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Author Topic: OWB #3  (Read 1197 times)

MarkP

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OWB #3
« on: December 19, 2018, 01:49:55 PM »

Hello........   I haven't posted for many years, but I come back to the site anytime I have a question on a build.  I've built 2 OWBs in the past, one I used for 10 years, and one for 3 years.  Both were similar, 1/4" sheet steel, 340 gallons, and did very well for me, heating a 1680 sq. ft. house and a 1000 sq. ft. garage with a 10' ceiling.  Home and garage were built in 2004 and well insulated.  I used 7-8 cord of wood each year in WV.  I have some health issues, and decided to install a new high efficiency propane furnace/AC for my house and a propane space heater for the garage. It works great, but I miss my wood heat.

With the cost of propane, and the lack of comfortable heat, I've decided to build a stove for my garage only.  I'm thinking a 3' X 3' X 4' water box(270 gallons) with a 2' X 2' X 3' firebox(90 gallons).  By my calculations, I should end up with a 180 gallons of water capacity.  (Larger firebox is a thought as well)  All new sheet steel will be used.  3/16" steel for everything except the floor and end walls/door of the firebox. Those will get 1/4" steel.  One of my stoves got a 8" flue, and one got a 6" flue.  Otherwise, the designs were similar.  The larger flue stove was much a better burning stove, and more efficient. 

With a smaller stove this time, I plan on a 6" flue out the top as the prior stoves have been.  Water will be pulled off the top and returned towards the bottom.  I plan on a White Rogers digital temperature controller and a simple, inexpensive Taco 007 pump for the 15', level run to my garage.  With years of OWB use, I have a general idea of what I want, but I'd like to have opinions from you on a smaller stove.

1.   Is there a calculation or rule of thumb for "gallons of water per sq. ft of space to be heated" that works
      well?
2.   Can BTUs be estimated for this size of stove?

I appreciate your input, and plan the build in the next couple weeks.  I can have it complete and fired in a week pretty easily.   
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Homemade OWB  (Smokey)
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One good woman that can split wood
Le Roy, WV

E Yoder

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Re: OWB #3
« Reply #1 on: December 20, 2018, 02:46:43 AM »

I've usually figured about 60 gallons minimum per 100,000 btu's. And that with a very tight damper that closes, with any air leakage you can boil way more water.
To estimate btu's things like the design of the exhaust path, draft fan size, etc would need to be known.
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E Yoder

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Re: OWB #3
« Reply #2 on: December 20, 2018, 02:48:26 AM »

If it's not pressurized I'd bring the pump/supply outlet off down low to lessen the potential for cavitation.
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MarkP

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Re: OWB #3
« Reply #3 on: December 26, 2018, 04:29:34 AM »

Thanks for the reply.  The damper I made in the past was a bit too airtight.  It would put the fire out, until I made some adjustments to it.  My shop dimensions calculate to needing 52,000 BTU for the lower level.  I hope to heat the upper level next year, and it's only 50% of the space, so 75,000 BTU would be good.  I'm torn between a smaller amount of water and quicker recovery, or a larger reserve and longer, but less often burns.  I've been happy with the past 2 stoves, but I think it may have been overkill, especially with your calculations as gallons per BTUs.

I'm using a 75 cfm draft fan going in through the door, one Taco 007 pump, and the stack will be 6" out the top, with a baffle in the top of the firebox.  As for pump supply outlet, I had my last one off the bottom area, but changed it after a few months, and got a warmer supply, with no apparent changes to the stove, as it was drawing off the top of the firebox.  I would keep at least 12" of water over the top of the firebox, and have it set on enough slope, to have any air in the top of the stove, going straight to the vent pipe.  With the vent pipe I used, I would rarely have to add any water.  I'd keep a check on it, and keep the temperature low, and increase as the ambient temperature started to drop.  Keeping it running during the summer for heating my potable water, I'd keep the stove temperature @130 degrees.

I'll do some more calculations, and check back in.  THANKS!!
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Homemade OWB  (Smokey)
Stihl 290, 2 Stihl 170s
One good woman that can split wood
Le Roy, WV

E Yoder

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Re: OWB #3
« Reply #4 on: December 26, 2018, 06:22:38 AM »

If the temp changed depending on where the supply outlet is, then I'd suspect that it's not mixing properly. Might need to be piped internally to the opposite corner of the tank.
Just a thought.
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MarkP

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Re: OWB #3
« Reply #5 on: December 28, 2018, 12:30:18 PM »

I had assumed the warmer temperatures in the top of the stove was because I had the supply line about 3" off the top of the firebox... the hottest area for water.  I had the supply line about 3' long, in from the back of the stove.  In the rare event I let the fire go out, I could build a fire, and have almost immediate heat on top of the firebox, before it had a chance to heat all the water to the same temperature.  At least that's how it worked in my mind....lol 

I just cut the steel today for the firebox and water box.  I'll be welding the next couple days, then I'll work on the plumbing.  I agree that front/back will create the best mix.  I appreciate all your input and thought. 

One question I have is a ratio between firebox and water box in cubic feet or gallons.  How many cubic feet of firebox, is best to heat a certain size (cubic feet) of water??  I like the idea of a larger reserve of water, resulting in less frequency of fires, but longer burn times, vs. firing often but for shorter times. 
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One good woman that can split wood
Le Roy, WV

BoilerHouse

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Re: OWB #3
« Reply #6 on: December 30, 2018, 11:00:24 AM »

You probably won't find any real correlation of firebox vs. water capacity.  Some like Garn have huge water capacities.  If I recall, one former member /dealer on this site claimed his best unit held just 33 gals of water, although I personally am in favour of larger storage to run the boiler full blast for longer periods.   One thing about water capacity, you can always add more storage later if you think you need it.  My boiler holds about 225 gal water, and I have a 300 gal external tank next to it. Downside with more storage is that it is not real responsive for a quick heat during the shoulder season. If I ever do a build 2.0, I would give some thought to variable storage.
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E Yoder

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Re: OWB #3
« Reply #7 on: December 31, 2018, 05:03:09 AM »

I just did some figuring and our coal furnaces run a bit less than equal volume of firebox v. water capacity. Gassers run not quite double the water volume v. firebox volume , I guess due to the greater efficiency (less firebox needed) and the desire for longer burn cycles and better gassing.
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MarkP

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Re: OWB #3
« Reply #8 on: January 06, 2019, 07:38:07 AM »

Thanks for the input.   My firebox to water capacity is 1:2.   Firebox is 90 gallons volume, and my total water is 180 gallon capacity.  Also, I run my pumps 24/7, so there is a radiant effect, which really makes a difference in the garage, as the heat exchanger and blower are exposed, and at floor level.  I have 2 small ceiling fans that are never turned off as well.

Welding has begun....  firebox is almost ready, and I have the base welded to the bottom of the water chamber, with the sides tacked on.  Top is cut and ready as are both the front and back.  Now to find the time to weld.... and weld.... and weld....lol   One thing I did on this one different, is add one inch of slope front to back, so any air in the water chamber goes straight to the vent tube, and will allow less air touching the top metal on the water chamber.  I know air will increase the chance of rust.

Anxious to get it fired up. My last two boilers were around 340 gallons each, and used 7-8 cord at the most, heating the house and garage.  I'm hoping this one will hit closer to 4 cord, with only heating half the area... garage only, instead of house and garage.  I'll keep you updated as it progresses.  THANKS
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Stihl 290, 2 Stihl 170s
One good woman that can split wood
Le Roy, WV

E Yoder

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Re: OWB #3
« Reply #9 on: January 07, 2019, 01:38:21 AM »

Post some pictures!
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WoodMOJoe

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Re: OWB #3
« Reply #10 on: January 08, 2019, 04:55:13 AM »

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MarkP

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Re: OWB #3
« Reply #11 on: February 05, 2019, 06:18:49 PM »

I've not been able to get  it finished yet, but hoping within the week or so.  I'll be posting pictures and dimensions on it..   I appreciate all your help.  I'm working on the door now, and trying to design a latch that works well and is simple.  Suggestions?
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Stihl 290, 2 Stihl 170s
One good woman that can split wood
Le Roy, WV

E Yoder

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Re: OWB #3
« Reply #12 on: February 06, 2019, 04:56:57 AM »

Some designs are easier to make than others. Some Crown Royal, the older Heatmaster's, and others used a latch sorta like a van trailer door uses. Pretty simple if you use a u bolt on the stove side.
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MarkP

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Re: OWB #3
« Reply #13 on: March 05, 2019, 11:04:10 AM »

I was able to get the furnace finished and fired up this past weekend.  I'm still doing some fine tuning, but it got it's first test today, with temperatures in the single digits.  It's doing a good job, and in the first 24 hours, I only loaded it one time.  I've loaded it once today, and I'll put wood in it tonight before bed.  The wood I'm using right now was cut about 4 years ago, and wasn't covered, so it's starting to rot.  No real good wood in the shed yet.  I'm happy with how it's doing.  I'm using a Taco 007, and it seems to be a bit weak.  It will keep the heat exchanger warm for a while, and then seems to lose a little heat.  Head is only about 2 feet, so the pump should be plenty to carry the water.  I'll have another one soon to replace it with.  I stumbled with the latch for a while, but today, I made a latch that's spring loaded.  Easy open and close.  We'll see how the spring holds up for the long haul.   


I have several pictures on my phone, but my phone isn't really user friendly for posting here, but I'll be posting pics as soon as I can. 
Thanks for everybody's help.   Pics to come soon!!!
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Homemade OWB  (Smokey)
Stihl 290, 2 Stihl 170s
One good woman that can split wood
Le Roy, WV

E Yoder

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Re: OWB #3
« Reply #14 on: March 06, 2019, 04:32:34 AM »

The 007 is a little weak for a lot of OWB situations. Too much restriction in piping.
Definitely post pics when you can.
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