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Homemade Outdoor Rocket Stove Furnace

Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
I stumbled across this forum while researching rocket stoves, this is a great place. By combining a lot of ideas here and elsewhere, I've made a outdoor wood furnace that works on the rocket stove principle. This is the second year I've used it and in that time it has changed a lot. I've managed to build a feed system that keeps the fire going with no attending to for up to 8 hours. It currently heats my home and water, and in the future, I'd like it to produce the electricity to run the heat reclaimer. I'm currently using a 15W solar panel for that, but have been less than pleased with its performance. Here's some videos that I've made of my most recent modifications:

The heat reclaimer which takes the heat that is conducted from the feed tube, turns it into hot air, and feeds it through the firewood. This preheats the fuel and provides preheated air for combustion.

This is how I prepare the wood for the fire. I've actually modified what I do a bit since I made the video. I now only use 2 loops to hold the wood and changed to a easier knot to tie. I'd like to find something beside baling twine, but it was what I had on hand. Temperature is adjusted by the size of the wood fed into it. Feed a bundle of kindling into it and you'll pop the safety valve. Feed a bundle with larger logs, and you'll maintain a nice even temperature.

Thanks to the many people here that have shared their projects - you were all an inspiration to me

**EDIT**

Here's a picture of the system on the outside for those without a high speed connection:


On the right, you have the feed tube made from 8" black stove pipe. It feeds the wood vertically into the insulated burn chamber. Heat conducted from the feed tube is captured by the heat reclaimer (the galvanized portion at the bottom). It preheats the combustion air which is forced past all the wood in the feed tube. This preheats the fuel as well. The flame goes horizontal for 2 feet in the insulated heat riser. There is 4" thick refractory cast around the burn chamber and heat riser. There is another 4" of concrete poured around that.

From there, the heat riser turns vertical and extends another 2 feet. By the end of the heat riser, the flame stops and all smoke has burned. This is where 110 feet of copper tubing is coiled inside. 50' of 1/2" pipe first, then 60' of 3/8" last. To capture as much heat as possible, I run the water from top to bottom. The exhaust is cool enough to hold your hand in front of, kind of like a dryer vent in temperature. Once the stove has come up to operating temperature, all smoke disappears and you can no longer even tell that wood is burning by the smell.

The water exits the bottom at 150 to 200 degrees, depending on the size of the fuel fed. By burning smaller wood, you raise the output. Larger pieces lower the output. It is quite easy to keep an even temperature by simply adjusting the size of wood being burned to correspond with the time of the day. To prevent disaster, there is a T&P valve at the bottom that pops off at 210 degrees. It is then pumped into my house where it is fed through a 18"x18" exchanger where it is turned into hot air. The cool side is then pumped into my hot water heater. This provides water hot enough for most daily tasks. If hotter water is needed, the blower can be shut down for 5 minutes to heat the water to 150. In the future, I'm going to install a different snap switch that will do this for me.

The water is then pumped back to the heater where the cycle starts all over again. If you have any further questions, I'd be glad to answer them. It is a pretty simple system.
Martin Seidel


Joined: Sep 19, 2012
Posts: 69
Location: Susquehanna, PA
    
    1
Wow, great job Geo! Your videos explain it well. I have been tossing around a similar feed design and must say that your stacked bundles are a wonderful way to change or blend fuel sizes "automatically". A 7 hour burn sounds good, I wonder how hot a thermal battery could get in that time. Nice to see that you seem to effectively preheat both the air and fuel without constricting or slowing the intake air flow.
Please keep us updated!
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
Thanks, I've been very happy with the feed design. I've ran it for a couple of weeks now with no jamming, so I think it is finally going to work out for me. Last year was a different story You can really put out some heat in that amount of time, on average I produce 384000 BTU in 7 hours to maintain 70 degrees and hot water. I'd love to have gone with a thermal mass setup, but I live in a doublewide, so I had to go hydronic.

I'd really like to find a way to run the pump and blower off this thing as well. The blower uses 500 watts on low and the pump runs 85 watts. The first thing that comes to mind is to find a way to produce woodgas with it and run a 1500 W generator. It should produce about 1000 W on woodgas, which will give me a surplus to charge my battery bank, run my lights, etc. It would only increase wood consumption by 1/2 a pound or so more an hour. Waste heat from the engine can be recycled back into the system as well.

I've got a small engine that I converted to run on steam, but I think the generator route will be cheaper and more reliable in the long run.

A homemade stirling engine is another option that I really like, but I don't have a whole lot of spare time, so I don't think I'll take that route either.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
I've been looking at RMH and RS for a few weeks now and I really like your design and it was something I was thinking of doing. I don't have much room in my house for an RMH but really like the idea of using a RS to produce some heat to supplement my furnace. I have some questions though. Does this system run non stop? I know you have a thermal switch on the fan to shut off when the water gets to be too cold but it looks to be on the HOT side of your heat exchanger. Wouldn't it always be hot enough to have it run unless the fire goes out? How do you regulate temperature in the house? If you stop the blower the water would get super heated... super fast

I'm basically looking for a system I can build that will heat my house primarily and use my furnace as a back up for when it's extra cold or the wood runs out. But so far I'm having a hard time trying to think of a way to regulate the temps inside the house besides how big the fire is in the RS.

ps. why the use of two different tubing sizes in the coil? If the water comes out of the heat exchanger then into your hot water tank does it then return from your hot water tank to the coil? Or is one coil for the heat exchanger and the other for the hot water tank?
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
The system runs pretty much non stop. I have made a few modifications since posting the first two videos on my youtube channel. I now put the thermal switch on the cold side of the exchanger. When the exit water is under 130 degrees the blower kicks off. This ensures my hot water supply is at least 130 degrees. When it climbs back up to 150, the blower kicks on. I over sized my heat exchanger to prevent disasters. In other words, the exchanger can pull more heat than the stove can put out under normal circumstances. The system also has a t&p valve outside in case the water gets over 210. If it does, the valve opens, which creates a pressure drop in the system. This opens a check valve from my cold water and allows the system to cool automatically. Temperature is regulated by the size and amount of wood you put in. By looking at the predicted hour by hour temperatures, it is pretty easy to load the wood in such a way to maintain comfortable temperatures. This also ensures you are not producing more energy than you are consuming.

On the coil, I had the 1/2" laying around, so I took advantage of that. Smaller tube enables more pipe to fit in, increasing the surface area for better heat transfer. So when I bought more tubing, I went with the smaller size. There is just one coil unit made of the two sizes. Water from the cold side of the exchanger goes to the hot water heater as described above. It then returns to the rocket stove for reheating. Let me know if you have further questions, I'd be glad to help.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
Excellent thanks for the info. I figured the HE must be over sized to insure nothing ever gets to hot. Is there a way you could show how it goes in and out of your hot water tank? I'm curious how it's under pressure so it can be used. As it's used from the hot water tank is more water introduced into the system from the check valve?
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
The plumbing is quite simple. I'll draw a diagram of it here in a bit. Got a waste water plumbing issue to fix for my wife, but I can describe it pretty quick in the mean time. Orignially, a cold water pipe just went to my hot water tank. Now, I installed a T fitting. The cold side of the exchanger goes to one part of the T, while the cold water supply has a check valve installed where it goes to the other part of the T. When the hot water is not being used, the water runs in a loop starting at the rocket stove, to the heat exchanger, finally to the water tank, where it starts all over again. If the hot water is used, the cold water rushes in, bringing the pressure back up until the check closes. The pressure is held constant while the system is running in a loop. I'll get a schematic up as soon as I can.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
Cool that's exactly what I was thinking. I don't need the diagram your wording was excellent. But it never hurts to put up for others that come across this and we all know a pic is worth a thousand words lol. Thanks for all the info. It's cold where I live now but I'm thinking of starting some testing next summer and if all goes well maybe I can have a stove finished by fall... I won't hold my breath though lol
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
Here's a link to the diagram I made of my system as well as a better description of how it works. This season has been unusual for me, it is cold a couple of nights, followed by a lot of mild weather. If you start in the summer, you should be able to get all the kinks worked out by the fall time. I look forward to seeing your build. Once you switch to smart wood burning, it is hard to go back to anything else.
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I am working on a similar concept, outdoor rocket stove hydronic heat. Is that hot water tank in your previous link connected to your home plumbing? What is keeping the valve on the house tank from blowing open before the outdoor one in the case of an overheat/pressure. I am trying to keep my rocket stove as separate as possible from my other utilities. When the system cools down does the hot water heater take over to heat the home?
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
The hot water tank is connected to my home plumbing. A check valve prevents hot water from crossing over to the cold side. The T&P valve at the rocket stove will always be the first to go off, because it is nearest to the heat source. When it does go off, cold water goes directly into the hot water tank, cooling it off first. The hot water goes to the heat exchanger where it looses a lot of heat before going to the hot water tank. In my safety tests, the T&P valve at the tank never so much as weeped. The one at the rocket stove went off like a steam powered rocket!

When the system cools, the hot water heater can take over and keep the house 20 degrees warmer than the outside, at least in my house. Needless to say, that isn't much of an option when it is 20 or below outside. At worst, the system has cooled for an half hour or so when I get up in the morning (if I sleep more than 6 hours), so the inside temperature is still in the 60s. I just throw in a couple handfuls of crumpled paper, some shredded cardboard, and some kindling. Fill the tube up with the wood bundles, and your back in business, no stirring around coals or anything.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
Thanks for the diagram but aren't you missing the hot water side "T" that goes to the house plumbing and to the rocket stove?
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
I just simplified the diagram a bit. All the house plumbing is installed off the hot side of the hot water tank. The rocket stove cold side actually Ts off the home hot water plumbing at my kitchen. The benefit of that is twofold:

1. less pipe to run, as it is near the stove and
2. almost instant hot water to the kitchen due to the circulating pump
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
BAH!!! it hit me last night that this system uses water and water will freeze if I don't keep the burner constantly fired. Because I'm looking for a setup that I don't have to keep fired all the time means I can't do water! So no hot water tank for me I may have to use something else... glycol? Then I can just use a HE in the furnace and heat just the house. I'll have to get my buddy here who does HVAC for a living to see if he can setup my furnace fan to come on and off when needed.
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
I ran a closed loop system last year. To prevent freezing, I added plain old automotive antifreeze to the circulating water. It worked fine, no problems with freezing. You can buy liquid to liquid heat exchangers as well if you decided to heat your water with it at a later point in time. This will keep your system isolated from the rest of your water, something I forgot to mention to John.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
Yup I've seen those too.

Did you cahnge anything about your system to run in closed loop or just isolate it from the house water with some valves? No expansion tank or anything?
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 32
When I ran it closed, the water was completely isolated from the house water. I had a valve at the top that I could add water/antifreeze to it. The water flowed from the stove to the exchanger, through the pump, and back to the stove. Everything seemed to work fine without an expansion tank and still does now that I've tied it into my hot water system. I still intend to add one, I'm sure the thermal expansion over time would wreak havoc. They are fairly inexpensive, so you almost can't afford not to.
Mark Bomhoff


Joined: Dec 12, 2012
Posts: 10
Ya it's cheap insurance
 
 
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