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[wrh51]

So after much deliberation and research I decided to build my own OWB. Rather simple design, tank in a tank. The outside water jacket is 4'x5'8" . The fire chamber will be 2'6"x 4'6". It will have a blower. I am unsure about the exhaust design. I have noticed some come straight down through the top and position the inlet about 2/3 of the way down in the fire box. I want to run my exhaust out the rear. That way I can weld in the pipe on the firebox from both sides and pressure test the firebox. Then slide the fire box with exhaust into the water jacket. Weld in the firebox and exhaust and pressure test the water jacket. I am thinking that the exhaut will still be in the bottom 1/3 of the firebox. After the exhaust exits the water jacket it will tee up with an access on the back of the tee for cleaning. Does anyone see a problem with this? I have my stand fabricated and the water jacket welded to the stand. The next step is cutting holes for the firebox to sit in the water jacket and the holes for the exhaust. I also plan on offseting the firebox in the water jacket. Meaning there will be 6" of water under the firebox and 12" above. My theory is that this will give me a larger quantity of water where the most heat is. At the top of the firebox. Make sense? Any input would be appreciated. I will try to get some pictures up tommorrow. Thanks in advance for any advice.

[Bull]

Welcome to the site.Sounds like a simple design, make sure you read the post about pressure testing and don't forget the pics.

[rosewood]

the tank in a tank concept is pretty  simple and very effective.exhaust is a area where thier is lots of heat to capture. have you considered running your stack this way..exiting vertical from rear2/3 in box up in to horozontal flu ,this would be a few inches off top of tank ,running all the way thru each side ...front and rear with clean out caps.then their could be a vertical flu ,located toward the front run up and out.you could use 6x6 sq tube making it easier to work with. you could build this like you mentioned with the exception of the vertical exiting flue.to do this you will have to cut a large enough opening in outer tank at top to get access to weld vertical flu at horozontal point.  this would give you somewhere about 5-6' of hot exhaust in water. and if you lowered box in tank to 3'' this could give you some more water area above firebox.with this setup you migh want to use a draft inducer,this would also help when loading ..no smoke roll out.or if you use forced draft you could have that exiting flu come into firebox  with a bypass flap.hope i dont confuse you to bad with all this..lol  you asked for comments!

[jackel440]

Sounds like you have your design already set with your burn chamber stand welded in.I agree with rosewood.would be nice if you could make a double pass in your water jacket to extract the btu's you will lose going straight out the back.

[wrh51]

Rosewood, After re-reading your post I think I understand. If you are looking at the tank from the side, profile view. The exhaust would exit at the top rear of the firebox.( The vertical pipe in the firebox would be in the bottom third of the chamber) The exhaust would then tee inside the water jacket toward the front of the tank and the other side of the tee would go out the back with an access port for cleaning. The exhaust would travel horizontally through the water jacket to the front and tee again. The pipe would turn vertically out of the water jacket and to atmosphere. The other side would exit the front of the tank with and access port for cleaning. I like the idea but it as you point out it would require a fair amount of additional cutting and welding.
What if the exhaust exits the firebox horizontally and turns up vertically INSIDE the water jacket and then exists the top of the tank. This will give me about one foot of horizontal and 3 feet of vertical piping inside the water jacket. It would allow me to still preweld to the firebox and the weld of the exhaust to the water jacket would be at the top where a potential leak would not be a big problem. Would I extract much heat out of the vertical pipe or would the exhaust escape too fast without much heat being captured?
Thanks for the ideas.

[wrh51]

I finished welding up the firebox into the water jacket on Sunday. Aired it up and had about 3 or 4 pinholes at weld bead starts or stops. Welded them up and soapy water tested everything. My door is fabricated. I need to finish the hinges and put the stack on it. I am trying to do this one piece at a time. I have now turned my attention to the inside plumbing and circulation system. Keeping with the theme of simple, my intent is to keep the plumbing one simple loop. It will flow as follows: OWB outlet to house, circulation pump, heat exchanger, out to OWB inlet. I will be running 1" lines. The OWB is going to be less than 100 ft from the house and the OWB outlet will be about 10-12ft higher than the pump and heat exchanger. I am probably going to use a Grundfos pump. I have experience with Grundfos products in other applications and they seem to have a good reputation as circulation pumps. I have done some research about which model to use. UPS15 series and the UPS26 series are what I have been looking at. It does not appear that the UPS15 pump will give enough flow at what head I will be pushing. I definately do not want to undersize my pump but from what I understand, oversizing it can cause problems too. Any input is appreciated. Thanks.

[peacmar]

I am not at all familiar with the grundfoss pumps. But this is what I have and it works well and I know I have room to spare. My boiler is 60 feet from the house and has a taco 011 mounted on it. The 011 pushes through 60 feet, to the house through 1" pex lines, through 40 feet in the house to the air exchanger that has a 18"x18" coil, also has a parallel loop of 3/4" pex lines about 50' long total that go to a water to water exchanger and side arm heater in series. I split off in parallel at the fan coil. I get sufficient flow from the pump and know this because when the heat is on and the bathtub (largest water demand in our home in terms of gpm) I see a water return temp that is about 30 degrees less than feed temp at the boiler. My fan coil and water heat sit about 10 feet lower than the boiler.

This is all just for comparison though. What you really need to do is calculate your required gpm flow rate (10,000 btu per 1gpm at 20 degree Fahrenheit temp drop) and calculate your head loss and then study a flow chart for the pumps your considering. Grundfos should have one available that shows all their pumps. Follow the head in ft lines and the gpm line and pick the one nearest the point they intersect. Slightly larger is good if you didn't calculate all your fittings individually as these add to head loss. Every tube and pipe and pex manufacturer, along with fitting manufactureres, along with heat exchanger manufactures all offer flow and head ratings, charts, and calculations for their products. Add up the total feet of tubing you will use, calculate the pump head in feet. Add up all the fittings values for head loss in feet. Add in the head loss value of your heat exchanger to the above two, and you have your total pump head.

Here is a good read on how to go about this all and has examples. They also use the standard 10,000 btu per 1 gpm figure in their examples.

http://www.pexuniverse.com/content/how-size-circulator-pump

[wrh51]

A belated thanks for the info peacmar. Can anyone point me inthe right direction on heat exhangers? Water to air that is. Any recomendations or things I need to be looking for? I know what size. I just need a good brand. Thanks!

[Fourced]

http://www.ebay.com/itm/OUTDOOR-WOOD-FURNACE-BOILER-HEAT-EXCHANGER-18X18-/390359614789?pt=LH_DefaultDomain_0&hash=item5ae33f0545