Fire & Water: Some Thoughts on Wood Stove Design and Efficiency

February 18th, 2011 by Roy W. Spencer, Ph. D.

Fire & Water: Some Thoughts on Wood Stove Design and Efficiency

Sometimes I have to get away from the climate stuff for awhile. This is one of those times.

Also, each year at this time my wife asks how we can get our swimming pool to warm up quicker this spring. Even after 20 years, global warming hasn’t helped a darn bit.

She also always mentions wood heat as a possibility. I have always discounted the idea as too involved a project.

Well, this year we’re gonna git ‘er done. Last year I built a homemade solar pool heater. This year we are going to add some of that concentrated, carbon-based fuel to our energy portfolio.

After all, we DO have lots of wood available to us behind our house. Mature hardwoods, and the old trees just fall over and rot. I believe one of our white oaks dates to before our country WAS a country.

So, how to make a wood stove that can heat swimming pool water? Over the years, I’ve had enough experience with wood burning fireplaces, free-standing wood stoves, thermodynamics, radiative and convective heat transfer, buoyancy of heated air, etc., that I think I could help come up with a good stove design.

And ‘Uncle Lou’ (my wife’s sister’s husband) up in Sault Sainte Marie, Michigan has a lifetime of building and welding and fixing and fabricating. So, he’s helping me design a stainless steel wood stove with an outer water jacket that I’ll pump pool water through to heat the pool. We will use stainless steel to help keep iron out of the pool water.

Meanwhile, I’ve been reading about the newer EPA-certified stove designs – which is all you can buy anymore — that provide a hotter fire with more complete combustion of wood, rather than losing the gases and smoke out the chimney like the older “smoke dragon” designs do. I had no idea that (dry) wood could be so completely burned that there is little or no smoke at all. Cool!

The modern advance in wood stove technology is, simply put, to create a hotter fire with sufficient oxygen supply to burn all the wood and its byproducts.

To achieve this, the firebox is better insulated, and a pre-heated supply of air is made available in the upper portion of the firebox through perforated stainless steel secondary burn tubes so the wood gases and smoke can burn.

I’m sure many of you have these stoves, which are the only ones sold for inside residential use anymore. The secondary burn tubes produce beautiful, “ghost” flames, helping to ignite the wood gases and smoke that used to just go up the chimney.

So, this got me to thinking about the optimum stove design that would provide maximum efficiency, that is the maximum amount of heat energy from burning the wood transferred into your home (or my swimming pool water).

The goal is pretty simple: burn the wood and its gases as completely as possible and let as little heat escape out the chimney as possible. But even after hundreds of years of experience, people are still debating the best way to accomplish that.

I was thinking about the efficiency of a car engine as an analogy…but it is totally wrong. 100% efficiency for a car engine would be for all of the energy created by burning fuel to go into the mechanical work of pushing the pistons, turning the engine, and creating motion, with zero waste heat.

The wood stove is just the opposite, though. We want to create as much “waste” heat as possible, with as little mechanical energy as possible used to “push” the air through the system.

So, what are the limits to a 100% efficient wood stove?

First, you must recognize that you have to lose SOME heat out the chimney. It is the warm air in the chimney which provides the buoyancy (lift) needed to draw more air into the firebox. But the greater the volume of air flowing out the chimney, and the higher its temperature, the lower the efficiency of the stove for heating purposes.

Next, the higher temperatures required in the firebox for more complete combustion means more insulation, which means a reduction in the rate of heat flow to the room — which is opposite to the whole point of heating with a wood stove in the first place.

Now, I realize a hotter fire which is burning fuel more completely might actually lead to an increase in heat transferred to the room….but, all other things being equal, more insulation MUST, by itself, reduce the rate of heat flow compared to less insulation. Simple thermodynamics.

It’s an interesting dichotomy, trying to increase the efficiency of these stoves. On the one hand you need to MINIMIZE the loss of heat from the firebox in order to attain the high temperatures required for more complete combustion. But you also want to MAXIMIZE the loss of heat by the stove to the room. That’s the whole point of using the stove.

But this really isn’t a dichotomy if you realize that you are only insulating a portion of the stove – the firebox – to achieve the high temperatures and more complete combustion. If you can then route the hot gases leaving the firebox through a different part of the stove before going up the chimney, you then have the opportunity to extract the extra heat you generated from more complete combustion at the higher temperatures created within the (well insulated) firebox.

In other words, the firebox portion of the stove is primarily the energy generation portion of the system, and the rest of the stove that the hot gases pass through is the heat recovery portion of the system.

What is needed is a way to provide the hot gases leaving the firebox a greater opportunity to transfer their heat through the stove to its surroundings. A longer path through the stove, with multiple baffles conducting heat to the outside of the stove, would be one way to accomplish this.

Another would be to have a system of fins inside. Either way, you need to get the hot gas to come in contact with as much stove inner surface as possible, to maximize conduction of the heat to the outside of the stove, before all the heat goes up the chimney.

Now, obviously, you can’t remove so much heat from the exhaust that the air in the chimney is no longer buoyant, because then you will lose the stove’s “sucking” power for the fresh air it needs to burn the wood. An insulated chimney will help keep those gases as warm as possible through the entire path length of the chimney.

The air supply is of particular interest to me. (After all, I am a meteorologist. We know air.) Why doesn’t a bonfire, with an unlimited supply of fresh air, not burn all of the wood gases and smoke completely? It’s because as soon as a flame develops, it gets turbulently mixed with cooler ambient air, reducing the temperature of the mixture below what is necessary to burn the wood gases and smoke.

An analogy is the entrainment of environmental air into a convective cloud, which reduces the clouds ability to produce precipitation…a key component of the atmosphere’s heat engine.

So, in the modern wood stove they put tubes heated by the fire in the firebox to deliver an additional “secondary” source of air – very hot air — to the upper part of the firebox where the hot gases and smoke naturally collect. The pre-heating of the air is necessary for combustion of those gases to occur.

But after thinking and reading about this, I don’t really see the need for a distinction between “primary” and “secondary” air sources for a wood stove. All that is needed is a sufficient supply of pre-heated air to the whole fire. The secondary burn technology seems to me to be a retrofit to fix a problem that could just as easily have been fixed by reworking the primary air supply.

So, Uncle Lou and I have been discussing a way to preheat ALL of the air that enters the firebox, one that includes as its first ‘stop’ the window in the door, since a steady stream of hot fresh air is also needed to keep the window clean.

Of course, this is all in the design phase right now. Unfortunately, as Bert once told Ernie on Sesame Street regarding building a lemonade stand, “It’s easy to have ideas. It’s not so easy to make them work.”

So, if you don’t hear a progress report in a month or two, you’ll know the project was a failure. At least I don’t have to worry about burning the swimming pool down.

So, now the REAL stove experts out there can chime in and tell me where I’m wrong in my newbie analysis of wood stoves. It’s OK…I’m used to it.


60 Responses to “Fire & Water: Some Thoughts on Wood Stove Design and Efficiency”

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  1. Christopher Game says:

    I suppose, perhaps too ignorantly (I am definitely NO expert), that the heat is desired to be transferred to a flow of water, cold from the pool, warm back to the pool? I seem to recall that a counter-current arrangement can be good: the coolest water is exposed to the coolest exhaust air flow, and then, slightly warmed, it passes to warmer exhaust air flow, and so on? Perhaps the same principle will apply for the air pre-heating? (I am not remotely suggesting that you are wrong in your “newbie” analysis.)

    • The sides and top of the stove are a water jacket, so about 2/3 of the stove is, in effect, immersed in water. A pond pump pumps a few hundred gallons of water through it per hour. I calculate the temperature rise should be about 20 deg. F.

  2. sordnay says:

    I’m wondering if you have thought about including the swimingpool water pump (some sort of an stirling motor) within the stove desing…

  3. Richard LH says:

    An interesting problem.

    My thoughts, for what they are worth, fall in this sort of line.

    1. The exhaust temperature should be at or near the pool temperature for maximize the heat transfer to the water to occur. This will make natural drafted stoves difficult.

    2. The top of the boiler water jacket should be level with the base of the pool to allow for full thermosyphoning of the water though a small water pump can overcome that.

    3. Naturally drafted stoves, because they rely on the outside air temperature to create the draft, are always going to less efficient than forced draft stoves. (There is a need to consider the energy cost of the blower required of course).

    So in terms of pure efficiency then I would think that a positively drafted stove with an input air jacket followed by a water jacket in that order down the exhast would yield the greatest efficiency. Electricty requirements from Solar of course :-).

  4. TWW says:

    http://snorkel.com/

    These people put the stove under the surface of the water to heat hot tubs. Can swimming pools be big hot tubs?:)

  5. I hope it works out well for you. I enjoyed reading this.

  6. R Archibald says:

    Why all the work
    take a look at (island hot company)

    at islandhottub.com

    by the why I am a regular reader

    • OK, that is getting close to what I am talking about. It’s a thermosiphon design which can’t be used for my in-ground pool, but I’m sure a pump could be added.

      Great find…I’m glad to see someone has built such a thing. It seems like such an obvious design.

  7. Hmmm – how big is your pool, i.e. how many m³ of water do you need to warm up and to what temperature above the ambient. – Is it an “indoor” or is it an “outdoor” pool, or is it maybe a bit of each? – It is worth remembering that as soon as the pool-water starts to warm up, the atmosphere, knowing what it’s duty is, starts to steal the heat which was meant solely for the water.
    It is quite easy in spite of starting off building a stove that is by far too small for that kind of purpose to end up with a monstrosity that, for practical purposes, is far too big. And once your bit of “backyard forest” is gone – ? – It might be just as well to design a “dual fuel” stove while you are at it.

    Anyway, don’t think too many complicated thoughts about how to utilize the exhaust gases. Just incorporate a heat exchanger as a part of the chimney-stack.
    I am sure uncle Lou, can knock one of them up quite easily. All he needs are a few copper or brass tubes, for the hot exhaust gases to go through, a couple of ¾ “ brass plates (to be cut to size and drilled to accommodate however many copper/brass tubes are deemed to be necessary). Oh yes, the tubes will have to be expanded, end for end, into the holes in the plates and the now finished tube-stack will need some kind of housing for the pool-water to circulate through, – and that housing is going to need a connecting tube for water in from the pool – and a water outlet pipe to connect to the stove jacket from which the water must be pumped back to the pool.

    You will also need an air pump (a fan) to supply as much air, and therefore as much oxygen as possible to the combusting wood (or later, when your private forest is gone, – coal) because as you know the O2 in CO2 has always been there but by making sure you pump as many Os in as possible you get fewer single Os marrying up with the Cs.

    And my last tip is to make the chimney-stack 20 or even 30 meters high to make sure you keep any bad and unwanted fumes away from your own and nearest neighbours’ properties. – Keep complaints at a minimum – (I remember back in the 60es there were some talk about the Germans should be made to add an extra 5 meters to all their industrial smoke-stacks in order to send soot and bad gases off to the North Pole)

    Oh, there is nothing more refreshing than diving into a cool pool on a hot summer’s day. – Or so they say.

  8. Leonard Weinstein says:

    If you cover the pool with clear drop cloths, the evaporation cooling would be eliminated so that sitting in the Sunlight would much more rapidly heat the water. Circulation would mix this more uniformly.several floating balls could hold the clear sheet up from the surface and cut air cooling conduction/convection. Once it was warm enough, remove the cover.

  9. Or, maybe you were right in the first place when you said: “I have always discounted the idea as too involved a project.”

  10. DocMartyn says:

    I fail to see why you are heating water using your stove. You want to heat the water in your pool, this water presumably contains chlorine and anti-bacterials, not something I would want to heat to high temperatures. Moreover, we want an absolute maximum temperature of water entering the pool, we NEVER want to have the situation where a pump failure can allow very hot water or steam to hurt someone.

    Now what you want is to heat Roses Metal (50% bismuth, 25–28% lead and 22–25% tin) which has a melting point of 99 °C.

    You use your wood stove to heat your Roses metal, pumped using an automobile oil pump, into a LARGE heat exchanger, which heats you pool water. Heating a large thermal mass of pool water forefils the safety factor.
    The nice thing about Roses metal is that you can liquefy it by adding boiling water (or Brine).

    So have your stove, insulated, above the firebox have welded steel coils that act as you primary heat exchanger. The pipes run along a trench (which can be filled with hot water) to be covered with insulator, leading to the second heat exchanger, in a pit next to your pool.

  11. RiHo08 says:

    The “modern” woodburning stove has at least two chambers: 1) a delayed combustion chamber which burns the wood with low intake of air to create gases which rise to: 2) chamber 2 with plenty of air burning the gases generating high amounts of heat and sometimes 70% efficiency. The key to all this is “balance” (Karate Kid); balancing the amount of air into the lower delayed combuster chamber with the larger amounts of air into the upper heat chamber. To achieve balance, one has to fiddle, fiddle a lot to get the right inflows of air. The real problem in terms of indoor pollution when the stove is located inside a closed space like a home, is that there is no steady state. First one lights the fire to start the combustion process and so one needs lots of air until the logs have a high and steady combustion. Then there is a “steady’ state which really isn’t steady since the logs are not a uniform source of fuel as they have varying degrees of moisture, combustable surfaces, etc, etc, etc. During this process one has to open the firebox door to “knock the logs together; ie, stoke the fire.” After this “steady” state, the fire begins to burn down, less surface of logs requiring an adjustment of incoming air to both the lower and upper chambers. Eventually, one has to “open the firebox door” to load more logs in which of course leads to a mix of embers with fresh yet to be ignited logs.
    If one wants to achieve the 70% efficiency of combustion, one has to “tend” the fire all of the time as the conditions keep changing from moment to moment. You can load the firebox, start the fire and leave the fire to die out; of course, if the woodburning stove is indoors one gets very very warm and then cool off to very very cold. Now if you go to bed at the time the fire begins to die out, then you are under the covers for, a time. Getting up in the morning can be a chilly experience. If you are heating a swimming pool, then you need to cover the pool surface all the time except when you are directly using it. The heat of the woodburning stove will not sustain a large thermal heat sink. If the swimming pool is indoors and the woodburning stove is indoors, be prepared for a “dirty” interior and more respiratory symptoms for occupants as once the firebox is opened, all those gases from delayed combustion are really really toxic, soot borne. The “sweat” smell of a woodburning fire are the aldehydes including formaldehyde and acrolein. If you live in a valley, be prepared for air inversions which play havoc with outdoor air pollution and the fire tender needs to fiddle their day away.

  12. Terry says:

    Roy

    I do know a bit about this so here goes with some advice before you get too far down the construction path.

    1 The best design is to have the firebox completely separate from the heat recovery stage. That minimises flame chilling and poor combustion (wasteful). So you are looking to have a dual chamber design.

    2 The firebox needs to be lined with refractory bricks. Normal bricks will not do

    3 All air should be pre-heated after the heat recovery stage, and yes it is a good idea to “wash” the window with clean air.

    4 Good Wood combustion is best achieved using overfire air, rather than underfire air. So your firebox should have a solid bottom.

    5 The height of the flue will determine the natural draught and thus the air (and air to fuel ratio that is about 1.5 above stoichiometric for wood). For a typical 10kW burner you will have about 10 to 15m of flue. A damper can be used to regulate it (and the burn rate) but dampers are best used on the inlet air not the flue gas (they dont last long)

    6 Keep ALL cold surfaces out of the primary firebox.

    7 Depending on how technically advanced you are going to get with this, you will be aiming to have a flue gas O2 level of about 8 to 10% or a CO2 level of about 12 to 10%. A cute little combustion pgm is available to help at
    http://www.mediafire.com/?h3ca8tuppd0ee1t

    Cheers

  13. David says:

    The traditional reason for the two separate locations for air injection is that size of the fire and the cleanliness of the exhaust can be controlled independently. When used in heating a house a slow steady fire is useful. The fire is kept small by limiting air to the firebox. Restricting air this way tends to make the fire smoky. Heated air to the exhaust burns the smoke yielding independent control of heat output and cleanliness. Pool heating may not require this kind of independent regulation as a hot fire will just heat the water quicker.

    • yes, this is something I was wondering about. I guess what you said makes sense…as long the secondary air supply is “out of reach” of the primary portion of the fire…which seems to be the case in the Youtube videos I’ve seen of the secondary burn when the primary air supply is restricted

  14. Rick says:

    Roy,
    After reading your post I did some research and found “Rocket Stoves” which are very simple and efficient. They have a well insulated generally vertical firebox and, for heating water, a water jacket around the flue above the firebox.
    The proportions have been worked out for maximum efficiency and there is a book (about $5) that covers their design.
    Google Images has a lot of interesting designs.
    Rick

  15. John Galt says:

    Dr. Spencer,

    You’ve certainly gotten some advice on this project. I’m no expert, but I’ll leave a few comments anyway. You mention that insulating the firebox will reduce heat transfer. That’s not entirely true, the higher temperature differental will drive the heat transfer back up. As I’m sure you will know, at some point the heat of combustion flux (power, I guess) must equal the transfered heat flux or the temperature will keep rising. Now, in pratice, what is going to happen is more heat will leave via the hotter flue gas, which is where your secondary combustion and heat exchanger comes in. Since you are using water as a working fluid, it should not be hard to extract heat from the reburned flue gas.

    One thing I’d try to consider is that you may find you are condensing water from the flue gas. From a efficency point of view, this is good, that’s a huge latent heat. From a pratical standpoint, you need to provide some drainage path for it. Also, no matter how complete the combustion is, you’re probably going to have some tar-like byproducts in the flue gas, which your heat exchanger is going to condense as well.

    Going off into the ‘too complex’ catagory, I wonder if there’s some way of actually turning this thing into a heat engine or heat pump. Most involve moving parts, which one would like to avoid. There are no moving parts in the Electrolux process, used in the gas refrigerators still found in camper trailers. Might be an interesting project to apply for a gov’t grant for.

    I lived about 400 mi South of you a few years ago, on the Gulf. There are a some pools there with geothermal heat pumps. Simply big electric powered units which use well water (the water table is about 10 ft.) as the source and the pool water as a sink. They seem to compete well with gas fired heaters. Gulf Power provides very reasonably priced power, I expect you enjoy similar rates.

    • regarding the rate of heat transfer, yes I specifically addressed the fact that more insulation will lead to a hotter fire because of reduced heat conduction to the outside, which will lead to an increase in heat conduction because of the greater temperature differential. But you can never attain the same rate of heat transfer you had with LESS insulation. Otherwise, we would be putting on thicker winter coats to cool off more!

      Now, if you end up burning the fuel more completely, which probably does happen, it might overcome this effect.

      But even THEN you STILL have more insulation to transfer the heat through! That very fact means that if you want to recover the heat you “lost” by adding more insulation, it must be after the hot gases have left the firebox.

      Think about it…if you could achieve sufficiently high temperatures for complete combustion in the firebox with NO insulation at all, you would leave the insulation out, wouldn’t you?

      • Mike Edwards says:

        If you are going to recover the heat from the flue gases just bear in mind that those gases do generally contain some proportion of unburnt material, which can settle out in the flue as a tarry substance if the flue temperature is low enough.

        The tar inside the flue is then a possible source of a flue fire if at a later point it catches fire. This does happen for real in house stoves where the temperature of the flue gases is not high enough (eg when damp wood is burned).

  16. bubbagyro says:

    Roy:
    After having an almost disastrous flue/chimney fire, I switched back to the old baffle-free Fisher stove design and have had little creosote problems. The trick to using efficient stoves is to have bone dry wood only of a certain type. I do not have the time to do this.

    So, on balance, I think that using an inefficient stove suits me best. I can burn pretty green wood without a worry about my house catching fire.

  17. Terry says:

    Re Flue height

    1 or 2 meters will not work. You wont get sufficient natural draught to make it burn. On a wood stove you can get away with about 25Pa, so you will need a minimum of about 5m for a flue temp of 200C, unless you are going to use fan assisted air supply. Cheers

  18. BFL says:

    Don’t know how big your pool is, but I looked at various heating methods for an outdoor ~20,000 gal pool to extend the pool usage season. This is about 167,000 lbs and @1 btu/lb and 10 deg delta would = ~1.7million BTU not counting evaporative wind chill and heat loss through the sides and bottom and pipe circulation. At ~5500 BTU/lb for wood and an overall efficiency of 50% this is about 610 lbs of wood just to get the water up 10 deg. At 3000 lbs dry oak per cord this is about 1/5 cord. If I recall correctly a piece of split firewood for a fireplace weighs ~12 lbs and this would be about 51 of those pieces (an awful lot of wood ‘just to start’). I also considered an electrical heater as the next best bet and at about 500KWH x .10 = $50 dollars which is not bad since an immersed electric heater should have very high efficiency. However the wall,piping and evaporative losses are significant and could amount to a couple of hundred dollars (or more) per month in months outside summer.
    In the end I stuck with direct solar heating (sunshine).

  19. Joletaxi says:

    Hi Dr. Spencer

    It’s a little bit difficult for an “low average” educated people to try to give advises too a PHD.
    But,I’m an Belgian citizen and I’m very concern to get the best efficiency for my wood furnace(anyway,I’m enforced too do so,US is robbing all the oil everywhere,and thanks to the subprime,I ca’nt afford it anymore…I’m jocking..or maybe not….)

    First,I think it is not a good idea too make the poolwater circulating directly in the furnace
    The t° in the heatexchanger will remain too low, and You will experience very quickly tar spoiling ,and loosing largely the efficiency.On the order hand, poolwater;when heated can be very corrosive, and You will need an high grade alloy(US citizen are rich, but maybe a little less?)An electric pomp will be needed,and in case off electric leak, things can go worse.

    So the best way is too have un primary circuit,with an little circulating pomp, and an 3 ways valvle too maintain the best compromise T° between efficiency and good burning.
    On the primary circuit, You can provide all the savety device(in case off an pomp defect.
    I’m convinced that the flow generated by You regular pom will be too high, an again, the t° in the furnace will remain too low.
    The heat exchanger for the poolwater is not expensive(You will find a lot on ebay) and very effiecient.
    I’m amaze off the results off the EPA “regulator?
    RiHo08 is perfectly right on that.An other great achievemnt off white colars in offices with whool full carpets!

    I’have,in that old good days, make myself some furnaces an openfires.
    The best results where achieved with a full double skin furnace,and a expanse chamber too let the smokes remain and exchange a while before exhaust.
    Scheme was very simple.
    With that system, t° in the chamber was good controled by the exhaust valve,and when fully charged, it burned quietly for a full nigh,giving a large amount off heat, that mean the best efficiency.The tar spoiling,You ca’nt avoid that, was falling apart every dayn due too the changes in T°,and the fernace remained very clean.
    In the front,I managed two glass door,very pleasant.
    I’m pretty confident that is just an occasion too play with a new toy, because the remarks off BLF are tough.
    When I build this furnace,I was also gettin the conventional centralheating system in place.
    The results ware so good, that I never bought the oilburner!

    Anyway, thanks for your great job,and let’s hope that global warming is occuring,it has been very cold these last monyhs here

  20. Mike Edwards says:

    Regarding the use of a pumped system.

    You will need to think carefully about the safety of your design. Most woodburning stoves in the UK that are used to also provide a hot water feed do so using a “gravity feed” system, where the hot water jacket is piped to a coil within the hot water cylinder which is placed somewhere above the stove and where the water circulates in that circuit purely through “gravity” ie density differences between cooler and hotter water. Such systems are also typically “free vented” through the use of a header tank well above the level of the stove and hot water cylinder.

    One reason for this approach is to avoid the problem of what might happen if the pump in a pumped system failed. Overheating of the water in the jacket, a potential build up of pressure if the water starts to boil and a possible explosion are concerns to consider.

    The hassle involved with gravity feed systems put me off using a water jacket with my woodburner – so we simply heat the house with it, rather than the water. One 5kW rated woodburner keeps our 5 bed house nicely warm on all but the coldest days.

  21. Charles Higley says:

    Didn’t Ben Franklin’s stove design have large wide baffles or a longer exhaust gas path that increased heat exchange with the metal and to the room? My impression is that by actually decreasing the rate of exhaust gas flow he increased the heat of the fire and achieved a better burn.

  22. Fred Harwood says:

    Free wood heat does not exist 😉

    The quantity of wood required to do the job is large.

    You live in sunny climes, and pool heating is one job the sun does well.

    Have a look at:

    http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13230

  23. Dan Lacinski says:

    Roy,

    Pre heating the air can reduce the amount of oxygen in the air. All things equal cold air carries more oxygen. If you pre heat the air you need to compress it to keep the oxygen up. You need oxygen not just air. Also, you will be a slave to the wood burner. Keep working on solar and reduce the amount of heat lost in the pool.

    Dan

  24. Kevin says:

    Dr. Spencer,

    Welcome to the field of engineering. I think what you will find while designing your pool heater is that some rules apply (these are not made up the the engineers, but they have been detailed by the scientists);

    1) You can slow the “speed of heat” (ie: insulation)
    2) You can increase the “speed of heat” (ie: copper .vs. steel)
    3) You can achieve better performance by spending more money (within limits)
    4) The laws of Physics always determine the upper limit of rule #3
    5) You cannot achieve any form of “energy gain”

    Once you have broken rule #5 please let me know because I would like to invest in your pool heater.

    I like to think that engineering is largely a game of “push the gelatin around”. Every complex system I have worked with is very much like a balloon filled with gelatin. I can indeed push on one side and achieve a little higher “performance” on that side of the balloon, but the balloon always bulges someplace else and I have to spend more money for parts, energy, labor, maintenance, initial engineering, consumables etc.

    Cheers, Kevin.

  25. Terry says:

    Hi Roy
    Re your comment
    “But after thinking and reading about this, I don’t really see the need for a distinction between “primary” and “secondary” air sources for a wood stove. All that is needed is a sufficient supply of pre-heated air to the whole fire. The secondary burn technology seems to me to be a retrofit to fix a problem that could just as easily have been fixed by reworking the primary air supply.”

    That is not entirely true. One of the reasons for having separate prim and sec air, is so that you control the rate of burn with the primary air, in somewhat of a starved air condition. The secondary air is to burn the residual combustibles. If you add all the air in as primary then the fuel burns uncontrolably and without adequate residence time producing smoke and unburned gaseous fuel. So to control the burn rate, you limit the primary air first, then finish the job with the sec.

  26. Mac says:

    Roy,

    Perhaps you have already considered a different source of “free” heat, your HVAC system.

    Many btu’s are discarded with the exhaust air from your AC condensing unit outside your home. It would seem that most of that heat could be recovered with a “freon” to water heat exchanger placed at the compressor discharge, before or instead of the “freon” to air exchanger that is currently there. The “pool heater” could be placed in the heat discharge circut of a gound water heat pump system as well. Neiter option would add homeowner work load or fuel consumption. All you would need for equipment would be a pump for the water and a heat exhanger capable of withstanding the high pressures seen by the “freon” system.

    This would of course void any warrenty on the AC system but if competently installed there would be no reason to believe it would adversely affect the system.

    I have hot done the calculations regarding AC system heat discharge v pool heating requirements as both of those depend on local conditions.

  27. TWW says:

    Since it snowed up here today and I will have to dig out the firewood to haul it into the house before using it, I got reminded of your heater project. Beyond the engineering part is the hassle of cutting, splitting etc the fuel (I hope you have a big woodlot). My main reason for writing this is I remembered the little outdoor furnaces people use up here to heat their houses. I quick search on ‘outdoor furnace’ found a link where the ad claimed their stove could heat ones swimming pool. What you want to design may already be available. This is a sample: http://www.outdoor-wood-furnace-boiler.com/

  28. Vlasta says:

    Roy
    I have been reading your site with interest and I admire your explanations about the none existant human induced global warming , I do read RealClimate jokers too .
    On the other hand I wonder , why you spent so much time with the stove . Surely its correct .
    But there is a more efficiant way to heat a swimming pool .
    Nobody can make money out of this , becouse it was already pattented . Build a ‘fence’ around your pool , comprised of an inch pipes and let the sun do the job .

  29. Vlasta says:

    I dunno
    It pays to read all the posts , Fred above posted the same thing . I actually seen that on a roof before , but that was a plummer’s idea to heat his hot water needs .

  30. Alan says:

    Wouldn’t it be more fun to heat your swimming pool with coal? 🙂

  31. Chris says:

    Fire and Water does not mix. It is a bad idea for many of the above reasons to try to heat the pool water with a wood stove.
    However, here in Alaska solar is not an option. My brother got a hold of a large wooden keg about 7′ dia that at one time was used for tanning hides. He wanted a hot-tub. So he cut it down the middle and used one half for a nice sized water tub. So how to heat the water. Realize that we were/are poor money wise. Oil heat or propane is not an option but there is lots of wood available. He had a welder friend build him a stove that was designed to be submerged right in the tub. Only the loading door (about 2 or 3 inches above water line and maybe 10 inches round) and 4 inch dia flue 3 foot long stuck out above the water. Now here is the shocker…it was built out of aluminium plate and bolted to the bottom of the tub.
    Usually my brother would drain the tub after each use but if it was not too cold he would leave the water in and float 2 inch foam board on top. Sometimes he would have to break up some surface ice when he wanted to heat it up.
    It was a little hard to get a fire started as you might guess. But dry tinder and paper and it would get burning. It heated surprizingly fast once the fire got going. I enjoyed several nice visits with his family in his hot-tub. As far as I know he is still using it.

  32. Barry Gramm says:

    Nothing to add on the stove subject, but good luck. I just wanted to comment on “global warming” and did not see another vehicle to reach you. As I see it, your interest is limited to “climatology”, “atmospherics”, or “meterology.” The subject of “climate change” requires a much wider approach (I have read that you said you only ever took one course in astronomy and didn’t think much of it). The earth naturally produces only a miniscule amount of energy through volcanic activity. Nearly all of the heat energy effecting the earth comes from the sun, so the earth is reactive in heat energy, not active. Not to minimize it in any way, but climatology is concerned only with the complexities of what happens to this energy after the sun sends it to us. The active factor involves the solar system relationships to the earth (sun, moon, Jupiter and Saturn primarily, as well as the sun’s energy output itself) as these are what drive the amount of heat energy the earth receives at any given time, or over galactic time periods (cycles). These relationships also happen to drive some of the dynamics that concern you in climatology (such as gravitational forces producing the ocean tides and the various currents of water and air). To come up with a comprehensive explanation for “climate change” would require a top climate scientist such as yourself to team up with a top astronomer/astrophysicist to bring all the relevant factors together into the subject. Seems like a great research project to me, and probably a real good book too! I enjoy your work!

  33. Kevin says:

    Mac, not to diminish your suggestion, but to heat your pool (water has a high thermal capacity) with the waste heat from your AC unit (air has a low thermal capacity) would require maxing out the AC inside your house for a minimal heat gain in your pool. Slightly warmer pool == Frigid house & a huge electricity bill.

    Cheers, Kevin.

  34. J. Lanier says:

    One of the best sources of info on wood gasification boiler is Fred Seton….http://www.rohor.com/. I have no affiliation and nothing to gain by pointing you in his direction. His web site, is not flashy but the information is solid and plentiful. If you call, chances are he will pick up the phone. Good luck.

  35. DeNihilist says:

    Dr. Roy, check out this site Heating Help –

    http://www.heatinghelp.com/forum-category/76/THE-MAIN-WALL

    These people are the best of the best when it comes to hot water heating. Enjoy!

  36. M says:

    You might find these sites helpful. Masonery heater association and Hearth.com.
    I love these kinds of exersizes.
    peace

  37. Tom Blackwell says:

    I have built just the device you describe. Started with a Royall wood boiler that has wet back and sides, and added a heat recovey section on top that consists of 48 2″ diameter pipes with the leaving water from the boiler surrounding them. The flue gasses enter this section after secondary air has been added. The water side of this contraption heats a storage tank containing 2,000 gallons of water by thermosyphon and there is a copper heat exchanger bundle in the storage tank for pool heat. The boiler was pretty well worthless before the addition of the HR section, because it contained only radiant heat transfer surfaces and the flue gas temperature was above 800 degrees. The chimney is 40 feet tall and is 8″ welded pipe inside a masonry chase. So–we use wood to heat the storage water to heat the pool which acts as a large boiler to heat the atmosphere by evaporation.

  38. Bob Thomas says:

    Hi Roy – interesting article. I am more interested in using a slow combustion stove to produce steam and drive a turbine to generate electricity in the winter.

    Any ideas?

    Bob

  39. Davdi Hogan says:

    Dr. Spencer:
    Whew – you unleashed a torrent!! Here in upstate NY, I’ve used a Switzer (Dundee, NY) indoor water storage boiler for 15+ years. It is a water storage 2-pass fire tube boiler with 800 gal capacity. There are 5 zones in the house with conventional baseboard convectors. The theory is to run full blast for about 3 hrs (daily in this weather), shut down and run the 210F water down to about 140F. It is a closed system with a 30 psi press relief valve. The firebox has upper and lower draft doors and a 2-spd draft inducer fan at the inlet to the chimney. Everything is controlled by mechanical switches in the exhaust. After these 15 years the inside masonry chimney has zero creosote buildup. My observations on the subject of wood burning are:
    1)

  40. Davd Hogan says:

    (cont)
    I screwed up, apparently a tab is illegal
    1) the typo, my name is David
    2) I like the idea of a heat exchanger in the pool, if the boiler water is closed, then SS is not needed, the water quickly becomes inert and mild steel will last forever.
    3) I don’t worry about efficiency, keep the chimney hot, use room air for combustion and no preheat.
    4) Wood smoke condensate is evil, comes close to being the universal solvent.
    5) We live on 100 acres, but not all wooded. The Emerald Ash Borer is here and will result in an abundant supply of firewood for the next 10 years or so.
    6) My wife likes to work in the woods, this is probably my strongest argument for burning wood.
    Sorry this got so long, good luck with your project. I’ve also built several lo-tech heating stoves, great fun.
    DH

  41. papijo says:

    Hello,

    I am a now retired enginner living in France (sorry for my poor English) specialised in the design of waste and biomass powerplants. Here are my ideas regarding your project.

    1 – Closed combustion chamber : resulting gas temperatures
    Closing the combustion chamber is a very efficient way to control the quality and the rate of the combustion. However, limiting the air flow (and the chamber wall heat
    losses) will result in flame temperatures much higher than experienced in the case of an open air fire. It’s easy to understand that if you divide by 2 the combustion air
    quantity of your open fire, then the resulting temperature is multiplied by two (roughly) .

    With a stoechiometric combustion (0% excess air, 0% oxygen in the smoke), and a reasonably dry wood, the combustion temperature will exceed 2000°C (3600°F) which is enough to melt any steel, the wood ashes and ordinary grade refractory bricks !

    Fortunately, the combustion gases of your oven will never be perfectly mixed and you will not reach such high temperatures, but you will reach unusual high temperatures, unless the problem has been anticipated. I made a few other calculations:
    40% air excess (5% oxygen in the gases): 1550°C (2800°F)
    120% air excess (10% oxygen in the gases): 1150°C (2100°F)

    In industrial boilers, the totality of the walls is covered with water pipes in order to “cool the combustion gases”. Your project to have some heat exchange area in the
    combustion room is OK, but will certainly not be enough. An operation with an elevated air excess will also be needed, even if the efficiency is not “optimum”.

    2 – Walls heat insulation
    Refractory bricks are not in general “insulating” bricks. If you do not wish to have a nearly “hot red” steel casing, you should install an insulating material (provided that the temperatures are not too high, rock wool panels are OK, not glass fiber) between the refractory bricks and the steel walls.

    In addition, a “soft” material such as rock wool will accomodate the thermal expansion of the refractory bricks (up to 0.8 to 1%) at high temperature.

    Never install the insulating material on the outer face of the steel walls (unless the steel is able to withstand the gas temperature, which is the case only downstream the heat exchanger at the stack level)

    3 – Primary and secondary air
    This is needed. The primary air is used to control the heating rate (there must be a system to control the air flow) when the secondary air is used not only to finalise
    the combustion, but also to cool and well mix the gases.

    It is important to prevent the secondary air from reaching the burning wood, thus becoming an additional “primary air”. If you have a fan, or a good stack draught, it is
    easy to blow this air at the upper part of the flames. An other solution would be to have cold primary air which shall stay at the bottom of the combustion room and a
    hot, at least 150 to 200°C (300 to 400°F) secondary air which should stay above the cold air.

    In any case, final air flow adjustments should be possible during the start-up, and possibly a long time after !

    4 – Stack gas temperature
    The heat recovery efficiency is certainly important, but it is not the only criteria.

    The gas dew point is also an important problem. The theoretic condensation temperature of your gases should be about 50 to 60°C depending how dry your wood and what your excess combustion air are. However some pollutants such as sulfur, chlorine (if your home is close to the sea) and other chemicals resulting from a poor quality of combustion may rise it. I would not recommand to have the water temperature at the heat exchanger inlet (nearly equal to the tube metal temperature) lower than 75 to 80°C.
    Stainless steel is not considered as a good protection. When lower cooling temperatures are wanted (in so-called condensation boilers), then the only solutions are to use corrosion resistant tubes: teflon coated tubes, glass tubes, carbon fiber tubes, etc. , certainly too expensive for a homemade exchanger.

    Of course, the above considerations apply to an installation operated several thousands of hours per year. If you plan to use it only a few hours and if you accept to
    repair or replace periodically the damaged parts, maybe you could be less conservative, but I have no experience (in the old times, I have seen recipients of water made of galvanised steel used to heat up water on wood fires, and they lasted … many hours !)

    In addition, tar will attach on the too cold surfaces and drop the heat exchange coefficient.

    The available draft is also an other problem. If the draft is not enough, of course, the hot gases will flow upwards to the stack, but the air flow entering the combustion
    chamber will flow very slowly and the heating capacity will drop. The solution is then to use a fan for the combustion air, but then the danger is to have a gas pressure in the combustion room higher than the ambiant and combustion gases leaking in your house ! (increase the stack diameter)

    The maximum possible gas temperature must also be taken into account in the case of a water pump failure and a dry heat exchanger (or tar burning inside the stack). The stack pipe must be heat insulated (rock wool) and installed inside fireproof walls up to above the roof of the house for house fire prevention, with no combustible inside (pay attention to structure wood and floors)

    5 – Cleaning and repair access: needed everywhere

    6 – Water circuits arrangement
    If your water contains lime, it will be quite impossible to use direct heating. At metal temperatures above about 45°C, the lime will make scale deposits in your exchanger. (Example: if you have a water with 30 mg Ca/lit, then a 20 m3 pool may produce about 1 kg of scale of which at least 80% will stick to the exchanger).

    The solution is to have an intermediate loop circuit. This intermediate loop is also required if you wish to rise the water temperature at the heat exchanger inlet as recommended in the above item 4. A system must be installed to control the water temperature with the sensor installed preferably on the heat exchanger close to the water exit and the actuator actng on the primary air damper (example: http://www.woodboilers.com/admin/uploads/public/t05000en.pdf).

    Chlorine contained in the pool could be a problem even in the case of stainless steel if the tubes are frequently dried. The drying must be as fast as possible and well done.

    The circulation of water must be upwards in the “low speed” heated parts (in fact the heat exchanger), so that any bubble can escape. In downward pipes, a speed of 1 m/s is OK. All the pipes should be given a slope (about 1%) and a drain valve in the lower points (icing conditions)

    There must be no valve (or any flow restriction) from the inlet of the heat exchanger up to the free air, so that in the case of boiling the steam can escape freely with no pressurization (and possible explosion) of your installation. (boiling will allways occur when the pump fails)

    6 – Some figures:
    Dry wood (after drying at 105°C): 0% H2O – NCV about 18000 kJ/kg
    Wood “well dried” in ambient air will have about 20% H2O, wood fresly cut around 45% H2O
    Stoechiometric combustion air volume: 4.6 Nm3/kg

  42. Terry says:

    Good advice Papijo. See my earlier entries with some similar comments. Your practical advice is spot on.

  43. wayne job says:

    Sir, buried in the fire bricks of the hot and efficient part of your heat exchanger should be some copper piping to extract some heat, the grating below your fire can be copper tubing that extracts some radiant heat. Above your fire chamber a copper water jacket that can extend about six feet up the flue will ensure you capture most of the heat. This is very old and efficient technology. 1950ties but works a treat.

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  46. Dallas says:

    That is a good design, but if you want to approach maximum efficiency you should think co-generation. Use the flue to heat a smoker or dehydrator. 🙂 It may end up looking like a Rube Goldberg contraption, but homemade jerky is pretty tasty 🙂

  47. Dr. Spencer,

    A couple of ideas.

    Firstly, make the fire box long enough to take whole fence posts.

    Secondly, have a look at some considerations regarding EPA regulations and recommendations regarding the recent history of the relationship between wood-stove design-principles and EPA regulations, at http://www.chimneysweeponline.com/hocats.htm

  48. David Russell says:

    Dr. Spencer,

    Many good suggestions have been offered above, some have expressed concerns about corrosion and safety.

    While you didn’t say what stainless steel you were considering, or what water treatment regimen you use for your pool, I would like to add these warnings and observations.

    Never make your water cooled fired heat exchanger out of an austenitic stainless steel; because austenitic stainless steels are prone to intergranular corrosion and stress corrosion cracking under these conditions.

    Austenitic stainless steels do not conduct heat as well as feritic steels.

    Make sure that you comply with applicable codes of construction and provide adequately sized relief devices on the water side.

    Chlorine, other halogenated compounds, oxygen or oxides in boiler water will be corrosive to any metal.

    Seriously consider using closed loop cooling for the wood stove with a heat exchanger for the pool water.

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