# Loop in fire box of outside hot air wood furnace?



## dave_dj1 (Feb 7, 2012)

Hi all, I found this great site via another site I frequent. I am an avid DIY'er and have an idea about putting a water loop inside the firebox on an outside hot air wood furnace to supplement heating my house. The wood furnace is 48' from the house, the firebox is aprox. 20" in diameter and 38" deep. I started with a Fawcette hot air wood furnace and built an insulated steel framed box around it and insulated it. I heat my detached garage with it but would like to help heat the house as long as I have it burning. Right now I only run it if I have a project in the garage.
My idea is this: I built a double loop of 3/4" black iron pipe, two 36" lengths and two 30" lengths, I used 2.5" nipples between the elbows, it sort of looks like a trombone...LOL.
I want to put this in the firebox up high and to the right as you open the door (I think it will be the most out of the way place for loading). Then plumb it underground to my basement where I will have a Modine (size is aprox. 12" x 12". I already have the Modine and the circulator (taco 007-f5) I am running 3/4" pex (insulated of course) to the house. One of my questions is should I install a small storage tank, say 30 gal.? or just run the loop as is , firebox loop, Modine and circ. There will be a TEE on the top of the firebox loop so it can be an open system, I will devise some sort of "tank" that will hold a little H2O/Glycol as a reservoir and escape route.
thanks,
dave 
If anyone has any questions on my wood burner I will do my best to answer them.
Here are a couple of pics of my wood burner.
This is my second winter with it and it works way better than I ever thought it would.


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## Duetech (Feb 8, 2012)

Welcome to the forum dave_dj1. I have a "ceramic effect" boiler (according to the seller anyway) which is just basically concrete with pipes running through it. There is no room for a reservoir in the boiler so I have a 50+/- gal barrrel that I use as a storage/reservoir tank. Open system.  My circulator (pump) runs 24-7 by basic concept and heats my domestic hot water (dhw) and supplies the heat needed for my home. I do not know how many pipes are in the concrete structure but there cannot be very much elaborate piping in the 4" thick walls. The fire box is 2' x 3' x4' and a half load of wood will usually run 12+ hours as long as old man winter doesn't have a grouch on. The Taco 007 model is the one I use and it will push water quite fast. The circ is connected low on my reservoir and pumps to the boiler which is 80' away and back through my heat exchangers and back in to the reservoir through a home made difuser. My water temps never get above 160*f. By restricting the flow from my pump with a boiler type valve I can slow the water down enough for it to collect the heat I need fairly quickly. Water temp is controlled by an aquastat that controls the draft blower on the boiler and I would strongly reccomend you adapt your system to work with some form of boiler and water temperature control for safetys sake. It takes time for my boiler to heat the concrete up in order to heat the water. Your propsed system will be much quicker to respond and may be prone to over heating and the quickest way I know of controlling water temperature is controlling the draft. I use one inch pex but my boiler piping outlet is 3/4" and my total sqare footage is 1700. Best wishes on your project.


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## dave_dj1 (Feb 8, 2012)

Cave2K thanks for the input. Your words are very encouraging as most people I have talked to about this seem to be of the opinion that I won't be able to make enough heat with such a short run of pipe in the firebox.
My house is a small easily heated ranch house, aprox. 900 sf. The draft on my burner is controlled by T-stat. When the garage get's up to temp (65 deg) the T-stat shuts down the draft so the fire never really burns out of control.  I am planning on having the Taco run 24/7. This will be an open system.
Another thought I have is to put the Modine fan motor on a speed controller, I may need to slow it down as to not draw the water temp down to much too quickly.
The only reason I thought of having storage is that I will have a sort of buffer for longer heating (Modine) and more time to heat the water (pipes in firebox).
Luckily for me I have acquired the parts over time and my only real investment now is the underground pipe and insulation. My neighbor has a small backhoe so digging the 48' trench won't be any expense.
I am also not looking to heat solely with this set up, if works out that I can great! I will be happy to cut the oil consumption by 25% or maybe 33%.
They tell me there is too much frost in my driveway to dig across it right now so this may not become a reality until next year. I have all the underground pex and am pretty much ready. I was hoping to get some time in on it this year but unless we get an extremely warm spell I don't think I can. Temp here this morning is about 8*F.
thanks,
dave


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## Duetech (Feb 8, 2012)

My house temp can get up to temp via the air/water exchanger in my oil furnace plenum but some times there is a surprising wait to the next call for heat so waiting for the T stat to control the draft could potentially cause me some slow heating trends. The 50 gallon reservoir I have allows the boiler to get warm before shutting down. Of course your pipes will get a coating of creosote that would slow the heating action of your pipes and your unit will function a bit differently than mine but even a low forced draft would cause my unit to over heat so my unit would have to stop the forced draft. The manual draft on my blower is never set and left over 1/3 open and that amount is not sufficient to heat the home even though it slows the cooling trend. Your proposed system might work well with a variable speed blower. My aquastat is set at 155*f and I use 2 temp guages to confirm the setting. Once a fire is going good though natural thermal draft will drift my system up to 160*f even after the blower stops and my system can coast with little smoke evident until the boiler starts to cool from heat demand and too little oxygen. The concrete of my unit is a form of insulator so I can get a hotter fire in the fire box than a standard OWB and the concrete will hold heat longer than a standard wood furnace. A lot of the heat I get is from infra red and perhaps a little catalytic action during blower down times. You will have to gauge the amount of pipe that will work best once you get this thing going, and it is true you may need some more pipe but I think once you start heating the garage and house you will find that you could run in to some potential over-heat situations with the water side if you don't throw in a secondary thermal off (aquastat) switch (putting a boiler control valve in the system to impede water flow may give you the information you need to determine if you need more pipe. Slowing the water speed down in this manner is like adding head pressuer to a system.) It takes a little while for 30-50 gallons of water to heat close to boiling in a passive system so I think you are on track with the thought of extra storage and the extra time it takes to heat the water could help you guage any overheat threat and piping amount. 

 I like the box you built around your stove by the way and am planning something similar for my gasser. DYIing may not be the preferrence but a lot of the times it's the difference.


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## dave_dj1 (Feb 8, 2012)

Cave2K thanks again.
I already have an overheat issue in the garage as it's small, about 20 x 22 and well insulated, I just open a window or door to "regulate" the temp. LOL
One "problem" with a wood hot air unit is that the blower comes on when the airbox reaches the preset temp on the fan limit switch, thereby blowing heat into the garage whether you want it or not. I don't shut the draft down to the point where the fire smolders, it's always burning. That is part of the reason I thought I could take advantage of it and use some of it in my house.
thanks,
dave


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## dave_dj1 (Feb 13, 2012)

Well I have the loop built, not installed yet though, waiting for some time and warmer weather. 
I also have my pex pipe and jacket ready to go, same deal, waiting for the opportunity to dig the trench.
Hopefully someday soon I'll get to test her out. 
wish me luck!
dave


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## Duetech (Feb 14, 2012)

Even in my boiler I have found that the only time there is a "smolder" going to happen is when there is wood that has not gotten coked or turned to a near charcoal state. Once wood gets to that point you mostly have a coals type fire and your exhaust is mostly clear. I know your trouble with the wood furnace though as I had one for about 15 years. At the colsdest part of the winter it was not enough and at other times it was open the windows or doors. An odd trick I learned was to add wood and turn the draft down. The fresh wood was a heat sink and slowed the output of the furnace. The reduced draft stifled some of the coked wood. It all eventually burned but with a reduced fire and then it was time to restoke. Best to you.


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