|
Post by muddome on Mar 21, 2012 8:05:01 GMT -8
Last fall I built a rocket mass heater for my earthbag home and was thrilled with my initial results. However after a few burns it started experiencing problems. The draw would stop, reverse and rocket backwards!! blowing smoke and fire into the house. Then it would correct itself and start working well again for a bit. This led me to think that there was a pressure change problem somewhere and not a blockage, because the airflow in both directions was pretty strong. So now I've removed the whole thing and am preparing for a rebuild. There was no blockage as I suspected and my problem may have been that which is outlined in the helpful hint thread: donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=337My new setup will be a 7 inch system with a horizontal run of 20 feet. It will be in a 10 foot bench with the pipe returning to near the barrel before going 7 feet vertical and then out horizontally through the top of the wall. My riser is a 28-3/4 inch steel pipe with a 1/2 inch wall thickness. I have a question about optimal burn tunnel size. The book says it should be slightly wider than tall. I'm thinking of going with 5.25" tall by 7.25" wide. Is this a good proportion of length x height? This ends up being 38.06 sq in vs a duct and riser size of 38.48 sq in. Does this size seem OK? Also I have a couple options available for the heat exchange barrel. One is 34" tall with a diameter of 22.5". The other is 29" tall with a diameter of 18.5". I'm unsure at this point which one to go with, any suggestions?
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Mar 21, 2012 10:35:25 GMT -8
In my eperiments for preparing to build a baking ofen I have got the best results with a long tunel of two times the diameter. With it the fire reached incandescence (dazzling white heat). With shorter tunels the fire became only orange, about 400°C / 700°F less and there was a lot more smoke.
|
|
|
Post by Donkey on Mar 21, 2012 15:36:10 GMT -8
Last fall I built a rocket mass heater for my earthbag home and was thrilled with my initial results. However after a few burns it started experiencing problems. The draw would stop, reverse and rocket backwards!! blowing smoke and fire into the house. Then it would correct itself and start working well again for a bit. This led me to think that there was a pressure change problem somewhere and not a blockage, because the airflow in both directions was pretty strong. So now I've removed the whole thing and am preparing for a rebuild. There was no blockage as I suspected and my problem may have been that which is outlined in the helpful hint thread: donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=337It's pretty likely that will do it.. While you're in there, make sure that your insulation is in good order.Also, you might have an issue out at the end of the chimney. Did you have windy days when the stove was acting up? I remember that in the book, though I don't really know that it's important.. Better to get close on the cross sectional area than anything. Seems like one average fire-brick flat plus one on edge (without mortar joint) is 7 inches (At least the bricks I've been using). It's got me making the things 7X7 more often than not.. Seems that you're probably inside acceptable fudge factor.. Gotta try it and test it before you make it permanent. The little barrel will be too tight to easily build an 8 inch rocket stove in.. Gotta make room for the insulation!
|
|
|
Post by muddome on Mar 21, 2012 17:59:21 GMT -8
Thanks for the replies. Karl, Two times diameter would give me a tunnel of 14 inches. A little longer than I previously had it , but I will give it a try. Donkey, Yes it was windy, but it ilways is here. It was hard for me to tell if it was gusting though while in the earth bag house, it is pretty soundproof. Also, just to clarify my system is 7" diameter. I thought maybe the smaller barrel would provide quicker heat, but I'm guessing that as much insulation as possible is more important.
Another thing I'm curious about is the ash pit at the bottom of the feed tube. Is it really that important? It would be so much easier for me to build on the existing flat surface.
|
|
|
Post by Donkey on Mar 21, 2012 19:09:36 GMT -8
I tend to forget to build the ash drop.. As you've said, it's easier not to. Oops.. I wasn't paying close enough attention.. You have a 7 inch system, 38.48 square inches. your dimensions (5.25X7.25) are 38.06 square inches. Seems like it should be OK. It's a little smaller than system size, you could go a little larger but it should be fine. I wouldn't lengthen the burn tunnel. For the J-tube configuration, especially if you have a run of bench, keep the burn tunnel as short as possible.
|
|
bob
New Member
Posts: 2
|
Post by bob on Mar 22, 2012 17:02:07 GMT -8
my rmh works like a dream on calm days ....however gusts of wind reverse the flow..the book speaks of three t-sections of flue to confuse the wind...any thoughts before I make this outlay
|
|
|
Post by Donkey on Mar 22, 2012 19:28:30 GMT -8
If the top of the chimney is out in the clear (above roof line, etc) you could use a rotating chimney cap instead.. They have a vane on them and rotate to face the exhaust out of the wind. They work pretty good.
|
|
|
Post by canyon on Mar 22, 2012 23:11:16 GMT -8
Another option which I recommend is the Vacu-stack made by ICP on the east coast I think (Massachusetts?) made in stainless steel without any moving parts and although you have to order it, it is reasonable in cost for what you get.
|
|
|
Post by muddome on Mar 24, 2012 14:40:52 GMT -8
I haven't decided on a chimney cap yet. I like the one Donkey mentioned as well as the one Canyon mentioned too. The Vacu-stack seems like something that would work pretty well, and I may be able to fab my own. Originally, I had planned to get something like a Master Flow turbine vent but I can only find them in 12" or 14" sizes. My other option is to make something myself, using a lenz style wind turbine suspended below the exhaust T that drives a fan blade within the T, blowing air upward during any wind.
|
|
|
Post by muddome on Apr 13, 2012 18:47:05 GMT -8
A bit more progress has been made on my rebuild. The perlite is in the insulation barrel and the clay cap is in place. Tomorrow I will fire it up and find the sweet spot for the exchange barrel height. First I will have to add more cob to build up a base for the heat exchange barrel. The following two pictures show the patch I made in the insulation barrel. Originally it was destined for another rocket stove project that never saw completion. I used some mesh to be able to smooth some fireclay onto and a piece of roofing tin. It was pop riveted into place.
|
|
|
Post by muddome on May 8, 2012 8:22:06 GMT -8
I've replaced the insulation barrel shown above with the inside of a water heater. This was discussed in my 'Barrel Gap Frustration' thread. Thanks to all for their input. This picture shows how I dealt with the transition area. Icut the sides of the opening in the barrel at an angle. Not entirely sure if this helps. My stream profile here is about 30 inches. You can also see the vertical stack next to the barrel. I want it to steal a bit of heat from the barrel to keep things going in cold weather. This picture shows everything with the bulk of the mass in place. The picture shows three levels to give an idea of scale. The two yellow ones are each 4 feet long The silver standing up is 16 inches tall. I started burning it a bit to get things dried out and was a little dissapointed. I was getting smoke back. At first I thought it was due to everything being wet and quite cold (about 8 C). Then I added a two foot chunk of vertical stovepipe to the outside of the structure. Things went extremely well after this minor change. Originally I thought that 7 vertical feet inside with a two foot horizontal atop it (to get throught the earthbag wal) would suffice. Not so. That little bit of vertical on the outside has made all the difference. The rocket noise is constantly there and the draw is incredible. After burning for close to two hours there wasn't even any ash in the burn tunel. It had all been blown into the ash pit at my exchange area. Very little made it past this point into the ductwork.
|
|
|
Post by muddome on Jun 15, 2012 13:33:04 GMT -8
I'm getting quite close to finished now, just plaster left to apply and a chimney cap to put in place.
The sound on the video doesn't really do it justice. The 'rocket sound' is actually quite a bit louder and the crackling of the wood, not so loud. Oh well, what can you expect from a telephone. I remember when the damn thing was attached to the wall.
|
|