Up in Smoke

[Laylow]

Up in Smoke

I was wondering how much heat is lost up the flue in wood burning heaters and how much could be recovered if creosote condensation wasn't a problem.

.02 BTU is required to raise the temperature of 1 ft³ of air 1°

Typical chimney draft might be about 72 CFM or 4320 CFH (based on 100% efficient 50,000 btu design that burns 7.215 lbs of wood per hour).

Winter design temperature for this example is 32°F

Lowest recommended flue temperature is 250°F

Delta T x CFH x .02 = # of BTUs exiting flue pipe

In this example 218° x 4320 x .02 BTU = 18835.2 BTUs

That's 5.52 KW or 7.4 HP

If we had a perfect heat exchanger and used 78°F room air to recover the heat then:

(250+78)/2=164  250-164=86  86x4320x.02=7430.4

If we then preheated the combustion air:

(164+32)/2=98 164-98=66 66x4320x.02=5702.4 for a total savings of 13,132.8 BTUs

Of course, if your exhaust contains creosote, you can't let your flue temperature drop below 250°F.  Some chimney professionals say not to let it drop below 325°F.  

Can anyone confirm, refute, or add to these numbers?  I was only able to find one source that described how many BTUs were required to raise the temperature of 1 ft³ of air and it didn't mention anything about humidity or air density.  Also, what is a typical chimney draft in a regular woodstove?  I imagine this could change quite a bit with the delta T.  Another figure that I found says that the air:fuel ratio for wood is typically 466ft³ per pound of wood.  If we burned 7.215 lbs of wood an hour that would be 56 CFM.

[maker of toys]

mm m m m. . . . don't forget that wood-burning devices usually supply considerable excess air, so your pre-heat  would actually net much more savings over the historical method of using indoor air for combustion. . .  and probably over your estimate above, based on 0'C intake air.

also, reducing the excess air would likely have an effect on creosote generation and condensation. . . but I'm not sufficiently informed to guess what that effect would be.

-Dan

[maker of toys]

ooops. . . forgot to mention that you can probably figure on room air being closer to 68'F, as anyone that has 78' air is probably thinking about turning down the heat. <G>

[willib]

we used to have a wood stove going all winter.

we made it from a beer keg , it was very air tight, and even had a baffle..

if we had welded fins to it i'm sure we could have gotten more BTU's out of it..

i seriously dont think that it used anywhere near 72 CFM, because we only had four small ( 7/16 to 1/2 " ) holes for combustion air intake.and we even had them closed off to some extent when it was going hot..

You also have to take into account (somwhere in your calculations) that superheated air is very thin..

[Tom in NH]

My wild guess is that you lose about 25 percent of your heat up the chimney of a wood stove. But there are so many variables it seems impossible to come up with a number that means anything. I remember standing on my roof and feeling the heat come out of my chimney when the wood stove was going. It felt a lot like a furnace hot air duct with warm air rushing out.

The chimney will absorb a lot of the heat and radiate it back into the house where the chimney runs through the house. The temperature of the flue will vary tremendously depending on what you're burning and how you have the damper adjusted. Burning at a rate of 7 pounds per hour will mean different heat outputs depending on the its moisture content. I can't imagine being able to control my fuel consumption so precisely. Maybe you could with one of those new pellet stoves, but with my wood stove the fuel consumption rate probably fits a bell shaped curve (i.e. a little at the beginning, then growing to a maximum, then slowly tapering off until the stove embers die).

The heat that comes off a chimney fire of burning creosote is almost unimaginable. If you could figure out how to bottle it and sell it, you'd give the Saudis a run for their money.

Your questions got me curious. I've burned wood for probably 30 years. My main issues with it included the following: how to adjust the damper so I'd still have coals glowing in the morning, what size chunks to throw in, what kind of wood to keep that burns hot and slow, how to get a fire started quickly and easily, where to find a secure source of good wood, how to keep it dry, how to get it cut and split without handling it too many times, and how to keep from burning my house down.

It absolutely never occurred to me to think about BTUs and stack temperature and air flow up the chimney, however I did recognize it was conventional wisdom to accept that some heat has to go up the chimney in order to keep the draft going as well as to minimize creosote. So I'm curious. Why are you trying to come up with these numbers? Are you trying to design a stove or something?

As one final point of interest, we started burning propane last winter. We still have the wood stove, but seldom use it now because it got really hard to find anyone to sell us firewood. I've noticed that when I do touch off the stove, I have all kinds of smoke problems that I never had before when the stove ran all the time. My conclusion is there must have been an enormous amount of heat stored in the chimney masonry that prevented the downdrafts. Now, the heat's not there and I have a terrible time with my stove back blowing smoke.  --tom

[terry5732]

Most chimney fires are from poorly made or deteriorating chimneys NOT build-up. A properly constructed chimney would not ignite a structure if it had a fire in it. Check out the double wall sheet metal chimneys made for zero clearance installation. Your intake / make-up air comes in through the outer jacket while exhaust goes up center. No sucking air throough every little leak in the house. Intake air preheated by exhaust. I'm sure I just upset a bunch of firefighters who really seem just hate any kind of fire.

[ghurd]

I had a chimney fire 20-some years ago.  

I fell asleep on the sofa. Woke up to a 'Whooosshhh' sound and an orange light in the room.  Maybe 10 feet of single wall pipe in the room, every inch bright red.  I closed the damper. All was good.  Inspection by authorities said no damage.

It was a newer built home. New chimney, double wall stack, etc. So I think you are correct.

Scared the crap out of me just the same!

G-

[richhagen]

In the high rise public housing developements in Chicago, they used to have fires in the incinerator shoots at at least one highrise almost daily.  It was rumored that when the shoots would get clogged, kids or sometimes an unscrupulous person responsible for clearing them would start a fire to prevent having to clean it.  (If you saw how nasty those things got in the heat of the summer, you would understand why someone might have incentive to take a shortcut)  Not one of those buildings ever burned down, and I'm not aware of any of those incinerator fires spreading to other  parts of the structure aside from garbage piled near a clogged incinerator.  The shoots were a combination of metal and concrete, and not sheet metal, but heavier guage stuff like 3/32 or 1/8th.  There was nothing combustable in the structure near them.  The only very serious problems occasionally caused by those fires was smoke escaping into galleries (open hallways) and then apartments.  The point is that the design of the chimney, to withstand, and disperse the heat, and the design of the structure, in addition to regular maintenance seem like the best defenses.  Rich Hagen

[Laylow]

The 72 CFM number is something I derived from http://www.repp.org/discussiongroups/resources/stoves/Anderson/GasifierLAMNET.pdf  I'm not sure that I'm interpereting the data correctly.

This is from http://hearth.com/econtent/index.php/articles/glossary_of_common_hearth_and_heating_terms/  AIR-TO-FUEL-RATIO--The ratio of air and fuel, by weight, in a solid fuel appliance. It should be noted that 1 pound of dry air occupies a volume of 13.315 cubic feet at 70 degrees F. Therefore, 35 pounds of air, the typical amount of air necessary to burn 1 pound of wood in a fireplace, occupies 466 cubic feet.

I keep seeing this 35:1 ratio in building codes but this is the only thing that seems to say it is the actual air to fuel ratio for burning wood.  This equates to the 56 CFM per 7.215 lbs.

I don't know what a good burn rate is to use.  I chose 7.215 lbs/hour because that would 50,000 btus of fuel.

I think this would be a full efficient burn which at my house wouldn't put any of the heat in the house.  If I want heat, I have to close the damper.  Closing the damper increases the amount of time the smoke spends in the flue which contributes to creosote build up.  It also drops the flue temperature down which can contribute to creosote condensation.  It can cause incomplete combustion which means more smoke and unburned fuel up the flue.

Tending a wood fire seems to be more of an art than a science and it can change with every wood stove.  I tried burning some black locust once.  It's a very dense wood so the btu content is high.  Once it's lit it burns forever, the trade off is how slowly it releases the heat.  I needed to work on the stove so I was trying to get this piece of black locust to burn out but it just wouldn't die.  12 hours later I look in on it and it still looks like a full log.  I poked at it and it turned out that it was like a big piece of burning charcoal.  I broke it up and covered it with ashes so I could take the flue apart.  If you could figure out how to use it as fire wood it would be the perfect tree to plant for a woodlot.  It turns out that woodworkers really like it for its grain and structural stability.  Supposedly untreated posts can last for 40 years.  The wood is flourescent green and orange.  Maybe the thing to do is save a couple pieces to throw on the fire around 7:00.

Yes, I've been thinking about building a gasifier furnace which is in a couple threads previous to this one in the heat section.

I have the same problem with cold chimneys.  What's even worse is a wet chimney.  I try to keep a little something going all the time just to help keep some heat in the chimney.

I hope no one has the idea that I am against wood heat.  Even in an inefficient woodstove wood is the cheapest fuel source available.  There's also a lot of potential for energy conservation.

[Tom in NH]

I would agree that a poorly constructed chimney will cause your house to burn down if you get a chimney fire in it. But it is a very dangerous proposition to believe that if you have a well constructed chimney you won't get a chimney fire. Creosote buildup most definitely provides the fuel for chimney fires. A spark from a piece of paper or other light glowing material flying up the chimney can ignite it and you will have a raging inferno, well constructed chimney or not.

I have a well constructed chimney of lined masonry. I had one chimney fire in it, which i got when I was late with the annual fall chimney cleaning and I was burning a wad of newspapers in the stove.

The fire created a roaring sound in the chimney. I could adjust the damper to increase or decrease the rate of burn, but it would not go out until every bit of creosote was burned up (My chimney was as clean as a whistle after that fire). There was a blue flame shooting up 2-3 feet out the top of the chimney. The chimney got so hot I couldn't touch it. Thank god I didn't have a horizontal run of metal stove pipe in the house, or I probably wouldn't be here right now.

Sometimes wood members near the burning chimney will get hot enough to smolder, and then will ignite even a couple days after your chimney fire has gone out.

Wood heat is a wonderful thing, but if you're thinking of burning wood, clean your chimney. Most people who have never had a chimney fire will clean their chimney once a year at best. Once you've had a fire, you'll do it at least twice a year, and you'll be very careful to run your stove hot so as to avoid creosote buildup. Don't believe that just because you have a sound chimney, you'll be safe from a chimney fire. -tom

[TomW]

AMEN!

[GaryGary]

Hi,

To confirm your 0.02 BTU/F-ft^3:

Air has a specific heat of 0.24 BTU/lb-F, and air weighs 0.075 lb/ft^3, so

(0.24 BTU/lb-F)(0.075 lb/ft^3) = 0.018 BTU to 1 ft^3 1F  -- close to your 0.02

Water to Steam Loss:

I think that one thing you might want to account for is the water that gets turned into steam (water vapor).  It takes 540 BTU to turn 1 lb of water into water vapor, and if the flue temperature is is above the water boiling point, the heat you put into making steam just goes up the flue.  If the flue temp can be be less than 212F, you can condense the water and recover the heat you put into it (like a condensing boiler or furnace does).  You would need to provide a drain for the water and contend with corrosion problems.

I'm not sure how much water comes off of your 7 lb/hr of wood -- there would be some moisture in the wood, and also water is probably one of the combustion products? (or not? -- I don't know how much Hydrogen (if any) there is in wood?)

Heat Exchanger:

In the heat exchanger calc you limit the cool down of the outgoing flue gases to 164F -- not sure why you would need to do this?  You have a more or less infinite supply of room air to blow over the flue -- ie it does not have to work like a counter flow heat exchanger with one flow heating and the other cooling?  Of course, this might be hard to do in practice.

Gary

[Laylow]

Gary,

Thanks, this is exactly the kind of stuff I wanted to know about.

I didn't pay very good attention in chemistry class so I wasn't very comfortable with calculating from specific heat.  I'll have to keep the formula in my little notebook.

I used the btu rating of 6930 BTUs/lb of wood with 20% moisture content.  It probably wouldn't be practical to try to recover heat from water vapor on this small of a scale but it is very interesting to think about.  I've been thinking about condensing the exhaust anyway though so maybe it could still be worked in there.  Combustion does require some water but I don't know how much.

For the heat exchanger, I was just going for simplicity.  You're right though, there is a lot more heat that you can pull out of the flue but I don't know how much.  The volume of air to be circulated needs to be calculated as well as heat loss of the living space.  Then you have to decide on a maximum supply air temperature and flow.  I don't know, that might be a fun problem to figure out.  There's probably a handy HVAC calculator somewhere that would give us all the answers.  What's the temperature rise and airflow required to replace X btus/h for x volume of living space?  Then figure out how efficient the heat exchanger would have to be to keep up.

[mikey ny]

Most of the new oil and gas boilers and furnace's are using an exhaust gas condensing system to obtain the maximum energy from the fuel that is burned. They actualy use pvc pipe as a vent or chimny. Most of them are 90% efficient. The control systems on these units are somewhat complicated, using redundant safety switches and relays. I am pretty sure some of us on this forum could duplicate the concept of one of these units and apply it to a wood burning boiler of some type. I was thinking of building a condencing coil and building it into the water jacket of my tarm boiler, then drawing the flue exhaust through the condencer with some type of fan. The high temp that goes through the condencer is a concern to me, but since it is already in the water I dont think it would be as big a concern as if it were not in the water. I envision an exhast fan coming on at low boiler temp and a damper in the flue closes at the same time so most to the exhaust goes through the condencer and out through a steel exhast tube back up into the chimny above the damper. When water temps reach a pre-set limit the fan shut off and the damper opens to let the fire breath up the stack. Fabrication of the condencer could be tricky, it would have accesability for cleaning. I may be able to find something in the scrap yard to modify with any luck.

[GaryGary]

Hi,

"Most of the new oil and gas boilers and furnace's are using an exhaust gas condensing system to obtain the maximum energy from the fuel that is burned. They actualy use pvc pipe as a vent or chimny. Most of them are 90% efficient. The control systems on these units are somewhat complicated, using redundant safety switches and relays. I am pretty sure some of us on this forum could duplicate the concept of one of these units and apply it to a wood burning boiler of some type. ..."

As you say, if you had a condensing "furnace" for wood, you could capture the heat from the water vapor in the exhaust.  In wood this water vapor would come from two sources:

1. - the moisture content of the wood (maybe 15 or 20% if its "dried"?)
   
2. - the hydrogen in the wood gases that combine with oxygen to make water -- this is listed as about 6% by weight here: http://www.woodenergy.ie/biomass_fuel/biomass2.asp
   
   

On a gas furnace, I believe that the hydrogen content of the natural gas is about 10%, so it seems like between the two sources of water in wood that the potential gain for a condensing wood furnace might be greater than what is achieved in a condensing gas furnance?

Seems like somebody must have tried this?

By the way, on the post above I said that you would gain 540 BTU/lb for each lb of water vapor in the exhaust gas that you condense to water -- this should have been 1030 BTU/lb -- oops!

This link has a plot of heating value of wood vs water content:

http://www.fao.org/documents/show_cdr.asp?url_file=/docrep/008/j0926e/J0926e06.htm

Gary

www.BuildItSolar.com

[mikey ny]

 So it really does seem to make sense to use or build a condensing wood appliance since it is almost impossible to collect enough wood and dry it properly before you burn it. Space is always a factor in storing enough wood to dry in addition to storing enough seasoned wood to burn. Idealy you need to store one season in advance. So if you had a condencing wood appliance the moisture content would not be as big a concern. I have not yet found any wood condencing stoves that are avialiable for purchace anywhere. I  am surprised that the wood burning industry is not up to date with the latest combustion technology. On the other hand if there were one availiable, no doubt, it would cost an arm and a leg. This whole concept really interests me since I have a bit of experiance installing and trouble shooting the gas fired condencing furnaces. This may be the ultimate summer project for me. I have access to a lot of scrapped furnace's and boilers to pick parts off. I would make a fire box similar to my tarm boiler. Using the under burn style adding a little air into the bottom of the exhaust will burn or maybee gasify some of the smoke first, i would then capture that 400 to 700 degree heat escaping up the stack and run it through the condencer placed in the water jacket to achieve optimum efficiency.

                                                   mike

[GaryGary]

Hi,

That does sound like a worthwhile (but not easy) project.

I think that doing some more searching for what has been done in the area of condensing wood burners would be a good idea.  Its hard to believe that no has worked on this.

I'll also try some searching on this, and let you know if I find anything.

Gary

www.BuildItSolar.com