OWB design. More water? Less water?

[AIM]

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?

 

[John D]

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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[John D]

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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[Stihl310]

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

 

[AIM]

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?

 

[Stihl310]

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.

 

[Stihl310]

I could agree that a storage system would be beneficial if you had space to put one...

 

[woodman6666]

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

 

[Stihl310]

I wish I had done the foam insulation... My only regret

 

[geobckmstr]

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???

 

[shane438]

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!

 

[woodman6666]

Would like to see some pics please!

I have posted this before but here you go.

 

[Stihl310]

I have posted this before but here you go.

Holy moly!!!!!

 

[shane438]

i have posted this before but here you go.

wow!

 

[AIM]

I've lost a post somewhere??? I posted earlier and now I don't see it. Oh well.

I still can't quite wrap my head around why oodles of water would be BETTER than just the right amount.
(going back to the single guy with the 80 gallon HWH) He just wouldn't need it.

I do want to say also that I am not REALLY arguing here. I'm just playing defense lawyer until I can weigh/ponder all the debates and come to a verdict in my head.

 

[atlarge54]

My entire system including 800'+ of indoor pex containts less than 20 gallons of water. I replaced the mass of water with pea gravel and copper tubing. One big advantage, the system is pressurized which allows less maintainance. Big negative, current copper prices are expensive.

 

[woodman6666]

I've lost a post somewhere??? I posted earlier and now I don't see it. Oh well.

I still can't quite wrap my head around why oodles of water would be BETTER than just the right amount.
(going back to the single guy with the 80 gallon HWH) He just wouldn't need it.

I do want to say also that I am not REALLY arguing here. I'm just playing defense lawyer until I can weigh/ponder all the debates and come to a verdict in my head.

I encourage anyone to build their own boiler. I know you are not arguing, I wish you well with your project I just like to tell people what I have learned from building things. I will say that 2-3 week burn times and loading the boiler with the skidloader, and being able to cut 10ft logs and never splitting anything and being able to use stumps and gnarly crotches in my boiler has made for a very easy heating in my shops. Again good luck.

 

[leon]

boiler etc.

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?

Yes it would be very efficient because you are heating a large water mass or
making steam WHICH HAS THERMAL MASS AND ENERGY TO RADIATE!!!!!!

The amount of water available will allow you to maintain highertemperatures
more easily simply because the water absorbs heat.


About your boiler design, the fire box should be large, cylindrical, and half
its actual depth should be filled firebrick over the half the coal grates
(using a piece of channel iron over the grate frame to cover half the grates and still allow you to shake them)
to act as a heat sink.

The coal grates will allow more combustion air to enter the fire and burn very hot very quickly.


The full sized firebrick acts as a heat sink and will radiate
heat into the boiler, as well as absorb heat and then radiate
the heat EVENLY into the walls of the firebox that are not
exposed to flame.

The remaining heat energy which is being kept very hot
by the firebrick than passes through the shell and tube
heat exchanger and then out of thre stack at a much
lower temperature with little smoke left to be seen



The water stove should be long and cylindrical in design to take
full advantage of a shell and tube heat exchanger to radiate more
heat into the water supply like a steam locomotive would to make
hot water.


The heat exchanger is simply gasketed and bolted in place if the
need arises to repair or replace it.

The jacket of the stove should be insulated of course with an industrial grade
of fiberglass insulation and then wrapped in chicken wire to hold the insulation
inplace-it works and works well.


The rear of the shell and tube heat exchanger will have a gasketed access door to
allow the cleaning of the heat exhange tubes with a small steel brush- two inch in
the example.


The combustion air should be drawn from under the grates to aid in combustion using
a single fan to pressurise the combustion air to exit the fire box and then pass through
the heat exchanger tubes to the stack.

A second smaller fan should be used to induce a draft from behind the shell and tube
heat exchanger and pushing a small volume of clean air out of the stack to aid in combustion
and smoke exhaust.


This can easliy be done with a 48 inch diameter boiler reservoir of X length with a shell and tube exchanger
using a rectangular firebox with coal grates and half filled with fire brick as I mentioned previously.

The one I doodled/ figured out was 48 inches in diameter and 20 feet long and had a potential capacity of 2,000
gallons or so but not taking into account the area reduced by the shell and tube heat exchangers.
All the tube heat exhangers would need is schedule 80 pipe and good welding and proper gasket installation with
several access plates on both ends to allow the removal of the heat exhanger if needed by removing the larger plates
which allow the passing of the smaller plate(s) containing the shell and tubes.

A pair of guiding rails installed in the boiler from end to end would allow the installation and removal of the shell and
tube heat exchanger if ever required and less water would be lost as slightly less than half the water would have to be
lost in the event of a tube faliure.


The firebox could be entirely surrounded by water but making the firebox dry with thick boiler plate
below and forward of the water supply allows you to use firebrick to aid in combustion and reduce
heat extremes affecting the water jacket. The firebrick is radiating heat at all times as well a absorbing
heat and creating a very hot fire reducing smoke to a very low amount(because it is being burned and not
wasted going up the stack.

This also allows you to use less wood as less smoke is lost out the stack and reduces or negates the effect of wood ashes
against the boiler as there is no steel in contact with water for the ashes to eat at as all the water is above and behind
the firebox.

The boiler could be made from spiral welded pipe of 48 inch diameter to create the boiler shell and the ends could be
welded on after the face was machined properly to accept the sheel and tube heat exchanger and the mounting plates
individual gaskets for sealing the water jacket.

my thoughts anyway.

 

[Stihl310]

But to the OP's point if you don't have a need why then heat thousands of gallons of water.... You are going to have heat loss regardless.

I can see if you're heating a shop.... Just having a hard time seeing the efficiency in hearing massive volumes of water when all a guy might need is a 200 gal capacity

 

[kyle1!]

I encourage anyone to build their own boilerQUOTE]

When you have time :msp_rolleyes: would you explain how to build a boiler? Most of us probably heat more then 2000sq ft on up and a boiler would be a good option. Thanks

Brian