Waste heat reclaimer or stack robber

[mikey ny]

 We all know stealing the excess heat off the chimny can be dangerous because of creosote build up and chimney fire's, but are there any other reasons not to. I still can't stand to see 600 degree's go up the stack. My chimney is schedual 40 steel pipe and it is in the detached garage, so a chimney fire is no big concern to me. Since my chimney is 6 inch steel pipe I would like to put another 8 or 10 inch pipe over it, maybe about 5 ft. long and cap the bot. and top around the 6 inch chimney. I would tap in a 3/4 inch fitting at the top and one at the bottom and run it back down to the boiler, thus having a water jacket around the chimney.  I think I would also weld a series of fins on the inside of the 10 inch pipe to help absord as much heat as possible. Since my boiler is a natural draft type I wonder if a cooler chimney will affect the draft, or will it suck the heat out of the boiler also? Is this just another idea that is not worth the effort?

[Stonebrain]

If you have good woodburningpractice,there

will be little creosote.Slow burning woodfire

without flames is catastrophic for creosote.

If you don't want to cool the smoke because of creosote

you have to count with a very poor rendement.

(creosote is wood that didn't burn)

The real dangerous thing is coalmonoxide.

If the fire is very low and choked it will make coalmonoxide

If at the same time there is no more draft because the

chimny is too cold,the coalmonoxide will find

his way out of the stove.Many more people dy because of

coalmonoxide than because of chimnyfires.

Keeping the preceeding ALLWAYS in mind,

you shouldn't cool down

1)if the stove will not work well because of lack of draft.

  (and will eventually become dangerous)

2)if you get condensation (=water) in the chimney.

If you get close to condition 2) without danger and with little

creosote you will have maximum rendement.

cheers,

stonebrain

 

[Darren73]

hey stonebrain,

The gas is carbon monoxide, it can be formed by incomplete combustion of any carbon based fuel.

take care

Darren

[electrondady1]

just in the process of creating an outdoor wood stove .

there is a secondary chamber to cool the smoke/gas to the point were you can put your hand on the stack .

i would think a lot of this creasote stuff/water vapor is condensing in there.

i'm thinking it will just drip back into the fire.

what happens when this stuff catches on fire.

[vawtman]

Hi Mikey

 I remember back when i was a kid crawling up on the roof trying to drop weights down the chimney to open it up. I remember my stepdad dumping kerosene at the base to clear it.Please dont do that.The chimney was exposed on the west side making things worse back when we had real winters.

 Back then the stove was a new type that could handle large logs and when the home didnt call for heat the inducer would shut off and the fire would smolder.Not good

 In ending accept the losses and cut more wood.

 Cant believe im still here.

[mikey ny]

Great input guys,

         My particular wood boiler is a bit undersized (as most should be) so it burns at wide open throttle almost all of the time so there is little or no creosote because it gasify's it on the way out.I have some water storage tanks also to pick up any excess heat generated on a warm day. I have great draft now but I wonder how it will be affected by a lower chimney temp. The whole project will not take a lot of time to complete and the materials are just about free so, based on what we all know I think is will be a worthwile effort.

                         thanks again

                                           Mikey

[bretco]

Hi Mikey,

Sounds like we both have similar systems. I built my woodstove/water heater (not boiler) out of 3/8 plate steel, lined completely with water jacket and plumbed to 500 gal stainless water tanks in basement. Anyway, my chimney flu is lined with 6", 10ga (.109' thick) tube we use at work for meteorological (wind measuring) towers.

Coming out of the firebox is 8" dia, gradually reduced to the 6" over 30" (cone shaped part) inside that cone I put a welded up heat exchanger "box" with fins on the outside. It's plumbed in series with the water jacket. So I'm sucking some of the flu heat into the water. Now, on the outside of the 8" to 6" reducer, I jacketed it completely, but not for water heating, but for AIR heating. I force cold air up through it and exit the hot air via aluminum dryer hose to a duct in our living room. The air comes out typ. at 180-200F. and can maintain our little 900 sq ft ranch here in Vermont at 65-66 when it's 20 outside. If more heat is need, we turn on the basement blower, through a truck radiator and use the water stored heat. It has been working well for over 3 yrs now and my next system is currently underway.

Anyway, creosote has not been a problem. Once a month, I go on the roof and run a brush down the tube once, and it looks like a gun barrel. I've been burning mostly pine and cottonwood (free).

Anyway, like to hear more about your system

take care,

Bret

[gale]

This not a new idea, actually there are quite a few factory made designs.  Here is a website that talks of heating water off the stove pipe.  http://www.woodheat.org/dhw/dhw.htm another site about using the heat from a wood stove more effeciently, http://www.backwoodshome.com/articles2/lee90.html This one follows my ideas to increase effeciency with thermal mass.

I truely do not believe that you could steal enough heat off a stove pipe to be a danger.  Cresote builds its deposits, even when the fire(pipe) is hot.  I cannot believe you can steal enough heat to even have water reform.  It is sort of like having a cold muffler on your car, Or better yet a school bus.  some of those exhaust pipes can be 30ft long, and the tip is almost touchable with motor running for a hour, no extra carbon is built up, and no extra water is produced.  Wet fuel and a cold burning AKA oxygen starved fire produces creosote.

[elvin1949]

 Sound'a like a jet airplane engine up close.

Produces high heat ie-over 2000 degrees.Not nice to be around.

later

elvin

[Clifford]

I was actually wondering about the effects on updraft when heating off of the chimney pipe.

I was actually thinking about boring pipes through the middle of the chimney, but it sounds like a jacketed approach with circulated air or water would be better as it would make cleaning easier.

You certainly want some kind of a cap and screen on the chimney to catch paper & sparks.  It might also be an argument for flame resistant roofing (metal).

Anyway,

My question then was how the modern Catalytic Converters that many new wood stoves & fireplace inserts incorporate would affect the creosote buildup.

Of course, there is always the lighting phase before the Catalytic Converter kicks in.

---- Clifford -----

[mikey ny]

Bretco,

       My system consists of an old HS tarm boiler I removed from a basement in Vermont a few years ago. It is an underburn so it gasifies to some degree. Althogh it is an older model it still work real well. It is out in the garage ,the water  is piped into the basement through 2 inch pipe to my central hot water heating system. With a few relays it will change over to the gas boiler when the wood boiler temp gets too low. That rarely happens though. I presently have an air to air heat exchanger around the chimny to collect and re-direct hot air to the combustion air intake on the boiler. So far I have not had any creosote problems. The temp leaving the boiler is between 4 to 650 degrees and after the heat exchanger it is in the 2 to 300 range. I have 3000 sq. ft. old house and we burn about 10 to 12 cords a year. It also makes most of my domestic hot water in the winter.

                                                 Mike

[scesnick]

I also have a boiler in my detached garage and have thought about this to heat the garage. I have an AHS multifuel boiler ( coal/wood/oil backup) I burn about 90% coal. And I was wondering, Since coal does not put out any creosote, would there be any problem adding a " Majic Heater" into my 8" pipe to use some of the heat going up the stack? The only problem I would see is maybe it would cool my stack temp too much and cause a draft problem.

I have a combustion blower though, so maybe that would eliminate any draft problem that might occur...

[gale]

You are not going to have a draft problem, because you will be unable to pull enough heat off the pipe to cool the exhaust that much.  On a furnace or a hot water heater, with a draft fan, it is only needed when the pipe is stone cold, once the metal warms a little the fan is no longer needed.  As long as the exhaust gases are 40F above room temperature you will draft.  The only way you are going to cool the chimmney stack that much, that the pipe stays cold all the time and cooling the gases that much, is if it is heating water below room temperature, like water directly from a well.  

There is one exception, like when you start your stove and have trouble drafting with cold pipes, the pipes are going to stay cold longer.