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Not getting the heat I expected - and other parts of my learning curve

 
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I"ve built my first rocket stove mass heater a few (3?) weeks ago. I have a few questions...

Creosote etc burns off at approx. 1000F - what temps should I be seeing on the top of my drum that would indicate that I'm getting hot enough to burn off the creosote?

the bench was built up over about 10 days - layer by layer - with fires running all day with each layer. I have cushions on the bench now, and the bench gets up over 100F under the cushions. I sleep on the bench the past few nights - it's great - but with outside temps down to about 15F, I wake up to about 40F room temp. I'm running the stove about 12 hours a dayl Does it just need more time to completely dry out the bench? I'm looking at minus 40F temps this winter, so I'm hoping the stove is going to get a lot more efficient. There is about 26 ft of 7 inch pipe in the bench, two 90 degree turns, and then the chimney goes  straight up through the roof about 3 inches behind the barrel.

I'm getting about 500 to 600F top center of the barrel most of the time, but I have had it up to about 1000F with the driest, smallest wood.

I love it, but it isn't living up to the ads for a 2 hour burn for round the clock warmth.

What do I need to do?

 
gardener
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Location: latitude 47 N.W. montana zone 6A
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Hi Bob; Welcome to Permies!


Sorry to hear your rmh isn't keeping you warm enough.
Lets see if we can help you make it better.

What is your home like?  How big? How tall are the ceilings? How well insulated?
-40 F is darn cold, you may need supplemental heat on nights like that.

Yes, your mass takes a good while to completely dry and warm up. What is your mass comprised of ?
How is your core constructed? Ceramic boards ,fire brick ,perlite / clay ?
1000 F at the barrel top is good. My 8" can get to 1100 F but more often its 600-800 F.

I guess we need more information about your home and your build. Photo's would be good.

One thing I can tell you,is that your bed is actually keeping heat in the core rather than releasing it into the room.
Folks who use their mass as a bed, report a several degree rise in room temps after rolling up the mattress for the day.  
 
gardener
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Others have mentioned that the top of the barrel for a 6" system is around those temps, assuming a typical spacing between the riser and barrel of a few inches. Can you get an oven thermometer and insert it into a tiny hole in the chimney and seal it, so you can get a good feel for the temp going up and out? You could certainly still be drying out the mass, depending on how wet it was to start and how humid the space is. The amount and temp of the mass relative to the space you're trying to heat also plays a factor in the temp of that space. Also to confirm, is there insulation between the mass and floor/slab to prevent heat being drained into the earth?

So lets say you have 5" of cob over the duct, and it takes 5 hours after a burn for the internal heat to move through that mass (typically it's 1" per hour). So then I'd wonder how warm the mass remains between that 5 hour mark and say the next 7 hours. Perhaps you need to burn a load or two every 12 hours to keep the mass warm enough to keep the space warm? It sounds like you have a surface temp reader, so I would take measurements frequently if possible, say every 2-3 hours, and you can see how frequently to burn. Also is the line of sight blocked between the mass/barrel and other areas you want to be warm? Since the RMH relies on radiant and conductive heat more than convective heat, blocked site lines would affect the comfort levels in those spots.
 
gardener
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Well, few things.

Where did you see that a two hours burn can heat the place for 24 hours. I would say more like 5  or 6 depending on your system and insulation.

Your mass is most certainly still drying. 100F° under the cushions is not much.

Mine goes to 115F° but it is 4 tons of concrete filled bricks. 8 inches thick. And it's a bell. This is after, let say 5 or 6 hours burning oak. And it is a batch, not a J tube.

Around the manifold,  beginning of your mass, you should see hotter temps.

650/750F° on top of the barrel is a good range.

What is the temperature you're getting at man's height inside your vertical chimney.  140F° is good 180F° is a bit worse. Over that you're loosing heat, so you haven't got enough heat extraction. 26ft plus two elbows seems rather short for a 7 incher.

The ultimate answer is how hot is your chimney.

What you're saying, is burning all day, and your house is at 40 in the morning, you musn't have much insulation.

HTH.

 
pollinator
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Not an expert on RMH. I have researched and modified building heat systems for many years.

> insulation
Important.
- Between the mass radiator (cob?) and the earth's heat sink.
- Between the mass and the exterior wall? Matters if the mass is at the outside wall of the room.
- Against the inside of exterior walls near the mass (well,  any exterior walls, really); think textile hangings, more reflective the better.

In general, how much are you  "heating the outdoors" because of building leaks? A little is beneficial for good fresh interior air; more, not so much. Where is your RMH getting its combustion air? It's getting at least as much air (from outside) as it's sending up the flue.

If the flue outlet is more or less reachable, what's the exit temps of the combustion gas? What's the gas look like? This relates directly to the "health" of your system and its efficiency. Somebody more familiar with RMH (of various types) needs to chime in here because I don't know what are good/better/best figures. My generic understanding is that lower exhaust temp is better, invisible gas (meaning complete combustion) the best.


Best luck,
Rufus
 
Bob Lewis
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Thanks to all who took the time to reply.
When I moved here, there was a 28 ft travel trailer - a 3 season trailer - single pain windows and maybe an inch of insulation. I added a 10ft x 26 ft room which has 4 inch urethane panels in the walls and ceiling. I also added those panels all the way around the travel trailer and a few inches into the ground, just to prevent any drafts - and then house wrap around everything. Built a roof over the trailer and addition with extra insulation above the trailer as well.

I knew the shack was not the best, and I wasn't expecting round the clock t-shirt comfort, but I thought the stove would do better than it was.
Thanks to you folks, I think I can expect better performance when the bench is completely dry. I have made the bench so that I can take off the end and extend the pipes another 3 ft - so that will add 6 ft of piping - but I might have trouble digging more clay and sand now, so that might have to wait for next year.

Last winter with a barrel stove was pretty rough - getting up every 3 hours to refill the stove and damp it down - was still only 15 to 20 F when I woke up. I've improved the insulation and draughts a bit this summer and built the rocket stove. I'm aware that the cabin could be greatly improved, but this is just a temporary home until I decide what and where to build.

Based on the suggestions, my cushions are now on a 9 ft x 3 ft x 3/4 inch OSB with 2x4 crosspieces underneath to allow some of the bench heat to escape into the room. The bench is built against the trailer wall, but I have now decided that I don't really need the trailer much, so I've closed it off for the winter.

Today - for the first time in a week, we have blue skies and full sun. The batteries are charging nicely, and my little greenhouse made from 5 old thermopane patio doors is up over 80F so the stove is off and the doorway to the greenhouse is open.

20191201_133241.jpg
[Thumbnail for 20191201_133241.jpg]
new stovetop
 
Bob Lewis
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That last post with the picture - I had made the barrel so the top was 2 inches above the chimney, but the top bowed up. I had wondered if that affected the efficiency - but also, my kettle didn't sit well on top, so we cut a 14 inch diameter hole and then welded a 16 inch diameter piece of 3/16 steel over it.

The pic below is what the bench looks like now.

20191201_134047.jpg
[Thumbnail for 20191201_134047.jpg]
sleeping/sitting platform raised slightly above the bench to let the heat out into the room.
 
Been there. Done that. Went back for more. But this time, I took this tiny ad with me:
Switching from electric heat to a rocket mass heater reduces your carbon footprint as much as parking 7 cars
http://woodheat.net
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