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Author Topic: putting water heater tanks on top of firebox for boiler  (Read 9960 times)

RSI

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Re: putting water heater tanks on top of firebox for boiler
« Reply #15 on: September 21, 2011, 10:55:41 AM »

I would not run the exhaust that far. It will plug up very fast and probably catch fire.
Since you are trying to heat water you would capture a lot more heat with a shorter exhaust that is wrapped in pipe. The water in the pipe wrapped around it will capture heat a lot faster.
It would be way better to build a water jacketed exhaust but it sounds like you don't want to do any welding.
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #16 on: September 21, 2011, 03:00:38 PM »

you are gonna have a long horizontal exaust pipe...you are gonna have to clean it often..or as it clogs up with soot and creosete the heat capturing you are looking for will be greatly reduced and also if you can't keep it warm enough, then the draw on the stove might not be good..then you will get a really hot pipe at the beginnning of the run and if this pipe is dirty with creosete you are looking for a chimney fire!?
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RSI

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Re: putting water heater tanks on top of firebox for boiler
« Reply #17 on: September 21, 2011, 03:34:04 PM »

you are gonna have a long horizontal exaust pipe...you are gonna have to clean it often..or as it clogs up with soot and creosete the heat capturing you are looking for will be greatly reduced and also if you can't keep it warm enough, then the draw on the stove might not be good..then you will get a really hot pipe at the beginnning of the run and if this pipe is dirty with creosete you are looking for a chimney fire!?
Yeah, I would be surprised if it didn't get a chimney fairly fast. And a chimney fire in thin pipe would not be good. It would burn through pretty fast. Also a horizontal run would probably be leaking smoke at every joint as there really won't be any draft.
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #18 on: September 21, 2011, 09:37:29 PM »

you are gonna have a long horizontal exaust pipe...you are gonna have to clean it often..or as it clogs up with soot and creosete the heat capturing you are looking for will be greatly reduced and also if you can't keep it warm enough, then the draw on the stove might not be good..then you will get a really hot pipe at the beginnning of the run and if this pipe is dirty with creosete you are looking for a chimney fire!?
Yeah, I would be surprised if it didn't get a chimney fairly fast. And a chimney fire in thin pipe would not be good. It would burn through pretty fast. Also a horizontal run would probably be leaking smoke at every joint as there really won't be any draft.

Your points are well taken, nobody wants a chimney fire.  I'm only 50 and grew up in the age of cheap energy, but everyone I know of my father's generation went to a two room schoolhouse where a horizontal flue heated the whole school room during the 1930's depression.  When I ask advice from old timers they pooh pooh about the realistic probability of a chimney fire from a horizontal run unless one is burning a lot of pine.
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #19 on: September 21, 2011, 10:26:56 PM »

them old school houses had no insulation and the stoves themselves leaked like hell..they nver had a chance to build up crosete..they were mostly full bore all day with kids stuffing wood in...and in them days the wood was dry..they cut it well ahead of time becasue they depended on it. And I would be willing to bet the the pipes were cleaned a time or two when the students were not around.  I had an iniside wood furnace for some years and i like to think i burn dry wood, i still had to clean my pipes (2- 90's and a 5 foot straight section) each january thaw or i could count on a fire in that pipe before spring. i finally learned to leave the door open for 6 or 7 minutes (that's about how long it took me to have a smoke when i did that) each morning after stoking it..taht in turn gave me a small chimney fire  every now and then before the creosete could build up and cause a big roaring fire so when i did clean out the pipes it was jsut soot...if there ws tar in them i just put them on the ground and filled them with news[aper and lit it..that cleaned them good

i am not trying to discourage you from your project but just trying to point out things to watch for if you go ahead with it. and please, if you do, report your findings here. Any information from first hand experience is always interesting to hear
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #20 on: September 22, 2011, 05:12:44 AM »

Bill and RSI, although it may sound like I was dismissive on the chances of a chimney fire, common sense says that you two are correct.  I will research what minimum temps I need to keep the chimney at to prevent creosote buildup and let that guide the amount of horizontal pipe or the amount of water jacketing in the exhaust.

For example, if creosote buildup is prevented by exhaust temps of, say, 400 degrees F, then I could stick a thermostat in the stovepipe and keep adding horizontal run or water jacketing until I dropped the exhaust to this temperature but no lower. 

Now how do I get a water jacketed exhaust?  I can buy it if there is a reasonably priced one out there, I could bend galvanized or stainless pipes to run inside the first ten feet of the chimney, or I could (have a buddy) weld a 4" stainless pipe inside a 6" stainless pipe and run water between the two.  What would you recommend for a guy that can't weld a watertight joint?   
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #21 on: September 22, 2011, 08:07:12 AM »

For design efficiency, I read that thorough mixing of air and wood gives efficient combustion.  So does a small diameter box with long length achieve this best?  Say 20"W by 20" H by 96" long?  If combustion air enters through the front door it would travel 96" before exiting the back of the firebox. 

Another thing I read is to keep the primary firebox hot enough for complete combustion, then put water jackets in the secondary chamber where wood gases burn at 1100F.  So after air passes through the long firebox it would then travel 8' through the second chamber where I've installed as many water pipes as I could fit (or afford).  Then perhaps I water jacket enough chimney for heat recovery but not enough for creosote condensation. 

This is probably a crude explanation, can anyone improve on this?  Am I on the right track?  I've ordered 3 Jay W. Shelton books on efficient fireplace design but they are from the 1970's.  Is there improved desgin out there?  Other than gasification, which is beyond my DIY capabilities.
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RSI

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Re: putting water heater tanks on top of firebox for boiler
« Reply #22 on: September 22, 2011, 08:51:21 AM »

Bill and RSI, although it may sound like I was dismissive on the chances of a chimney fire, common sense says that you two are correct.  I will research what minimum temps I need to keep the chimney at to prevent creosote buildup and let that guide the amount of horizontal pipe or the amount of water jacketing in the exhaust.

For example, if creosote buildup is prevented by exhaust temps of, say, 400 degrees F, then I could stick a thermostat in the stovepipe and keep adding horizontal run or water jacketing until I dropped the exhaust to this temperature but no lower. 

Now how do I get a water jacketed exhaust?  I can buy it if there is a reasonably priced one out there, I could bend galvanized or stainless pipes to run inside the first ten feet of the chimney, or I could (have a buddy) weld a 4" stainless pipe inside a 6" stainless pipe and run water between the two.  What would you recommend for a guy that can't weld a watertight joint?
I am not saying anything won't work for sure, just pointing out stuff that is questionable. I am not trying to stop you from making anything.

You would probably have to get jacketed exhaust pipes made. I would use 4-6 pipes 3-4" diameter piped inside a larger pipe. You will also need a way to clean it. (probably run horizontal and have a box at the end that opens. The hotter and cleaner you can get the fire to burn the less buildup you would get.
A chimney fire is not a problem with heavy wall pipe with water around it because it won't do damage and you will also capture the heat from it.

I am not sure what it would cost to build but you could get it all set and have someone that can do water tight welds just weld it up.
If you can find used pipes it probably wouldn't cost much.
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #23 on: October 09, 2011, 10:02:14 AM »

Here's an update.  Thanks to the good advice on this fourm, I've given up on the idea of putting water heaters above the boiler, there just isn't enough steel surface area to transfer a lot of heat to the water.  I'm casting a refractory boiler out of plywood forms and I've gotten rid of the long firebox, it now looks to be about 24" by 24" by 30" long.  The secondary combustion chamber will be the same dimension.  Given my limited welding skills, instead I'm running about 100' of 1" black iron pipe through the secondary chamber for heat collection.  These pipes run 18" horizontally, then turn back and forth six or 8 times to create a baffle system and let the turbulent air flow give up as much heat as possible to the pipes.  I will also put in thin piece of steel behind the pipes to help the baffle effect. 

I will start with about 10' of horizontal stovepipe before the chimney.  Once I get a fire going and can measure the temperature of the exit gases, I will add or take away horizontal stovepipe until I have about 300F exit temperatures, to keep creosote from building up. 

I'll still put some heat exchangers on the ceiling of the concrete building to preheat water before it passes through the boiler.  I am also building a solar water heater that is big enough to supply my summer hot water.  The OWB will supply my winter space heating needs.  Both systems will share the heat sink of two 275 gallon hot water reservoirs, but will be run by two different circulating pumps.  The OWB will require a 30 gpm pump whereas the solar collector will use a much smaller pump for a system of at most half a million BTU's per day.

I will keep everyone updated, it is never too late to give me the benefit of your good experience to keep me from doing extremely stupid mistakes.  Thanks for all your input so far,you've prevented me from some serious stupidity. 
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yoderheating

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Re: putting water heater tanks on top of firebox for boiler
« Reply #24 on: October 09, 2011, 12:14:08 PM »

I don't think you want to run water pipes through your secondary burn chamber, at least if you are planning on designing it to work like a gassifier.  Running water lines through it will cool the gas and you need it to stay as hot as possible to get a clean burn. Most gassifier designs I have seen didn't try to cool the exhaust until after the secondary burn chamber.
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peacmar

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Re: putting water heater tanks on top of firebox for boiler
« Reply #25 on: October 13, 2011, 06:58:17 AM »

When I started reading this thread I was fearful for your safety but I'm glad to see that everybody's efforts have turned this into a very promising endeavour. Might I add, that a refractory lined water tube boiler is nothing new of a concept. But make sure you use thick enough tubing. My experience as a pipe fitter for a couple years taught me two things that pertain to this: heavier wall is better! And absolutely no fittings inside the fire box! Schedule 80 mild steel pipe is very common as steam can still developed in hot spots. And anywhere there is a threaded fitting there will be different rates of thermal expansion so there will be leaks for sure. A welded manifold and bent water tubes are the safest and most reliable way to go.
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