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Trying to Understand Modulation

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Eric Johnson, Feb 1, 2008.

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  1. Eric Johnson

    Eric Johnson Mod Emeritus

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    Some recent excellent discussions about Veissmann's (as yet unavailable in the USA) modulating wood-fired boiler (I think it's the Vitolig 200) have got me pondering the whole concept of modulation. I guess, to start with the fundamentals, "modulation" can be defined as the boiler controller automatically adjusting the boiler's heat output to conditions. So, for example, a 100 KBTU/hour modulating boiler would actually be capable of producing variable heat output, effectively making the boiler many boilers in one.

    I know this is what the premium gas-fired boilers being sold today can do, but I guess I don't understand why. Presumably there's some advantage in running a gas boiler for a longer period at a lower output, but I don't what that would be.

    When it comes to wood gasification, I can see some real potential advantages to modulation, namely lessening the need for hot water storage and allowing the boiler to run for longer periods with the same amount of fuel during periods of low demand, without idling. The Veissmann literature says that the Vitolig 200 achieves modulation by varying the draft blower speed, which doesn't sound like a big deal, but I assume there's a lot more to it than that. I also notice that the blower on that boiler is mounted on the exhaust side so that it's pulling instead of pushing, like the fans do on the more common gassifiers like many of us have. Somebody explained this the other day and their explanation made sense to me.

    Any thoughts or clarifications would be more than welcome.

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  2. solarguy

    solarguy New Member

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    The theory behind modulation is quite simple.
    When there is a large demand on the system the boiler will ramp up to 100%
    of it's firing rate. This firing rate is determined by the system water temperature.
    As you close the gap towards your high limit setting the micropressor control
    will modulate the firing rate down to as low as 30%. This saves energy
    by allowing the boiler to sip fuel instead of binge drinking. When we size
    heating systems, we'll spec a larger boiler & fire it at 30% of it's rate.
    What we've found is when a boiler is allowed to modulate, very rarely will it operate
    at full fire on a yearly basis.

    Tarm's pellet boilers modulate down to 15% (I think) of their firing rate.

    With the right control setup Ekos are prime to modulate due to their VFD combustion air fans.
  3. Eric Johnson

    Eric Johnson Mod Emeritus

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    Thanks for that excellent explanation, solarguy. Now I see why everyone wants to get away from the bang-bang on/off approach. Striving to keep everything on a gentler course would seem to be more efficient and easier on the (increasingly sensitive and delicate) equipment.

    I guess the EKO does modulate to some extent since, as you point out, its controller slows the fan speed when you near the high limit. What I think you're saying is that with a more sophisticated controller, the EKO could actually modulate across a much wider temp range with otherwise stock equipment. Pretty cool. I think nofossil and Indiana Bill are already hard at work on that.
  4. solarguy

    solarguy New Member

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    That's correct. If you could run an Eko or any other forced draft boiler say at 40% of it's firing rate over a longer period of time it would burn more efficient & you'd get a longer burn.
    With the right control you could dial it for 40, 60, 80 or 100% just based upon water temperature.

    I haven't had a chance to really wrap my head around the mutiple firing rates yet because I ain't got an Eko to play with yet. But I've considered a 2 stage boiler controller. Stage 1 would run @ 40% of the firing rate, when it could no longer keep up, stage 2 would kick in & ramp the fan up to 100% or whatever percentage you deemed necessary. A 2 stage boiler controller is around 700 bucks & I'm sure there's a cheaper way to do this.
  5. wdc1160

    wdc1160 New Member

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    Eric,

    I posted a similar thread about this before. I still have questions myself. And, after having done some research... I am going to attempt several explanations- Most unfounded , and likely not believable.
    I don't think that it would be "easy" to make an EKO mod.
    Furthermore, I don't think it can be done in the same way as a boiler designed especially for moding(at least the vito200 and HMX.)


    Here is why solarguy hop in if you don't think is correct.

    Ironically some of my best explanations from this came from engineers at GW.
    They face issues with efficiency that I believe are similar, because of their not having the ability to induce draft. For all intents and purposes it is very similar in nature to purposefully smothering. They address the problem by advising their clients not to burn wood to dry and not to small.

    To paraphrase:
    They loose a great deal of efficiency when people out of habit use small splits. The small splits loose gas to fast. Its not calibrated for that.(high surface area). It also has no way to artificially induce draft (speed combust). So high surface are and low moisture can easily have gases escape from the stack.

    According to NOFO some of the testing that’s done on EKO uses very high surface area. It can induce a draft. Clearly an advantage when you want a raging hot flame.

    Slowing the reaction down is tricky. To slow and your flue gases are turned to creosote on the walls of your stack.

    During most of the discussions here the mechanisms that people talk about when regulating combustion is air control. Controlling the oxidizer.
    A 30% reduction in heat output in an EKO by (smothering) results in similar results as an OWB. You would infact be approaching this using the same strategy.


    I think you’ll find that the HMX and vito200 have other mechanisms in addition to oxidizer control.

    1. They try to trap flue gas by regulating what type of gas escapes the flue.
    They try to do this by allowing gas to move slowly through SS pipes and allow the gas to “settle” or slow it speed enough to stratify into different densities.

    2. I think they try to expose limited wood to the gasification process at one time; The EKO equivalent of blocking off nozzles on the fly.
    This is proprietary/ trade secrets stuff. They don’t explain this anywhere. This comes from dealers. Does it make sense to you??
    Or, are they pulling my short Italian leg?
  6. solarguy

    solarguy New Member

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    I don't know ABGWD4U. Some of it makes sense but I tend to think you can modulate anything so long as you don't go below the minimum firing rate as set forth by the manufacturer. In the case of Eko, it's 40%. I've read the EKO literature & I'm under the impression you can manually dial in the combustion air fan for whatever percentage you'd like to run it at. If that's not true let me know. If it is true than you should be able to modulate the fan anywhere within the boilers temperature range.

    The cresote issue I think would be directly related to return/boiler shell water temperature. If you modulated too low & allowed the shell temperature to drop off below 130 or 140 that would have an effect on the gasification process & in turn make it a smoker. I also think draft also plays an important part in the equation. If your chimney has a marginal draft than modulating down could have a negative effect so that would be an important thing to check before deciding to proceed.


    I like thinking outside the box, sometimes it works, sometimes it doesn't.....

    Have a good weekend!
  7. wdc1160

    wdc1160 New Member

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    I have seen that posted by many smart people solar guy. But, if it were that easy wouldn’t everyone be using a modulating boiler? If all that is required is a fan that modulates and a demand sensor. I would have the damn thing in by now.
    We have some terms that are confused I think.

    Here are some of the axioms I will givem to you point by point if you disagree. I'll show you who I stole the idea from and we will see if we can figure it out cause it drives me crazy not knowing such a key feature.

    First is the definition of modulate. I consider modulation to increase and decrease output without "significantly" reducing efficiency. for this discussion lets say 10 percent. Gas boilers modulate. Pellet boilers modulate to a certain point. At idle I think much of their heat doesn't make it to the system.

    I think viessmann is saying at optimum "lab" we can get 92 percent efficiency. I will hopefully be able to get 80.

    Any lower than 70 percent and it doesn't modulate, now it is sustaining.
    My contention is that from 13KW to 40 KW the Viessman corp claims their boiler is at least 70% efficient. All things being equal (wood, draft, etc…)

    I don’t believe you can do that by controlling the air alone.

    Here is why. Quick disclaimer… This is not my area of expertise.


    How can the vito go down to 33 percent output w/ 70 percent + efficiency.


    Idea 1.
    @ 70% efficiency you cannot break out of gasification ie you cannot start having chemical reaction at low temps (like normal wood combustion).
    We have all read, and all agree two types of chem. Reactions occur during gasification. Combustion of low temp molecules and high temp molecules (endothermic and exothermic-- yada yada).
    A ton of people have weighted in on this topic. I don’t want this to turn into one of those theory discussions. I take it as gospel truth that certain conditions must be present.

    Idea 2.
    Air starvation stops chemical reactions from occurring. Enough starvation and you break out of Gassfication. Also a ton of theory and conjecture about the amounts and finer points have been posted online about this. However, I have never been able to get the numbers to jive where an EKO running at @ 40 % output and still be gasifying.
    Nofo’s graphs somewhat slam this idea home --if you’re a skeptic. But he almost never runs @ 40 output unless he messes something up.




    Idea3
    @ 33 percent output a large portion of the wood just cooks. During the cooking flammable gases are given off. If conditions (ambient temp and O2) are not correct it cannot combust-- it goes up the pipe. Often we talk about wood Mo content.
    The Mo steals our energy for our chem. reactions and we don’t have the juice to get full temp during some periods of burn time. Not having air, not having energy/temp. These are the things as wood burners we control, and so we talk about them the greatest amount.

    We control these with our stacks, inducers, fuels type, mo content, and additives.
    When we are talking about gasifiers we are indirectly manipulating how and when our wood gas is released.


    Idea three is something I have never seen talked a great deal about. How to reclaim this gas; Prevent wood cooking in times of little need. How to preserve heat in the firebox to keep high temps for long periods so rapid gasification can be re-achieved.

    I think that these issue are going to be the most elusive when discussing modulating. Unfortunately. I only know 3 people have seen these in action. All say it is different. But, none can tell in great detail why.
  8. Bob Rohr

    Bob Rohr Minister of Fire

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    I know the gas boiler modulation folks struggle with ideal operating efficiencies across the modulation range. i had a good chat with the EBM Papst folks at AHR. www.ebmpapst.com They make most of the variable speed motors and control boards for mod con boilers. They recently bought a control company and are planning on the next generation of motors and control.

    The trick is the unknown and ever changing calorific content of the fuel we get. as more and more imported NG is blended the BTU content is all over the place.

    Theri new system is called LambdaConstant System, detects combustion quality and adjust accordingly. barometric pressure, wind effects, length of the flue, all have an effect on combustion.

    Sounds like a lot of technology to get the ideal combustion with a fairly stable fuel like gas or LP. I imagine wood fired combustion would be VERY hard to get a handle on.

    Time and money, I suppose. i can't imagine what the cost or complexity of this techonology would be. Ii wonder that it would be sell-able on a wood fired piece of equipment even if it could be made to perform reliably.

    hr
  9. heaterman

    heaterman Minister of Fire

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    Simple question/simple answer

    Think about your truck and how you drive down the road. Do you get somewhere by flooring the go pedal, getting up to high speed and then coasting with the key off, or do you "modulate" the gas to maintain a consistent speed? Which is the most fuel efficient way to get from point A to point B?

    A standard boiler of furnace operates just like the first scenario. A modulating boiler operates like the second.

    Got it?
  10. wdc1160

    wdc1160 New Member

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    Eric I appolgize for hijacking your thread.

    I understand what your meaning, but since a gas engine stops using gasoline when the key is turned off I think that the anaolgy falls short of some of the realities of gasification smothering.

    I think that it would be closer to using the break eventhough the vehicle has an extremely high idle rate. Say maybe it idels at 50-60% of max rpms.

    Its very hard to have solid fuel compared to liquid or gas, because they so naturally lend themselves to modulation. They simply do not introduce more fuel to combust. Its hard to imagine an internal combustion engine that works collectivly with all the gas in the tank at the same time.


    It certainly is. I don't think we know all the mechanisms they use.
  11. solarguy

    solarguy New Member

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    I agree with many of the points that have been made here & I second that statement
    about many people being much smarter than me but I still think it can be done. Maybe not to the extent of a fossil fueled boiler but any modulation that will extend the burn time in a wood boiler without causing cresote problems is an extra in my book.
    I'm flushing my OWB & next heating season we'll have an Eko in the basement. We'll start pushing the edge of the envelope, see what the old girl will do.
  12. Jersey Bill

    Jersey Bill Member

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    I have a KP Pyro 32, and very little storage (200 gallons). I am on the 3rd season. Its about as simple as a gassifier can be. The fan is on or off and the 2 air vents are either 1/2 or 2/3 closed. The rating is 109 MBH, but my best attempts to measure output show it at about 80 mbh
    I have been trying to modulate it down as far as I can because of the need to match output with need. The lowest sustained measured output in in the 40-50 MBH range with the gassification process working. Below that output, I can maintain the fire by keeping very little wood in the upper chamber, but I think that its too cool for gassification. The only way that I can check this is that the lower secondary chamber doesn't glow, so I am assuming that gassification is not "on". Also the chimney shows a light smoke.

    When its 24 deg outside, I can burn @ full output with the boiler output matched to the load, air closed 2/3, stack around 300. Below that outdoor temperature, I can push the output up by opening up the dampers to 1/2, and the stack temperature goes to 350.

    I am on the verge of buying a combustion gas analyizer. I have a line on a Testo 335 for $2500. It will do O2 and CO. NOx can be added later for another grand. An E- Instruments meter is $1000 less for the same stuff. Does anyone have any experience with these tools?

    I am trying to figure out a way to add 1000 gallons of storage to my system. The hard part is where to put it. I thought about the garage, but then my wife would have to park outside (and I would be sleeping there).
  13. Eric Johnson

    Eric Johnson Mod Emeritus

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    Wow, a KP Pryo. It's great to have you on the board, Bill. Where did you get it?
  14. wdc1160

    wdc1160 New Member

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    Anything that looks fishy in the description?
  15. Jersey Bill

    Jersey Bill Member

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    I got it from Zenon
  16. solarguy

    solarguy New Member

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    >Anything that looks fishy in the description?

    duh, I don't know, I guess so:)
  17. wdc1160

    wdc1160 New Member

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    solar, I was asking if you saw anything in my description you thought neeeded looking over?
  18. Bob Rohr

    Bob Rohr Minister of Fire

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    The EKO already has a modulating fan, and the control has outdoor reset potential built in.

    I think you would be better spending time and money on tweaking distribution efficiencies then trying to make them burn more efficiently?? power the entire system with a couple 40 watt circs for starters. Design around the lowest possible temperatures. Insulate all piping. these are inexpensive mods that show immediate and long term payback.

    I visited with dozen's of gasification manufacturers at the ISH Frankfurt fair. They all employee full time engineers to design and squeeze as much energy from solid fuel as possible. many have labs and equipment we have never even heard of to design with.

    I suspect a few more 5 points could be squeezed, but at what cost and complexity. I suspect the market would be very small if another grand or so of sensors and microprocessors were installed to squeeze more out of them.



    hr
  19. wdc1160

    wdc1160 New Member

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    sparks, this is stock from Viessmann. We haven't even started talking about making mods. We don't even know how it works????
  20. solarguy

    solarguy New Member

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    <solar, I was asking if you saw anything in my description you thought neeeded looking over?

    I'm sorry ABGWD4U, I don't understand the question. Sometimes I just don't see that deer under the stand.....
  21. wdc1160

    wdc1160 New Member

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    I was explaining what I am now beginning to believe is done to acheive modulation.
  22. solarguy

    solarguy New Member

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    Ok, I see the deer now:)

    For someone that claims this isn't his area of expertise you did a dam good job putting that thought into words.
  23. hkobus

    hkobus Member

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    Very good discussion and ideas. Working with the Eko 40 for the season now, I have to agree with Sparks. I think we may be better of to optimize the systems around the units rather than controlling the burn. There are just too many variables to control at the same time. I find that even through out one burn cycle the parameters change dramatically, not even accounting for issues like bridging and changing drafts and heat demands, and not to mention the fuel it self.
    Modulating the fan on the Eko doesn't seem to do enough, it takes to long for it to slow down and way to long to restart. If I were to make a change to this I think I would see if we could control the primary and secondary air independent from each other through out the burn cycle. They serve very different purposes, but I need to think and measure more about how to achieve that.
    As far a the Viessman units go, I believe they are build by other manufacturers under their own name with some minor mods to fit their own specs.

    Just my thoughts.
  24. wdc1160

    wdc1160 New Member

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    Very interesting. I am also in the camp of "Its complicated for me to do" I was hoping the manufacturer would do it. I was interested in how they could do it. But its less important that I understand, so long as they understand and can follow through. Do you think that the manufacturer could be mistating or embellishing that they can modulate??
  25. hkobus

    hkobus Member

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    I believe that most changes in equipment and inventions are innovation by users, that put this board and it contributors right in the heat of it.

    I think we are looking at marketing numbers and actual field performance and conditions that, at times, may not always be the same. That doesn't meen that we can't strife for optimal performance, I just have a hard time believing we can control all parameters or at least at reasonable cost. ( this must be the farmer in me coming out)
    The wood boiler market is also much smaller than natural gas or oil, and this will reduce the number of players and the amount spent on R&D;compared to the other fuel types. ( never mind our fuel being less user friendly)

    I also get a kick out of some of the pictures in some of the European flyers, where a boiler is placed in a laundry room with spotless white walls and open door to the living space. :lol:

    Henk.
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