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Problem: RMH low efficiency & temperature  RSS feed

 
Posts: 7
Location: Wales, UK
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Hi everyone,

I need your help with this one. I have built my first RMH using ernie and erica's 6" plans.

My heater works fine when it comes to lighting it, its draft if pretty amazing. The problem is that I don't get efficiencies nowhere near what many of you have reported. I burn more or less the same amount of wood to heat the room than I did before with the normal store-bought stove.

Symptoms are:
-I see smoke coming out of the chimney even after the system has been running for several hours.
-Outside barrel temperature rarely exceeds 350-400F when I use extremely thin pieces of wood and constantly play with it. Normal temperatures are more around 300F.
-The RMH fails to heat even a relatively small lounge (13' x 15') in a mild Welsh winter (Outside temperatures around 40-50F (5-10C).

I have included a link to video of the first lighting as well as photos showing the building progress.

I have used new refractory bricks, refractory cement, insulated everything with a clay-perline mix. Also used new stainless pipes for the exhaust. The heat riser is a piece of T304 stainless steel surrounded with a clay-perlite mix.


Video of the first lighting.


https://vimeo.com/76323708


Online album of the RMH in progress

http://imgur.com/a/pwHnW#0



Looking forward to hearing your thoughts on this. I wish to be able to fix what's wrong with my RMH and start using it again.


Dominic



 
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Posts: 2707
Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Dominic, way too much mass in the burn unit. This takes forever to heat to the proper temps i think. And for efficiency, you don't seem to have enough pipe in your mass.

Looking at your hearth, i would switch to making it a bell.

Check the chimneybell2 sketchup file. And read the thread.

http://www.permies.com/forums/posts/list/40/31382#247574
 
gardener
Posts: 1270
Location: latitude 47 N.W. montana zone 6A
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Hi Dominic: Thought i would throw in my opinion for what its worth. As i have only built one rmh so far i am far from an expert at this. Very clean looking build. If you are getting smoke you are not burning hot enough ! Why is the big question. From your video it looks like your burn tunnel is to small, this may not be but it looks so. How much room is in your transition area ? You don't mention any smoke back or drafting problems . You say the barrel temp is 300-400, at what point ? top or side of barrel ? Is your cob dry ? I had trouble with wet cob when i first started running mine. There are many people here who know alot more than me , give them more information and i'm sure that they will help you solve your problems. Tom
 
pollinator
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Dominic : W.o.w., that is an Ox cooker and a half, a beast ! We talked about half a year ago about magnetite bricks, and I hope that none of the bricks see here
are magnetite ! Starting with your 1st picture below those bricks I would have had a trough nearly as big as the bricks total foot print about 4'' deep filled with a
mix of perlite and Clay slip, and the bricks surrounding the Feed Tube, and Burn Tunnel should themselves be insulated, I do not see where you have used the
Perlite and insulating Cob !

I have not seen the 6'' plans from E & E, but it does look like you have deviated some from their plans and the amount of Heavy materials where you would be
better served with more insulating Cob Material ! This is what 'Max' is referring to !

If you and I can straighten this out so that I know what I am seeing we can move on and get this Rocket working ! Big AL !
 
Dominic Lemoine
Posts: 7
Location: Wales, UK
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Hey guys thanks a lot for the replies.

@Satamax. Really interesting idea to convert the whole hearth into a bell. Thing is, there is a massive oak beam holding the hearth together and I don't want to mess with it. Maybe a smaller bell?

@Thomas. My burn tunnel is the exact size that was described in the plans (well... give or take 10mm). So is the CSA. I have absolutely no backdraft problem. I can put a incense stick a foot away from the feed tube and see all the smoke going in! I have measured the barrel temp near the top on the side the barrel. I have measure roughly 100F higher on the top. Yep, my cob is dry. I made this system at least two good months ago.

@Big Al. No worries, the magnetite bricks have only been used in the mass. What you see is proper refractory bricks. I see what you're saying about insulating the burn tunnel. I have to say that the front half of the tunnel is insulated (not shown on the photos) with a clay-perlite mix. Maybe I could fill the space between the outside of the burn tunnel and below the barrel with clay-perlite leaving only the space for the air to get out? That would almost certainly allow the core to reach higher temps.

Yes, I have used slightly bigger bricks because the ones used in the plans were not available in the UK. And I thought I would be doing a better job using refractory bricks rather than too much cob! Silly me... It does takes a while to heat up but its not a question of hours. Merely minutes.

I think it would be good for me to add more clay-perlite insulation and see how it goes. I am not too keen on destroying the whole thing if a simpler solution is available.

Tell me what you think guys.

Dominic
 
allen lumley
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Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Dominic : As you are going to have to open up your Rockets burner base to add insulation, I would definitely open it up to create a bulge where you have
installed the 1st piece of Horizontal stove pipe between the burner base between the base and the 1st 'T' make it big enough to not need that 1st piece of
stovepipe, I have hopes that that will greatly improve your Rockets ability to make and throw more heat ! More pictures if you can ! big Al
 
Satamax Antone
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Dominic, in case you'd want to turn the hearth into a bell, the beam could be protected with metal and some insulation i think. Plus, the temps inside the bell wouldn't be that high either. Or you could make the front wall of the bell (the front plug of the fireplace) go behind the beam.

If you put incence a foot away form the feed and it draws that way, you have way too much draft, imho. That's cooling the room and you're loosing heat to the great outdoors. You realy don't have enough pipe behind your heater. Your heat recovery is very low then. And so is your mass accumulation.

About your core, you say it heats up in mater of minutes, but not the mass, the whole lot of bricks might take an hour or two before they're even warm inside. This forms a heatsink, and lowers the efficiency of your rocket.

If you come down to rebuilding it at some point, put the bricks on their edge, so then you have less thickness between theses and insulation, so less mass.


Back to the pipe, for good heat recovery behind a 6 incher you need between 30' and 45' of horizontal pipe. Or a 4 cubic metre bell. You don't account for the floor in a bell. If you want to do a test, a tractor fuel or home heating fuel tank of a 1000L is about 5m² internal surface area. (ISA) Which might do with the draft from your chimney. You've most certainly seen thoses rectangular or square metal tanks, made to keep fuel or other things sometimes. two holes at the bottom, and presto, you have a bell. Then you could accomodate this where your actual mass is. Lean it against the side wall and back wall of the fireplace, and cover the rest with dry stacked bricks. That could be an option too.

Well, hth.
 
Of course, I found a very beautiful couch. Definitely. And this tiny ad:
Food Forest Card Game - Game Forum
https://permies.com/t/61704/Food-Forest-Card-Game-Game
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