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Home Made / Re: putting water heater tanks on top of firebox for boiler
« 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.
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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Home Made / Re: putting water heater tanks on top of firebox for boiler
« 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.
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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Home Made / Re: putting water heater tanks on top of firebox for boiler
« 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?
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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Home Made / Re: putting water heater tanks on top of firebox for boiler
« 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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Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 21, 2011, 10:49:26 AM »
RSI, you have a good point to use two pumps, I was already thinking of a battery backup. What I haven't explained clearly is that I will zigazg the exhaust pipe for about 60' horizontally before it goes out the shed because I think there's as much heat given off a horizontal stovepipe than from a stove. If the heat exchangers at the top of the room are cooler than the water jacket (and you are probably right about this), I could run my return (cool) water through the exchangers on their way to the firebox so they function as preheaters.
I am just a nut on efficiency. If the horiztontal flue captures another 25% efficeincy by putting together $100 worth of 6" galvanized round pipe, it means that I have to cut, carry and load 25% less wood over the course of a winter. I could wrap a pipe around the chimney, but I think a whole lot of horizontal flue will capture a lot more heat for a little more work. Do you agree?
I am just a nut on efficiency. If the horiztontal flue captures another 25% efficeincy by putting together $100 worth of 6" galvanized round pipe, it means that I have to cut, carry and load 25% less wood over the course of a winter. I could wrap a pipe around the chimney, but I think a whole lot of horizontal flue will capture a lot more heat for a little more work. Do you agree?
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Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 21, 2011, 09:51:52 AM »Dave,
I can see you are like me and others I know.We like to figure out a way to take a design and keep improving it to make it work.BUT At what point do you have to say why are we reinventing the wheel
I think you will be wasting btu's out the block walls,burn chamber roof?,flue pipe..You could easily capture all those btu's in a stove with a 2 or 3 pass exhaust in the water jacket.
Jackel440, you and WillieG are exactly right again. I was fixated on the free water heaters and blinded to spending just a little money to get a much more efficient system with a water jacket and heat exchangers.
Following this good advice, I will now build my own water jacket from 200 feet of 1" galvanized pipe that will sit inside the firebox and also function as a grate. I will abandon the thin steel of an oil tank as a firebox and make it concrete and firebrick.
I will zigzag the exhaust flue so that it radiates heat into the concrete shed and install 120,000 BTU/hr of air to water heat exchangers at the top of that space for heat absorption after the water passes through the jacket. I think the water jacket and heat exchangers provide enough reserve capacity for a safety margin if I don't overload the firebox. At 350,000 BTU's per cubic foot of wood, I should fill the firebox with no more than ten cubic feet and set the airflow to achieve a 12 hour burn, so the fire would generate about 300,000 BTU's per hour times my conversion efficiency (66%?) will give me perhaps 200,000 BTU's per hour. Perhaps capturing heat off the water jacket and also the 120,000 air to water heat exchanger is overkill but good for safety and only costs $200. Although 200' of 1" galvanized pipe as a water jacket with a 15 gpm flow in the firebox may already collect 350,000 BTUs per hour (see next paragraph) so the heat exchanger at the top of the shed may be redundant.
If the O.D. of 1" pipe is 1.31" and that times pi times 12" per foot is 50 square inches times 200' of pipe is 10,000 square inches, or 70 square feet. With water on one side the heat absorption is about 2 BTUs per square foot per hour per degree F difference. If incoming water is 80F and the firebox is 580F, then I am getting a heat transfer of 140 BTUs per square foot (2 BTUs times 70 square feet) times 500 degree differnce is 9,800 BTU's of gain per pass. If my 30 gpm pump effectively flows half that after pressure loss to the heat exchangers and pipe walls then 15 gpm on a total volume of 25 gallons (500' of 1" pipe at 5 gallons per 100'), then the water will make 36 passes per hour (15 gpm times 60 min per hour divided by 25 gallon volume). So my heat gain should be about 352,000 BTUs per hour just on the water jacket without counting the air to water heat exchanger at the top of the room.
Inside the house I will have three heat exchangers totalling 350,000 BTUs per hour and 300' of pex tubing. I can add extra radiators or pex tso that the indoor heat exchange is greater than the outdoor heat collection for safety. Now my head hurts from all the math. What am I missing?
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Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 19, 2011, 08:59:50 AM »
Jackel440, you have a good point. I will look for water sleeves to see if I can build a stove around that, in which case the water heater tanks would just be preheaters to try and capture more BTUs.
RSI, you are also correct that the steel is too thin on a fuel oil tank, I was going to line the bottom and up part of the sides with firebrick but this still may not extend the lifespan very much. I was hoping to get a couple of seasons out of it.
If I can't buy a separate water sleeve, why not make my own out of a header and 3/4" galvanized pipe inside the firebox? I could probably fit 30 pieces of 5' pipe around the top of the firebox. The inside end of the manifold (header) would assemble easily but the opposite end of the pipes would have to stick out through holes in the firebox and then be adapted to copper, because clockwise threads on the pipe only allow one side of the manifold to tighten. The other side either needs counterclockwise threads or brazed connections. I could drill holes with small tolerances but some smoke would still escape the holes where the pipe came out. To the best of my knowledge you can't get counterclockwise threaded pipe to make both ends of the header tighten.
I hope my description gets the idea across without needing a diagram. Basically the inside of the firebox would have a header at the back end and many pipes running out the front to a collection header which sends water back into the house and the front would have to be copper.
RSI, you are also correct that the steel is too thin on a fuel oil tank, I was going to line the bottom and up part of the sides with firebrick but this still may not extend the lifespan very much. I was hoping to get a couple of seasons out of it.
If I can't buy a separate water sleeve, why not make my own out of a header and 3/4" galvanized pipe inside the firebox? I could probably fit 30 pieces of 5' pipe around the top of the firebox. The inside end of the manifold (header) would assemble easily but the opposite end of the pipes would have to stick out through holes in the firebox and then be adapted to copper, because clockwise threads on the pipe only allow one side of the manifold to tighten. The other side either needs counterclockwise threads or brazed connections. I could drill holes with small tolerances but some smoke would still escape the holes where the pipe came out. To the best of my knowledge you can't get counterclockwise threaded pipe to make both ends of the header tighten.
I hope my description gets the idea across without needing a diagram. Basically the inside of the firebox would have a header at the back end and many pipes running out the front to a collection header which sends water back into the house and the front would have to be copper.
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Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 17, 2011, 05:37:13 PM »
Once again Willie, your advice is sound. I will propose this setup to some engineers for their input on heat transfer rates. It wouldn't be hard to put the absorbing pipes or water heaters under a shield or tray to concentrate heat around them. I will zigzag the stovepipe anyway for about 50 feet and this will put off a lot of heat. Dull red color is around 700 degrees and I don't think the stovepipe will get this hot but I can see it getting 500 or more, which would generate a lot of BTUs.
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Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 17, 2011, 12:44:06 AM »
Jackel440 and Willie,
Thanks guys, you make excellent points and you both have a good grasp on what I'm trying to do. When my I was young my father taught me that the insulated wall of a house lost 1 BTU per square foot/hour/degree difference F. I assumed that the conductivity of steel would result in a much more rapid heat transfer than 1.25 BTU/hour/sq. ft., but it appears I was wrong. Even a 300F difference between the water and the room would only absorb 56,000 BTUs per hour at this rate.
So basically I need to triple my surface area. I can think of a few options: coils of copper tubing or radiators near ceiling height, weld fins to the water heater tanks, run galvanized pipes through the firebox. Hot water baseboard radiators at the ceiling are probably the easiest heat absorbers and they are cheap. I could probably use them exclusively and forget the water heaters, they sure would be a lot easier to install. What do you think?
My winter gas bill gets as high as $500 in February and my typical monthly electric bill is around $150. I have one gas water heater and one electric. I'll use boiler water to preheat my well water before it gets to the house water heaters and I suspect that they may never kick on in the winter. I'd like to do a small solar hot water collecter for summer hot water. Hot water cost is less of an issue now that my kids are small but when they're teenagers taking one hour hot showers it will be a bigger deal. My house is actually 4,000 feet with a full sized walkout basement, so although the footage sounds high much of it is protected from heat loss.
You guys are "spot on" with your analysis! What else am I missing?
Thanks guys, you make excellent points and you both have a good grasp on what I'm trying to do. When my I was young my father taught me that the insulated wall of a house lost 1 BTU per square foot/hour/degree difference F. I assumed that the conductivity of steel would result in a much more rapid heat transfer than 1.25 BTU/hour/sq. ft., but it appears I was wrong. Even a 300F difference between the water and the room would only absorb 56,000 BTUs per hour at this rate.
So basically I need to triple my surface area. I can think of a few options: coils of copper tubing or radiators near ceiling height, weld fins to the water heater tanks, run galvanized pipes through the firebox. Hot water baseboard radiators at the ceiling are probably the easiest heat absorbers and they are cheap. I could probably use them exclusively and forget the water heaters, they sure would be a lot easier to install. What do you think?
My winter gas bill gets as high as $500 in February and my typical monthly electric bill is around $150. I have one gas water heater and one electric. I'll use boiler water to preheat my well water before it gets to the house water heaters and I suspect that they may never kick on in the winter. I'd like to do a small solar hot water collecter for summer hot water. Hot water cost is less of an issue now that my kids are small but when they're teenagers taking one hour hot showers it will be a bigger deal. My house is actually 4,000 feet with a full sized walkout basement, so although the footage sounds high much of it is protected from heat loss.
You guys are "spot on" with your analysis! What else am I missing?
10
Home Made / Re: putting water heater tanks on top of firebox for boiler
« on: September 16, 2011, 08:26:45 AM »
No mixups Willie, I think you got the concept just fine. 
I incorrectly stated pressure of 40 psi because I confused that with my house pressure. Actually it is an open system in which a circulating pump draws from the indoor reservoir, cycles it through the hot room and returns it to the reservoir. My wild guess is that a half horse pump will create 5 or 10 psi. If power failed to the pump and water circulation stopped there would be two safeties. First, there are 6 pressure relief valves on the water heaters and second, steam would flow out to the indoor reservoir which isn't pressurized. This system could even flow by convection without a pump, but the flow rate would be so slow that I wouldn't trust it around a firebox capable of a couple hundred thousand BTU's per hour.
Basically I am constructing a "poor boy's" water pipe for heat transfer because my maintenance business gives me cheap water heaters. Instead of running lots of small diameter pipe through the hot areas the water heaters are large diameter steel pipes for heat transfer. Each water heater has about 24 square feet of steel and about 40 gallons. Half a dozen provide 150 sq. feet of surface transferring radiant heat into about 250 gallons of water and the circulating pump should cycle this a couple times per hour. If I pick up 50F degrees per pass that's 100,000 BTU's per hour.
I could be wrong on this point, but my gut tells me large water volume is safer than pipes. Because a 3/4" pipe is so efficient in heat transfer (with it's high ratio of surface area to water volume) I can see it causing steam much easier than high water volume in 30" diameter water heaters. If the stove ran wild I think 250 gallons under slow convection turnover would have more safety margin than pipes which steam almost immediately. But now you've got me thinking to put in a battery backup for the circulation pump in this event, thanks for stimulating my little brain.
You're right that fins would create better heat transfer but the thin diameter of the water heater may not take welding too well, and the labor and steel cost may be better used by plumbing an additional free water heater. I am relying on large surface area, large water volume and the turnover rate to do the job because it' cheaper than a couple thousand feet of copper or galvanized pipe.
Now I've either answered your questions or raised a bunch of new problems. Which have I done?
I incorrectly stated pressure of 40 psi because I confused that with my house pressure. Actually it is an open system in which a circulating pump draws from the indoor reservoir, cycles it through the hot room and returns it to the reservoir. My wild guess is that a half horse pump will create 5 or 10 psi. If power failed to the pump and water circulation stopped there would be two safeties. First, there are 6 pressure relief valves on the water heaters and second, steam would flow out to the indoor reservoir which isn't pressurized. This system could even flow by convection without a pump, but the flow rate would be so slow that I wouldn't trust it around a firebox capable of a couple hundred thousand BTU's per hour.
Basically I am constructing a "poor boy's" water pipe for heat transfer because my maintenance business gives me cheap water heaters. Instead of running lots of small diameter pipe through the hot areas the water heaters are large diameter steel pipes for heat transfer. Each water heater has about 24 square feet of steel and about 40 gallons. Half a dozen provide 150 sq. feet of surface transferring radiant heat into about 250 gallons of water and the circulating pump should cycle this a couple times per hour. If I pick up 50F degrees per pass that's 100,000 BTU's per hour.
I could be wrong on this point, but my gut tells me large water volume is safer than pipes. Because a 3/4" pipe is so efficient in heat transfer (with it's high ratio of surface area to water volume) I can see it causing steam much easier than high water volume in 30" diameter water heaters. If the stove ran wild I think 250 gallons under slow convection turnover would have more safety margin than pipes which steam almost immediately. But now you've got me thinking to put in a battery backup for the circulation pump in this event, thanks for stimulating my little brain.
You're right that fins would create better heat transfer but the thin diameter of the water heater may not take welding too well, and the labor and steel cost may be better used by plumbing an additional free water heater. I am relying on large surface area, large water volume and the turnover rate to do the job because it' cheaper than a couple thousand feet of copper or galvanized pipe.
Now I've either answered your questions or raised a bunch of new problems. Which have I done?
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Home Made / putting water heater tanks on top of firebox for boiler
« on: September 15, 2011, 01:38:48 PM »
Hello folks,
I am building an outdoor wood boiler. The building is cinderblock and concrete. I am using a used 275 gallon steel oil tank for a firebox and will run a 40 psi pressurized water circulation system with either five 40 gallon lowboy elec water heater tanks above the firebox or half a dozen 80 gallon elec and gas water tanks mounted horizontally above the firebox. This room will get very hot, as I intend to run the stove pipe horizntally for many feet (it will actually have a slight upward slope). A cast iron circulating pump will cycle the heated water through heat exchangers in my furnace plenum and radiators and then into a 1,000 gallon indoor reservoir lined with EPDM to repeat the cycle. The circulating pump will be plumbed below water level. The cold water supply to the boiler room will be pex but everything in the boiler room and the hot return will be galvanized.
Will elec and gas water heaters mount horizontally? Will 6" 24 gauge stovepipe and elbows handle the heat? Is there anything on the water heaters that will melt, like the PRV's?
I have an 8,000 square foot home and a plentiful supply of free wood. I plan to leave the circulating pump and the two zone HVAC fans running continuously all winter. I live in Virginia. Many thanks in advance.
I am building an outdoor wood boiler. The building is cinderblock and concrete. I am using a used 275 gallon steel oil tank for a firebox and will run a 40 psi pressurized water circulation system with either five 40 gallon lowboy elec water heater tanks above the firebox or half a dozen 80 gallon elec and gas water tanks mounted horizontally above the firebox. This room will get very hot, as I intend to run the stove pipe horizntally for many feet (it will actually have a slight upward slope). A cast iron circulating pump will cycle the heated water through heat exchangers in my furnace plenum and radiators and then into a 1,000 gallon indoor reservoir lined with EPDM to repeat the cycle. The circulating pump will be plumbed below water level. The cold water supply to the boiler room will be pex but everything in the boiler room and the hot return will be galvanized.
Will elec and gas water heaters mount horizontally? Will 6" 24 gauge stovepipe and elbows handle the heat? Is there anything on the water heaters that will melt, like the PRV's?
I have an 8,000 square foot home and a plentiful supply of free wood. I plan to leave the circulating pump and the two zone HVAC fans running continuously all winter. I live in Virginia. Many thanks in advance.
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