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Does a Pellet Boiler Need Storage?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by velvetfoot, Aug 30, 2012.

  1. velvetfoot

    velvetfoot Minister of Fire

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    I'm thinking it wouldn't, but could someone verify? Also nothing special for a chimney - right out the side of the house. Might save install cost anyway.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    A gasifier doesn't really need storage, but it will run better with it. I assume the same can be said for a pellet boiler, though the fact that it uses a much more consistent quality of fuel than a chunk wood gasifier, and can stop and start automatically as a result, rather than idling, probably makes the advantage minimal. Probably not worth the investment is my WAG.
  3. BoilerMan

    BoilerMan Minister of Fire

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    I'd say no for storage, however, if the place is extensivly zoned a 100 or so gallon buffer tank is benificial. I installed a PB105 for my uncle last year with four zones. One was a heated slab (big draw) one for indirect, ant two 20' baseboard zones. When a small draw baseboard zone was calling the boiler would fire for 20min and stop for 20 mine and back and forth. A buffer tank (old 80gal electric water heater for examlpe) relieves this short cycleing and reduces the cleaning of the boiler. I believe a buffer tank is benificial for all hydronic installs even oil.
    TS
  4. goosegunner

    goosegunner Minister of Fire

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    Maybe Heaterman will chime in, I believe he has posted before that he uses some type of storage even with his natural gas boiler.

    gg
  5. AndrewChurchill

    AndrewChurchill Minister of Fire

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    I'd say no as well. I have a Harman PB105. I believe the main reason wood gasifiers have added storage is it allows them to have only one or two fires a day and still have heat available during the times the fire is out.

    Since a pellet boiler is self feeding you don't really need the expense of adding storage. Might you gain some efficiency from using storage, possibly, but I doubt you would ever recoup your expense from the small savings you might see.
  6. velvetfoot

    velvetfoot Minister of Fire

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    The idea of a buffer tank to smooth out the short cycling is appealing.
  7. BoilerMan

    BoilerMan Minister of Fire

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    Heaterman will put it straight, but just about all hydronic applications benifit from buffer setups.

    TS
  8. SmokeEater

    SmokeEater Feeling the Heat

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    Velvetfoot, have a PB105 and as you all know it has an output of about 110,000 btu/hr on max output. Some days here in northern NY the demand from the boiler, including DWH, might exceed that number. My solution, and I don't really know if it will work as designed, was to install an indirect Smart 100 that the PB heats and use it as a buffer tank and a DHW source. My oil boiler backup also feeds the same buffer tank and can do so simultaneously with the pellet boiler to give me the opportunity to put over 240,000 btu/h to the buffer which supplies the heat demand and the DHW. I exchange the heat in the tank for the heat demand with a flat plate exchanger and a stainless Grundfos pump. The last winter wasn't really cold enough to "test" this design of mine, but I'm certain that time will come in the near future. Summary, in logical hypothesis, a fair sized buffer should be an advantage, and with a small footprint, over a hydronic system without one.
  9. heaterman

    heaterman Minister of Fire

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    I like to use at least a small buffer tank in the system somewhere to soak up heat when the unit reaches operating temp and shuts down. It prevents over temp situations and dumping your relief valve. Many of them operate at full fire rate and then turn down once set point it reached. If the temp continues to climb as is often the case when all the zones satisfy, you can easily get a pretty good spike in water temp which may trip your high limit or dump zone. Better to have even a small 30 or 40 gallon tank in the loop somewhere to act as a "shock absorber". I've used a water heater many times for just this purpose.

    As for serving the heating load from storage and in effect disconnecting the boiler itself from the load, you will find that very large volume storage is not needed when you are working with a pellet boiler with a decent control setup. Many of the better units have the ability to "turn down" or modulate the firing rate according to demand. This eliminates the need for hundreds of gallons of storage like you would see in a cord wood boiler to obtain optimum burn. Most of these units can get along very nicely with far less storage. My rule of thumb when heating out of storage with a pellet burner is roughly 100 gallons per 100,000 btu output.
    For heating systems that use scorched air, storage takes the "bumps" out of the water temperature cycle. These swings in water temp can lead to cold or lukewarm air blowing from the registers. In shoulder seasons especially, the boiler can be "off" or cold and have to play catch up when the system calls for heat. Very low "WAF" when cold air comes out of the register in the bathroom.
  10. EcoHeat

    EcoHeat Member

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    A pellet boiler like the Harman unit is going to benefit from a buffer tank because of reduced short-cycling. A smaller unit, like the EcoBoiler or similar is a closer match to the average heat load of average-sized homes, so short-cycling is less of an issue. However, even a smaller unit can benefit from a buffer tank if there are spikes in demand. An example of this is first thing in the morning after night-time set-back, when there may also be a spike in demand for hot water. A buffer tank is a reservoir of heat that can be drawn from to satisfy these spikes in demand.

    There are some alternatives that may work just as well, or nearly so. Having a programmable thermostat that starts heating the home before there's any demand for hot water reduces the demand spike. Another approach is to have a lag- or back-up boiler supplement through the demand spikes.
  11. peakbagger

    peakbagger Minister of Fire

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    There is an article this month in Northern Woodlands magazine about pellet based heating systems and how the NE states are encouraging their use. The folks from Tarm USA were quoted as recomending storage for pellet boilers.
  12. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Water storage yes/no is a requirement set by the pellet boiler manufacturer (to my knowledge)
    Froling - required
    Okofen - not required
    Windhager - not required
    I would say for a modulating pellet boiler it is not required.
    I can also understand that a small tank can absorb of the heat left in the pellet heater
  13. Former Farmer

    Former Farmer Feeling the Heat

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    No to additional storage on a PB-105. I added an old unused water heater into my boiler loop for additional storage thinking that it would help with the pellet consumption, but it actually takes more with the additional storage. The reason that it doesn't work is that the PB-105 will shut down after the ESP probe reaches a certain temperature. Every time the boiler cycles off in the automatic mode, the boiler will cool down some until the unit shuts itself down. This is because the combustion / exhaust fan runs until the unit cycles off and draws heat out of the unit and out the exhaust until the ESP probe senses that the unit is cool enough. The more storage that is circulating through the boiler, the more heat that is lost out the exhaust and the longer it takes the boiler to shutdown.
  14. TheMightyMoe

    TheMightyMoe Minister of Fire

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    Call for heat ends, circulator turns off, boilers goes into cool down, over heats, turns dump circulator on, dumps into storage, return keeps boiler warm, and it extends cool down? Lame.

    At what temperature does the dump turn on / off?

    Just want to know how it works.
  15. Former Farmer

    Former Farmer Feeling the Heat

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    I have a Lennox potable water boiler. It provides dhw, forced hot air via a water to air heat exchanger, and circulates water through a flatplate heat exchanger to provide heat to our in-floor heat loops. My PB105 is connected to the Lennox via a flatplate heat exchanger. The PB105 monitors the water temperature in the Lennox. The PB105 is set to turn on at 140* and turn off at 170*. I have the Lennox set to turn on at 130*. The Lennox unit is not designed to handle water temperatures over 170*. The storage that I added is tied directly into the PB105 boiler. I do not monitor any calls for heat on the PB105, only the water temperature in the Lennox. The circulator for the PB105 is on all of the time, to be able to transfer heat through the flateplate heat exchanger. The circulator for the Lennox turns on when the water temperature of the PB105 is above 130*. I do not turn off the circulator to the Lennox when it reaches 170* because if I get a call for forced air heat, the PB105 cannot start and get up to temperature quick enough so that the Lennox unit would not be wanting to fire.
  16. TheMightyMoe

    TheMightyMoe Minister of Fire

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    Ahhh... So it is situational. Was thinking why would you dump into a storage if it isnt adding heat.
  17. Chris Hoskin

    Chris Hoskin TarmSalesGuy

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    As others have said, any boiler is going to benefit from buffer and this is especially true of a solid fuel boiler. The least efficient part of any boiler's burn cycle is the start-up and a buffer minimizes on/off cycles and encourages long run times. Fröling did side by side testing and found that a boiler without buffer cycled 1700 times in a year/season and a boiler with buffer cycled only 700 times. Fuel consumption was also less with the buffer. A boiler like our Fröling P4 is self-igniting and can modulate down to 30% of its rated output, so buffer is not required, but best practice would still recommend an appropriately sized buffer tank. We recommend an ideal buffer volume of 20 gallons per 10,000 btu (boiler output) for the P4 pellet boiler, but, as I've said, buffer is not strictly required. By comparison we recommend a minimum buffer volume of 40 gallons per 10,000 btu and an ideal volume of 70 gallons per 10,000 btu for wood boilers when buffer is used.
  18. MaineEnergySystems

    MaineEnergySystems Member

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    Great Answers! Bottom line, PROPERLY SIZED and installed, a true modulating pellet boiler absolutely does not require mass storage. The size issue is huge. We constantly see oversized units installed. Probably the worst thing you can do for your emissions profile. Many a good system is creating more emissions than necessary because of this! An experienced heating technician that understands the benefits of biomass systems and the availability of various sized boilers will give you the best results.

    Another, often overlooked factor in all of this is the type of burner design. An underfed burner, will start and stop with little effort, and often no "re-start" at all. This is due to the presence of pre-pyrolyzed (wood that has already been through the necessary process to be ready for gasification or basically charcoal) remaining in the center of the burner, awaiting the next run cycle. Many other types of burners must completely dump out the burner by-products after each run cycle, and therefore start with “raw” wood at every start cycle. This “raw” wood / pellets is much more likely to smoke and create higher levels of co during the ignition cycle. For burners with the correct design, all of this talk of starts and stops being the “monster in the closet” just isn’t valid.

    Hope this helps you, and remember, if you have an indirect for DHW, this is storage in itself! Apply the above, skip the “storage” and save yourself some $$.
  19. boilermanjr

    boilermanjr Member

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    To be clear, Froling P4 pellet boilers do not require thermal storage. Still, we believe that thermal storage allows for the best operation of nearly all existing solid fuel euipment even if thermal storage is not required.

    I'll be careful here. None of us are interested in this conversation evolving into a he said she said between two boiler companies. I think more clarification would be helpful. First, whether a pellet burner smokes at the end of a burn cycle or at the beginning of a burn cycle, it makes more smoke when it cycles. Pellets do not become pyrolized without smoking. Different burner technologies have different advantages. It would be good for a neutral third party to construct a burner technology comparison chart.

    Dan is right about sizing. If we could always undersize pellet boilers, thermal storage would provide fewer benefits. The problem with central heating appliances is that we generally must provide heat sources that provide 100% of the heating load on the coldest day of the year. However, the average heating load throughout the heating season is about 30% of maximum for an average north country home. Consider that an average of 30% of maximum load indicates many heating periods below 30% load. While an average North Country home does not use 100,000 Btu/hr on a design temperature day, I will use that number for the following example. If a 100,000 Btu boiler has modulated down to 30% of its output, as many will, and is producing 30,000 Btu/hr. (the average load) and the house becomes satisfied, the boiler must shut off. Even a boiler producing 30,000 Btu/hr. and containing 30 gallons of water will rise 10 degrees in 5 minutes without a heating load. There is little time for a boiler to react. Because of the wide variability of heating loads we face in the north country, even modulating pellet boilers cycle on and off many times each season.

    I believe that it is undisputed that the dirtiest operation of a pellet boiler is during start up and shut down regardless of burner technology. This is also the least efficient operating period of a pellet boiler. Therefore, the less often a pellet boiler cycles on and off, the better it is operating. In typical North American homes that use bang/bang heating systems rather than modulating temperature, constant flow heating systems, cycling in pellet boilers is particularly prevalent. Modulation is not enough to handle periods where there is no heat demand at all. In Europe where Froling and Okofen boilers are developed, primarily low temperture heating systems are used and the boilers control the distribution. In these environments where the boiler knows what is calling and what the outdoor temperature is, it is much easier for the boiler to reduce cycling without thermal storage. We are working to advance these techniques, but these ideas aren't adopted over night. What I am trying to convey is that the decision about whether or not to use thermal storage is not so much related to the technology of the burner in the boiler as it is related to the system that the boiler will be installed into. There are many ways to mitigate cycling with the distribution design.

    One more point of clarification: It is not an either/or choice to buy a modulating pellet boiler or a boiler that will be connected to thermal storage. The best systems (regardless of existing boiler technology) will be connected to thermal storage and will modulate. These installations have dramatically reduced cycling. We tend to think of the of subject pellet boiler installation with/without thermal storage not as good or bad, but instead as good, better, best, and best+. There are so many other factors such as installation and sizing that will affect operation.

    In addition to the reduction of cycling, there are other advantages of thermal storage:

    1) A boiler will normally have fewer operating hours each year if it is connected to thermal storage.
    2) Thermal storage will allow a pellet heating system to respond instantly to a call for heat if the boiler has cycled off.
    3) While a domestic hot water tank is not generally considered thermal storage because it is maintained at temperature with little variation, thermal storage can also provide domestic hot water.
    4) On/off cycles use more power than during constant run operation.
    5) Less ash is produced.
    6) Heat exchage surfaces are cleaner.

    I hope this helps.

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