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Author Topic: putting water heater tanks on top of firebox for boiler  (Read 2882 times)

woodboiler dave

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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.
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #1 on: September 15, 2011, 07:11:56 PM »

to run a hot water heating system at 40 lbs. psi is beyond me..not likley legal and without proper design and placement of relief valves and such  most dangerous.

if your stove were to run wild for some reason, water at 40 psi would not boil until it reached about 267 degrees F.  most home boiler systems i have seen don't run over 12 psi or maybe 16 (building with upper floors)

it sounds like you are wanting to heat this brick building and have your tank (s) above the stove in this room to collect your heat?
if so, i am not sure how fast or well  air to water heat transfer will work, especially if your tanks are just round and have no fins to help collect the heat like a radiator or finned heat exchanger

perhaps i am not understandingn what you are writing? i have been know to get mixed up before?
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #2 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?


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jackel440

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Re: putting water heater tanks on top of firebox for boiler
« Reply #3 on: September 16, 2011, 02:20:27 PM »

I don't see how you are going to efficiently get enough heat transfer to the tanks staged above the burn chamber.The room would be really hot with the concrete walls absorbing alot of the btu's from the firebox.With the firebox seperate from the water tank you won't have much of a way to concentrate the heat transfer directly to the water.I think your performance of the heat transfre would be poor and you will be disappointed.
Now I might not be understanding what you are trying to convey in your post.But hey,I'm a millwright so I need a print  >:D
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #4 on: September 16, 2011, 06:07:29 PM »

good thing on the 40 lbs pressure (whew) so now i am thinking you will have an insulated room that you willbe heating up to about as hot asyou can get? yes/no

Then you will fill the area above your stove with used tanks to absorb the heat and move it to your collection tank where you will use this heated water to heat your home and then return it ot the "hot room" to reheat? yes/no

i think you will transfer heat this way at a rate of about 1.25 btu per hour per square foot of  tank surface area (a bit of a guess here) per degree F

you  will need to calculate the btu per hour you can collect in your tanks and seeif that can meet your demands on an average winter day...then you will need to calculate how many gallons of reserve you  need to make sure your heat loss on a colder than normal day will not drain your tanks btu capacity down to the point of "no heat" before your system can "catch up" again

your home is above average size for most calculating program i have seen but if you can dertermine how well it is insulated (or you allready know the heat loss of your home) you could get an average of btu per square foot of living space calculator for your geological area. once you know on averagehow many btu per year you will need that should help you in sizing your collection tank square footage surface area and how big of reserve tank you may need  where i live (ontario canada) it is said an average size average heat loss home requires about 100,000,000 million btu per year

hope i have been of some help so far
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #5 on: September 16, 2011, 07:58:31 PM »

dave, i have been re reading your post! 8000 square feet, wow your current fuel bill must be WILD?

you can figure out from what fuel medium you now use (electric, propane or gas) how many btu you used last year and we can convert that to an approximet wood consumption and how many btu you will need to capture and store?
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #6 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? 
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willieG

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Re: putting water heater tanks on top of firebox for boiler
« Reply #7 on: September 17, 2011, 11:10:51 AM »

dave i am using the collection rate from a wood boiler testing site...it may be off as this guy says that some manufactures claim the boiler is 100000 btu an hour but  that is in a fully stoked stove. they say that it is more around 50,000 btu and by square footage of the fire box that is what it comes too. perhaps you could search the net and get a better  handle on the true btu per sqaure foot per degree collected. as you say it may be higher, but before i started a project such as what you are about to embark on, i would like to know if i stood a chance. I do believe by coming here and asking questions you are trying to do that. keep looking and asking. perhaps even if you were to put your tanks very close to your stove and have some sort of reflective shield it may increase the amount of heat it would collect. this would now be some what radiant heat instead of passive heat and the collection rate may be more.? i really don't have an educated idea on this but it may give you food for thought and you could ask the right people?
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #8 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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RSI

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Re: putting water heater tanks on top of firebox for boiler
« Reply #9 on: September 17, 2011, 07:12:40 PM »

You are just going to have a fuel oil tank with a fire in it and let the heat radiate up to your collectors? If so, I would be surprised if you get much over a month or two before it burns through. A forced air furnace will get destroyed pretty fast if the blower fails.
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jackel440

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Re: putting water heater tanks on top of firebox for boiler
« Reply #10 on: September 18, 2011, 12:52:16 PM »

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 would think by the time you go and add fins to capture more radiant heat,add pipes throught your burn chamber.etc etc ........I think you might just be better off building a conventional boiler or gassifier.I think you need to step back and look before you get stuck in the "box"
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.All Btu's of the fire would be in the fire box concentrating its heat to water transfer.I am thinking to maximize the capture of heat to your water.
Now this is just my opinion so don't pay it no mind.If I can help you to see things in a new light for another idea cool. :thumbup:
Do you have a drawing design of what you plan to try?
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #11 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. 
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #12 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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RSI

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Re: putting water heater tanks on top of firebox for boiler
« Reply #13 on: September 21, 2011, 10:05:06 AM »

One problem I see with the pipes in the firebox is if the power goes out (pump stops running) when you have a good fire going they might explode. If you do it this way, I would put 2 pumps on it with separate battery backups on them.
I don't think the water to air heat exchanger would collect much if anything. Assuming your pipes collect near as much as you calculated, the water should already be hotter that the air will ever get which means it will be working opposite of what you want.

What about wrapping pipe around the chimney to collect some heat off it?

Several years ago, my brother had a similar idea to yours but ended up giving up on it after doing a lot of research and bought a used boiler.
He was going to make a concrete firebox and put old cast iron radiators an their sides at the top to collect the heat.
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woodboiler dave

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Re: putting water heater tanks on top of firebox for boiler
« Reply #14 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?
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