RMH Mass heat absorption / insulation problem :(

Hi

posted my greenhouse heating Rocket Mass heater on another thread a while back - I've got a problem with the heat absorption of the mass


I used compacted soil rather than cob as mass both around most of the barrel (5" thick) and for the bench and around the pipe through the raised growing beds.

Initially it worked well heating up quickly around the barrel and the bench steamed nicely as it dried out - however once there is a dry area around the vent pipe and or barrel the heat absorption sucks !!

The compacted soil was rubbishy stuff I dug out whilst making another bed outside the greenhouse. Basically clay base but with a lot of gritty sand - I figured it was a gritty cob mix by nature - its well compacted but now acts as an insulator. Was down at the allotment yesterday and 2 and a half hours after lighting the stove the barrel surround was just barely noticeably warm - hot as hell against the barrel but hardly warm 2 - 3 inches away. Same goes for the bench.

I tried using earthenware tiles each 1 1/2" thick on top of the barrel and the top of a stack of 4 was too hot to touch after 25 minutes .......

Before I replace all the mass (not that much actually - although wrecking the raised beds will be a pain) does dry cob retain its heat absorption characteristics???

or is the 2 hour plus heat up for 5" thick of mass sound familiar ???

Everything else about the RMH is working a treat but I can't run it long enough to heat up the mass !!

Roger

Back up a moment you have 5 inches of cob where exactly?

did you run the exhaust through the bed?

more info and a picture would be good.

edit: umm re reading your post i assume you have much of the barrel covered in dirt?

defiantly pictures

Ernie thanks for the interest photos follow

The mass around the barrel started due to space and safety considerations - I'm a clumsy idiot and would definitely walk into the hot barrel regularly

there's approx 5" of compacted soil between the 2 barrels the outer container barrel has insulation under it.

Short bench area next to the "barrel within a barrel" RMH with vent running through the raised beds - approx 20 feet

Its a 5" system - cos its what i could get - works great, boils the kettle lickity split but, now the mass has dried out, no more warm ass !!

if I can find them i have video too

Thanks again
Roger

few more

couple of vids during the testing process - I've got a kettle now

<iframe width="560" height="315" src="http://www.youtube.com/embed/ci4I1XISNKo" frameborder="0" allowfullscreen></iframe>


<iframe width="560" height="315" src="http://www.youtube.com/embed/XEUnWu3QsqM" frameborder="0" allowfullscreen></iframe>


OK how do I embed youtube vids again

Actually the way you have it set up is the problem. your flow is probably great and this stove roars like its gonna fly away. that is the sound of way to fast of flow. you have insulated the barrel and now you dont get any slowdown in the system. heat propagates over time so you need to slow it down a bit to allow the heat time to actually penetrate the thermal mass. Ill bet if you water that five inches of soil in the large barrel you will see the bed heat up again. better yet water both it and the bed.

Ernie

It has a pretty good draw but not that great tbh. After maybe 2 hours it slows down and if you over load the feed tube it smokes back (actually flames back) - I figured that was part the 5" system and part the insulating effect of the barrel mass reducing the temperature differential. I thought that was a problem but from what you say its normal

I have tried adding water to the barrel mass and it transfers heat well until it dries out - but that's just because water is super efficient at transferring heat isn't it? Its actually quite tricky to re wet the soil because its well compacted and the clay content seals it, but the top few inches were soaked.

The Ducting in the bench area gets really hot - I dug that area out already and recovered the duct with a shallow layer of the compacted soil - again great heat transfer until there is a layer of dry soil around the duct. By the time the duct is leaving the building and the raised beds the temperature is really low - barely warm - so the heat is being transferred - just don't know where ! certainly not feeling the benefit

Why don't you come over to Blighty and do a lecture tour - you could fix it for me whilst your here !!

I was impressed by the fast absorption and dissapation of heat by the earthenware tiles (seen in the pics) I tried the other day as an experiment - if proper cob works as well then its worth changing out the rammed earth mass - I'm just a little wary of filling the barrel with cob that would be a bugger of a job to remove if it doesn't work ............

I've a vague memory of a post by Paul re his sand filled "mobile" RMH that the sand acted as an insulator because of the air gaps within it - can't find the post unfortunately to find if he fixed the problem - maybe as the compacted soil dries it has sufficient air gaps to insulate the mass ?? Does the denaturing process of cob manufacture remove post drying air pockets in the clay

If its "simply" a problem of using the wrong material for the mass its a pain but its easy enough - what worries me is if its a more complex combination of problems....... and anyway why would compacted gritty clay soil be so wrong ??

Further thoughts much appreciated

I'll go smoke a pipe and stare at it for a while - you can't beat giving things a good stare at !!

thanks again

Roger

Well I was giving the pics you sent a good stare and had the pipe going then decided that i would deal with the easy bit first and thats the mass in the barrel not shedding heat as well as it should. the reason why the barrel is supposed to be uncovered is that it sheds lots of heat very fast and adds a kinda push pump to the way the system works. I understand why in that small space you covered the barrel. then we get into the reason we dont build stoves of less than 6 inch and that has to do with laminar flow in a pipe. with five inch you can fudge it a little but essentially 6 inch is as small as you can go with out running into flow problems. Then we get into some things about the type and thickness of your insulation and what the heat riser is made from. unfortunately i can see where the thing starts to backup when its been run for a while and it has to do with the barrel of dirt around the barrel Even clay soil compacted down is insulation in sufficient quantity. so if you want whats happening in actuality this is it your mass around the barrel is holding in enough heat that the heat riser and the barrel are reaching equilibrium (very close to the same temp) the reason why this gets better when you water the soil in the barrel is that water is a wonderful heat sink and transferrer of heat to the outer barrel surface where it radiates away and cools the inner barrel. what this all means is that you should remove the 5 inches of soil and the large drum and just be careful. But failing that your gonna have to water it from time to time.

the heat is going into your thermal mass but i am thinking that my question about the raised bed is what is the soil temp 3 inches below the surface? and do you have the back of the bed isolated from the outside of the green house or is it against the cinderblocks?

Hi

How do people figure anything out without a pipe ? preferably a whiskey and a pipe

OK - I get the insulated barrel thing ............... (and the 5" pipe thing too - I knew that going in but needs must - and hey its a greenhouse I'm not too bothered about the odd bit of smoke back) but adding mass around at least part of the barrel is desirable especially in a greenhouse where heat loss is a big factor - and anything that evens out peaks and troughs in temperature is good. So although I am the clumsiest git on the planet it was a deliberate act not solely linked to an aversion to hospitals

I'll have a play remove 3/4 of the compacted clay soil (that'll be fun ) and maybe try adding brick to add mass but allow airflow and hope to reach a best of both worlds situation.......... My only scepticism is even with the mass the rocket element works pretty darn well and if the mass was better at absorbing and dissapating heat - ie changed to cob?? then the balance of benefits would be towards the mass and accept limited (2 - 3 hrs) burn times and frankly thats about all they're likely to be anyway.

Feel vaguely like I'm questioning God here but you see the point - too much heat too quickly isn't desirable in a greenhouse situation and, even with added insulation, is going to be wasted quickly too, through the glass.

The short bench area and then the raised beds are insulated from the exterior walls (rock wool) with a pvc liner between soil and insulation. the duct is surrounded by compacted soil and then has good topsoil and compost above that - following your comments I soaked the raised beds yesterday, so we'll see if that is sufficient to improve heat absorption............... the apparent, observed problem is an insulation layer of dry "mass" around the duct that doesn't allow for heat transferring into the bench/bed. If that's true then watering the beds regularly should work - I'll let you know, I'll have another burn tomorrow. Again really it points to changing the mass around the ducts to cob...........

It doesn't of course fix the cold ass problem of the bench and as I don't fancy a wet arse, anymore than a cold one, every time I stop for a coffee, watering the bench doesn't seem like an option - I've already dug out that area anyway so I'll add cob gradually and see if that works better - I mean I guess it does work better or that book of yours is a scam

I've got a load of clay soaking so will add cob to the bench over the next couple of days .... be prepared to hear from my lawyers if it turns out the whole cob bench thing is an elaborate practical joke to get idiots like me covered in mud

Thanks again for the ongoing help

Roger

Ahh who needs that weak whisky..... I have black-strap rum! better taste and warming.. I am trying to do diagnostics on a stove via pictures the darn lawyers keep getting in the way of the screen if you get to many you wont know what your writing. Any how it sound good But i dont want to add more work for you I am of the warm butt group myself so i am trying to get the figures on how i fix every problem Including the butt shelf.
Two or three hours should heat things up well if you do it every day you also might get a but of foam rubber to put o the butt shelf so it holds more heat on the surface (Erica's foot warmer is what i call it) when you go in just take the foam off or sit directly on the foam. We like a soft seat to protect our delicate fundament from the ravages of cold cob.

A pipe and a tot to you sir

Ernie, do you have any experience with including scrap aluminium or metal shavings into the mix as a means of moving heat more efficiently away from the hot spots ? Do you ever include boulders to increase storage density or as decorative elements? And finally, what about water tanks, hot tubs and other vessels inserted into the cob?

I have used scrap. I dont recommend it cause its not fun to try an dig out later. I like (if i have a welder at hand) putting fins into the pipe that extend an inch or so in to cob.

Water pools in the cob mass can work well if your not trying for bath water. if you want to heat water; putting a coil around the first five feet of pipe from the stove and build a very well insulated box will work.
Dont however; expect to heat a large thermal mass as well. the water coil is plenty large and a short bench to get the rest of the heat will work fine.

its not to hard to get the heat to flow. its hard to balance a system so it will do everything we expect it to do. American expectations are far greater than other cultures. We cut our teeth on the most inefficient stove in the world and expect other stoves to do all the different tricks a box stove will do. what we dont know is that those tricks are the only things that made a franklin stove worth anything. our ancestors would have tossed franklin stoves in favor of anything that worked better and could be built from simple things they had on hand, added stuff or no. look at how long the fire place lasted as a cooking implement. We where still cooking on a trammel and baking with reflected heat in the 70's. since then we have lost much information on fireplace cooking that we need to relearn.

Ok I've removed most of the compacted soil from the barrel - not as bad as I feared - there was a dry 1/4" around the barrel then wet (watered yesterday) so it is a problem of the medium used for mass The dry soil gets super hot but 1/2" away is cold so it looks like compacted soil is a remarkably good insulator - counter intuitive but thats what makes the world a fun place to be -I guess this could be useful if I ever get round to rebuilding the greenhouse.

Sadly it means removing that mass from around the duct in the raised beds and replacing it with cob - bugger

Ok without the mass the rocket effect is marginally improved in general and after 2 hours there was no sign of slowdown - all good. Even better from my point of view it wasn't so wildly different that adding some mass to the barrel would appear a bad idea per se - There's always a compromise position tricky bit is finding it.

Tried adding cob in a column one side of the barrel - dried very quickly and oddly the dry cob was considerably hotter than the wet. Heated up very quickly too.

Tried adding brick on one side of the barrel - have to say another counter intuitive result ...... brick is baked cob but unlike the dry cob that was too hot to touch 2" from the barrel after 1/2 an hour the bricks where barely warm after 2 hours maybe because of contact area - close contact with cob, point contact with brick - I'll try cobbing the bricks into place tomorrow.

The next question is is it better to add mass in columns the height of the barrel with air gaps between them or to add continous mass to 2/3 (?) of the height of the barrel and leave the top (hottest area) open............ I shall play with that one

The results of any further pipe induced euphoric cogitations always very welcome

Ahhh Rum ........ New Years Eve in Belize a long time ago I got a bottle that must have been off, felt very poorly for some considerable time into the new year I fear I have trouble looking the stuff in the eye now Whiskey however goes down like mothers milk and calms the Seafaring Scottish blood that finds itself stuck in this Sassanach landlubber

Roger