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Thread: OWB design. More water? Less water?

  1. 01-07-2012, 07:08 AM #1
    AIM
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    OWB design. More water? Less water?

    Well I've been designing in my head my next OWB. My current one works Ok but has many design flaws in my opinion. The biggest flaw is the size of the fire box. 24" diam 40" long. My next one will be MUCH larger.
    The biggest question/concern for my next one is how much water.
    If I can heat my house with lets say 100 gallons. Why would I want more? Wouldn't I just be heating water that I never needed.

    I guess I'm thinking along these lines.
    If you were a single guy living alone would it make sense to have an 80 gallon HWH for 1 shower a day. Of course not. You would be using 20 gallons a day but yet keeping 60 gallons hot that you never used.

    I don't see why this wouldn't apply to my OWB.

    Another thought is this.
    Water by nature WANTS to lose heat. The instant the heat source is removed the water starts losing heat. The more water you have the more heat loss your gonna get regardless of how well insulated your system is.

    And yet another thought.
    Large quantities of water will result in fewer BURN times. This I agree with.
    (BURN= draft inducer ON)
    BUT...
    The burn times will be LONGER to heat the larger capacity where as less gallons will require more frequent burns.
    Why would 10 short burns be less efficient than 5 long burns?
    In my thinking the same amount of wood would be consumed but if you are heating more water than you need then you are losing efficiency. (this one is kinda hard to explain)
    EXAMPLE: 5- 20 minute burns to heat 300 gallons. MAYBE only 7 -10 minute burns to heat 150 gallons.

    Lets put this in a grand scale.
    Let's say you have a boiler system big enough to heat a school. Take that same system and heat ONLY your house. Would it be efficient?

    Am I thinking this through properly?
    Last edited by AIM; 01-07-2012 at 07:16 AM.
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  2. 01-07-2012, 07:27 AM #2
    John D
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    I dont know how many sq ft you are heating, how efficient your insulation is, how tight your home is or your climate,but I don't agree with your thinking on less water in the owb is better.I have 230 gallons in my shaver owb and if I had the chance to do it over I'd get one with 450 gallons minimum...and I'm heating 3200 sq ft ranch on an open (windy ) 21 acre plot. With only 80 gallons you have no reserve to heat your home on days when you threw in a large round or one that's not fully seasoned ...and they burn a tad slow ...IMO large unsplit rounds are the best for burning in most OWBs ...however they burn slow, so you need a larger water capacity to draw heat from or your temps will cool off too fast as your hone cycles for heat and hot water. This is happening to me...also a larger water capacity allows you to run a smaller draft opening or fan...allowing slower steadier burning which extracts more heat to the firebox than an intense fast burning fire which puts a higher percentage of heat out the chimney ...

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  3. 01-07-2012, 07:31 AM #3
    John D
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    I've talked to a lot of owb owners and I've never heard anyone say they wish there owb was smaller and held less water...as long as your insulation on the owb is good the loss from larger water capacity is more than offset by the benefits of having a much larger heat source and more stable owb water temps.

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  4. 01-07-2012, 07:47 AM #4
    Stihl310
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    I built my own a few years ago. What i did was use CB capacity and cu/ft of firebox to come up with a ratio of water to firebox.

    I think mine is right around 150-160 gallons and it has no problem keeping up w my 1700 sq/ft home. Actually I think I just about nailed sizing the unit as I only have to fill up every 12 hours and that's not even packing it to full capacity.

    I have a 30" dia firebox w a 40" dia jacket . Right around 40" deep with a damper and 10-12 cfm blower.

    I did a lot of research prior to building and honestly think the end product reflects that. I have had zero problems with it and it's going on it's third heating season
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  5. 01-07-2012, 08:12 AM #5
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    Quote Originally Posted by John D View Post
    I dont know how many sq ft you are heating, how efficient your insulation is, how tight your home is or your climate,but I don't agree with your thinking on less water in the owb is better.I have 230 gallons in my shaver owb and if I had the chance to do it over I'd get one with 450 gallons minimum...and I'm heating 3200 sq ft ranch on an open (windy ) 21 acre plot. With only 80 gallons you have no reserve to heat your home on days when you threw in a large round or one that's not fully seasoned ...and they burn a tad slow ...IMO large unsplit rounds are the best for burning in most OWBs ...however they burn slow, so you need a larger water capacity to draw heat from or your temps will cool off too fast as your hone cycles for heat and hot water. This is happening to me...also a larger water capacity allows you to run a smaller draft opening or fan...allowing slower steadier burning which extracts more heat to the firebox than an intense fast burning fire which puts a higher percentage of heat out the chimney ...

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    Let me put what I'm thinking differently.
    Let's just say that A guy is heating his house just fine with 150 gallons. No problems, always keeps up, etc.

    Would there be any benefit to him having a 300 gallon system?
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  6. 01-07-2012, 08:52 AM #6
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    More water is always better, IMO, but all of that water does not have to be designed into the OWB. Stand alone hot water storage tanks of 1,000 gallons or more can be a very good thing if done correctly. A stand alone tank that could be plumbed into the OWB that worked on a passive design has advantages like the OWB having a place to put its heat should power be down.

    I'm still waiting to see an OWB that worked entirely on passive design to heat a house. It has been done tens of thousands of time for coal based systems in the past. In fact my grandparents house was designed that way and used large cast iron radiators.
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  7. 01-07-2012, 08:55 AM #7
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    I can't see heating a ton of water only to lost btus that you aren't using... There are so many variables that it would be hard to entirely nail the exact size. But personally I'd rather have multiple shorts burns to heat up a balanced system than have extended burn times to heat up a large volume I'm not going to use.

    Although burn times are all going to be relative to the size of your firebox as well.
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  8. 01-07-2012, 08:56 AM #8
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    I could agree that a storage system would be beneficial if you had space to put one...
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  9. 01-07-2012, 10:26 AM #9
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    YOur answer is in this question Why are you building your own boiler? My guess is for longer times between loading. If I am right then you can store the energy you need for longer times either in water or in wood in the firebox waiting to be burned. The first boiler I built was 10 years ago and I still use it, my average time between fills is 2-3 weeks. Now you probably dont need something this large but what I am getting at is when you build a very large firebox you will end up with larger water storage just by design. I have built several now and I wouldnt change much from the first one. My boiler has 3400 gallons of water. The door is always the hardest part. Also I would definately have an insulation company spray foam the entire outside when you are done. Good luck
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  10. 01-07-2012, 10:28 AM #10
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    I wish I had done the foam insulation... My only regret
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  11. 01-07-2012, 10:38 AM #11
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    One of the most efficiant things is storing your heat,thus more water more storing of your heat, heat transfer is a slow process so fewer longer fires is more efficiant,the longer burns are also better for burning off creosete, MY Oppinion???
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  12. 01-07-2012, 11:01 AM #12
    shane438
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    Quote Originally Posted by woodman6666 View Post
    YOur answer is in this question Why are you building your own boiler? My guess is for longer times between loading. If I am right then you can store the energy you need for longer times either in water or in wood in the firebox waiting to be burned. The first boiler I built was 10 years ago and I still use it, my average time between fills is 2-3 weeks. Now you probably dont need something this large but what I am getting at is when you build a very large firebox you will end up with larger water storage just by design. I have built several now and I wouldnt change much from the first one. My boiler has 3400 gallons of water. The door is always the hardest part. Also I would definately have an insulation company spray foam the entire outside when you are done. Good luck
    Would like to see some pics please!
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  13. 01-07-2012, 11:15 AM #13
    woodman6666
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    Quote Originally Posted by shane438 View Post
    Would like to see some pics please!
    I have posted this before but here you go.

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  14. 01-07-2012, 12:53 PM #14
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    Quote Originally Posted by woodman6666 View Post
    I have posted this before but here you go.

    Holy moly!!!!!
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  15. 01-07-2012, 01:42 PM #15
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    Quote Originally Posted by woodman6666 View Post
    i have posted this before but here you go.


    wow!
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