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Heating incoming outside air

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by iceguy4, Jan 12, 2013.

  1. hoverfly

    hoverfly Minister of Fire

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    Just do not inhale, unless the pellets are made from hemp? :) Or does your stove have a name?

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  2. mepellet

    mepellet Minister of Fire

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    I am using Selkirk DT. When my stove is chugging along throwing out a lot of heat the piece that brings the outdoor air to the stove is hot. I like the looks of the Selkirk DT because it doesn't look like a dinky 3" or 4" pipe.
  3. fmsm

    fmsm Minister of Fire

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    As the box says "pellet stove cleaner". Funny thing I had my vent pipe cleaned just yesterday and there a a small amount of dry black creosote that came out. Maybe that chit really worked?
  4. DBCOOPER

    DBCOOPER Feeling the Heat

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    I asked for a reference and you post a picture of a book cover. Fire is an exothermic chemical reaction of rapid oxidation. We're not talking about steam boilers, but if there is an engineering calculation that's showing that preheating combustion air when burning solid fuel will provide increased efficencies in that book , I'm interested.

    I do know that if you provide a negative pressure and pass the byproducts of combustion thru water its not as harsh on your throat as if you don't.
    DexterDay likes this.
  5. iceguy4

    iceguy4 Minister of Fire

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    wow
    Can we call it fire?





    can we call it a Hooka
    smoke show likes this.
  6. jtakeman

    jtakeman Minister of Fire

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    AKA bong? Where's smoke, Me thinxs this might be up his alley! :p
    smoke show likes this.
  7. DemonGT

    DemonGT Member

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    Your stove may drop a few degree's with the cold outside air but on the plus side your not sucking in cold fridged air into your house through all the little cracks. ;)
  8. hoverfly

    hoverfly Minister of Fire

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    That book does not cover engineering aspects for preheating, which is odd since they talk about boiler design. However you can start here and here.
  9. DBCOOPER

    DBCOOPER Feeling the Heat

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    I have to believe that if you could increase efficency by preheating the air for combustion, the people that make and test these things would have figured that out. I think that the systems that have the outside air inside the exhaust do it for convience more than anything else. Just my gut feeling.
  10. iceguy4

    iceguy4 Minister of Fire

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    so what your saying is it takes no energy to heat the combustion air? I'm sure your not saying that. Any energy that is not used to raise the temp of the combustion air will go twards raising the efficency...it has to. No "gut feeling" just logic.
  11. Don2222

    Don2222 Minister of Fire

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    Yes, but that is wasted heat going out the exhaust, so why not use it to warm combustion air? Like burning trash for energy. :)
    StihlHead likes this.
  12. smoke show

    smoke show Guest

    [​IMG]

  13. iceguy4

    iceguy4 Minister of Fire

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    No don You mised my point. I know its gathering heat that would be otherwise wasted, I was asking him . if your combustion air is say zero degrees and it goes into the combustion chamber ...it is going to rob 100 degrees of the combustion energy.... that combustion air that goes in at 100 degrees wont.
  14. smoke show

    smoke show Guest

    sorry to intrude. Is that a fact?
  15. DexterDay

    DexterDay Guest


    I doubt that is fact....

    The flame that is say.... Around 1,500° isnt going to care if the incoming air is 70° or 20°… 95% of all air passes through the pot (give or take due to air wash and other burn pot bypasses) so that 95% of the air at a 50° difference isnt likely to disrupt a flame that hot??

    Maybe I am wrong. But OAKs provide cooler air. But my stove GAINED Efficiency when I installed it. Cooler air is more dense and contains more O2. Why do Super Charged engines and Turbo Charged engines try and get as cool as air as possible??

    Again. A flame that hot, will not care of such a minute difference when talking Thousands of Degrees.

    Maybe I am wrong. But until a fact comes up. The 50° air differential won't make a 1,100°-1,600° fire even sputter.
    DemonGT and smoke show like this.
  16. Gary Gileau

    Gary Gileau Member

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    I totally agree with DexterDay!! A lot of money is spent pre-cooling air to get more efficient combustion.
    DexterDay likes this.
  17. DBCOOPER

    DBCOOPER Feeling the Heat

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    No,what I'm saying is that combustion air doesn't have to be heated. All it has to do is provide oxygen to support the reaction.. Doesn't matter if its 0 degree air or 200 degree air once the fire is going. In every solid fuel fire I've ever seen, the more air(provided there is enough fuel) the hotter the fire. Its like turbo charging. Its how these things work. Kill the combustion fan while its running and its a smouldering pile of pellets, they won't even burn without enough oxygen.Sure the excess combustion air get heated and right out the exhaust it goes after transfering some heat to the heat exchanger.
    DexterDay likes this.
  18. phil san

    phil san Member

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    I have to agree. Have an enviro empress FI installed in an outside chimmney that takes it's combustion air from inside the house via two upper and 1 lower intake vents. 3 years ago I blocked off those vents ( rock wool and sheet metal). I cut a 3" hole in the exterior chimney for an OAK. The stove is much quieter, it is totally using cold outside air and I see NO difference
    In the heat output temp as measured by an IR gun. I am convinced I'm using outside air, no affect on convection temp and the house DEFINATELY heats up way better.
  19. iceguy4

    iceguy4 Minister of Fire

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    first off you are compairing apples and oranges. engines I know and I know them well

    I'll tell you why and no one can argue this point because I'm right
    cool air in more dense. when you compress air the heat becomes more dense too (hotter) If you take the heat in one cubic foot of air and make that 1/2 cubic foot the exact same heat will be in that 1/2 cubic foot. So all turbos, blowers, compressors will make the air hotter by doing just this. if you cool that air you can pack more into a cylinder. this makes the engine in effect act like a larger displacement engine and everyone knows there is "no replacement for displacement"
    a stove in NOT an engine and that is also a point you cant argue.

    this is the part you can argue

    pellet stoves supplie too much air (at least mine does) only 21% of what goes in is used ( the O2) the rest is of NO value to the process (N) so IMHO saying the more dense air contains more O2 dosent hold water since my stove supplies way more air then it needs for combustion.

    so pick my post apart and have fun with it
    SwineFlue likes this.
  20. StihlHead

    StihlHead Guest

    Well, colder air will require more energy than warm air to ignite the wood gasses. The differential is not zero... and I think you have to use absolute temp scale and not C or F to calculate actual energy differentials. That said I believe that the energy difference between warm and cold air burning is rather minimal. I do not think that air density alone (between warm and cold air at the same pressure) gets you much of a gain when comparing open flame to internal combustion burning. Denser (more) air does get you more energy when burning. However, most internal combustion gas engines have what, about a 10:1 compression ratio? That gives you a 10x factor for compressing air in a gas engine compared to open burning. So yes, there is going to be ~some~ energy gained using denser colder air, but not at nearly the rate as with a car engine.

    I also believe that your stove gained efficiency because you heat the OAK supply air once at the most efficient place to do that, in the wood fire. O/w you are drawing in air to burn that has been pre-heated in a less efficient way. The net effect is that the energy required to burn cold air is less than preheating warm air and then burning it. If you are using anything to pre-heat your OAK, you are robbing energy from somewhere. The way around that is to use heat that is being otherwise wasted, and that is at the flue. Pre-heating the OAK line from the radiant stove will be about the same as using inside pre-warmed air. It is secondary and less efficiently produced heat, and about 20% of your wood energy went up the flue to generate it. So the best way to pre-heat your OAK air is from the flue, where energy is being wasted and expelled from the system. Which is why a fresh air supply routed between a chimney and a metal flue is actually a good idea.

    From what I have read, the effectiveness of the fresh air secondary burners in EPA wood stoves is the more important factor in pre-heating air, and not the air temp exposed to mixing with wood gasses on the wood. For that reason most non-CAT EPA stoves have alternate routes to the secondaries rather than the OAK inlet. Ideally if OAK air is pre-heated, it should use waste energy from the flue gasses and not ambient energy from the heated house or radiant heat from the stove. Otherwise it is using less efficient heat to pre-heat the air before burning.
    SwineFlue and iceguy4 like this.
  21. DexterDay

    DexterDay Guest

    How hot is the flame? How hot is your air?

    Engines or Air pumps (pellet stove, yes an air pump, very similar to a Super charger)

    A pellet stove is a Giant air pump with a feed system. That's it. Nothing more. Nothing less.

    Not gonna pick your post apart. But your argument is basically that a regular OAK (cold air) will decrease my stoves efficiency?? Ludicrous.
  22. StihlHead

    StihlHead Guest

    I am in violent agreement with you on the main point here. I believe that an OAK is more efficient than a non-OAK wood burning device, overall. What I tried to point out is that the pressure difference between cold and warm air is going to be minimal in an open burning engine (which is what these devices are), when compared to an internal combustion engine which you use in your example (which have very high pressures). I also pointed out that warm air will take less energy than cold air to heat the wood gasses to burning temps. ~However~, when you take into account the fact that the (non-OAK) warm air has to be pre-heated, the efficiency drops below that of the OAK cold air supply wood burner (pellet, wood or sawdust).

    The main factor in an OAK improving efficiency is that you heat the outside air at the best place to heat the air; during combustion. From the perspective of the OP, if you have no OAK and use pre-heated house air to burn in your stove, it is basically the same as pre-heating air in the OAK if you use radiant stove heat or ambient inside house air to pre-heat the OAK (through a heat exchanger). The less efficiently produced heat from the stove is being used to pre-heat the OAK air, and the energy gains are negated. To get around losing energy that way, I suggest that if you are going to pre-heat your OAK air, use waste heat energy to pre-heat your OAK air from the flue as some have done here.

    Clear as mud?

    Added: Bottom line form energy perspective is that an OAK is better than no OAK. Heated OAK is no better than no OAK if heat is drawn from inside the house*. OAK pre-heated with waste heat is best, if it can be done. *except when used to reduce cold drafts in loose houses and relive pressure in tight houses, where OAKs are effective (though many web sites disagree with this).
    SwineFlue likes this.
  23. DexterDay

    DexterDay Guest

    Yep.... ;)
  24. hoverfly

    hoverfly Minister of Fire

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    Keep in mind that when it comes to heating appliance combustion theory Vs. engine combustion theory they function differently, but both use the same thermodynamic laws. One is to transfer convection, conduction, and radiation energy “heat” into your home, the heating appliance. For an engine it is to take expanding gases into mechanical energy while convection, conduction, and radiation heat is a waste by product. Now if you combine the two together you get what is called cogeneration. Where after mechanical energy is generated the waste heat is collected either to generate mechanical energy, like a steam turbine. Or some other purposes like heating your home. A typical pellet stove is not a cogeneration system, or a mechanical engine, it is just a heating system.

    One other thing, a pellet stove does not consume all the oxygen; it’s also used to carry out the combusted gases out as well. Unlike “in theory” engines do not have any oxygen left as it has been totally consumed.
    CtPaul and SwineFlue like this.
  25. pell it

    pell it Feeling the Heat

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    Imagine if C A T really spelled dog?
    smoke show likes this.

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