computer (not a PC - it's a single-board embedded computer without a monitor, keyboard, or disk drive). No problems at the higher temps, though I use thermocouples for the really high temps - combustion and flue.
  11. Eric Johnson

    Eric Johnson Mod Emeritus

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    Do you think it could be programmed into the stock controller, or a retrofit? Might be something the mfgs would be interested in acquiring, once tested out and proven. Then the liability/support is up to them (I guess).
  12. MarcM

    MarcM New Member

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    Ah... the trouble I'm running into is I'd not only like to monitor and record temperatures from various places in my heating system, but in the living (and service) spaces throughout my house, as part of a control for the heating system and as part of a comprehensive fire protection system. Unfortunately, adc and analog output boards are soo expensive for the kind of channel capacity I'm looking for. My best hope right now is to maybe "inherit" (not pilfer) something from work.
  13. Nofossil

    Nofossil Moderator Emeritus

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    Perhaps. I know that some manufacturers are following my site. If they'd like to add this kind of functionality, that would be great. I still need to get hard data on how much difference it makes.

    The analog input boards that I use are from Technologics - the TS9700. 8 channels for $60. There are cheaper ways. There's a system called '1 wire' that ends up being pretty inexpensive and well suited to the data rates that a heating system needs.
  14. sparke

    sparke Minister of Fire

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  15. MarcM

    MarcM New Member

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    I've considered them, but they're not that cost effective/versatile for my particular goals. I'm trying to integrate everything I want to do into a centrally located pc.
  16. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Efficiency-wise, not a whole lot.

    Reliability-wise, though... that's where I'm hoping we can convince manufacturers to go for this sort of functionality. It may not be cost-effective on the residential market, but in the commercial market, the reduced "wear and tear" on the boiler that results from designing for quick heat-up and things like that could justify the added expense.

    Joe
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