Reluctant to rocket

There is no insulation under the mass … that seemed pointless .. the floor IS extra mass n’est-ce pas?

(The stove is very deliberately at the heart of the house, away from the external walls)

Nick Ax wrote:

3-4 burns a day sounds like a whole lot more than .6 cords of wood a season - perhaps my expectations are unreasonable

 

if  you believe in this, and I think it is a most relevant bunch of numbers  

 
A cord producing about 20 million BTU's
and a standard cord of well-seasoned hardwood (stack of wood 4'X 4'X 8' or 128 cubic feet) contains the heat equivalent of about 20 million BTU's. By way of comparison this is more or less equivalent to the heat value in 145 gallons of #2 fuel oil or 215 gallons of LP gas.

20M x 85% efficiency (which is really good)  so now at 17M  divided by 4 heating months (short season for most wood burners)  or 4.25M BTU's per month.
or 141,600 Btu's per day...  or at .6 cords  that is only 84.000 btu's per day...

What are you planning on keeping warm on that little amount?  These are general figures and you can use what you think best for the starting number  But the end results, is how much you expect to heat and your expected heat loss.  

Pounds of wood to equal 20 million btu's  is a better way to calculate as wood does not vary to much per pound vs volume.   Again you can fudge numbers all you want.    All this being said---

A RMH of any type that works well, in my opinion is simply a stove that extracts the most heat for a given pound of wood that you can put through it. It is not magic,  It does not make   a pound of wood expel more btus, than any other stove, (even that can be said different if your bring the burn science that you gain)  but designed right, it captures more, in a simpler NO electric manner

Nick Ax wrote:There is no insulation under the mass … that seemed pointless .. the floor IS extra mass n’est-ce pas?

(The stove is very deliberately at the heart of the house, away from the external walls)

No it is not completely necessary to insulate under the mass and many people dont.
I think you will be ok on top of your concrete floor but the floor will act as a heat sink, stealing heat from the mass.
Having the mass slightly elevated with an air gap is possibly the most effective as it allow a thermal  convection to take place.


The more important part is under the core.

Hi Scott

I’m a rational man, an engineer (albeit, majoring in software) .. so no I don’t expect magic … just a clean burn and a cool exhaust temperature, I totally get that you can’t do much better than that - the whole journey started for me (as I imagine it does for many) contemplating just how much heat was going up my chimney.

I would like to heat more space, for longer, with less fuel than my previous “efficient” Morso stove … I think that is probably  happening now, but my feeling is not by a factor of 3 (which I’d settle for), let alone a factor of 7

Mr Wheaton seems like a nice guy, and I don’t think he practices to deceive .. which leaves me thinking I am not quite there with my RMH..

Today we flipped the riser over, so the (minor) constriction is now at it’s top, not it’s bottom - intuitively (and possibly wrongly) I feel that might help.. create a little more pressure inside the riser

I also sealed some fairly obvious gaps between bricks .. 1mm or so at riser end of the burn tunnel

I fitted (as best I could) my remaining foot square of 1” ceramic fire blanket on the back side of the end wall of the burn tunnel, and made a pretty shoddy attempt at insulating the sides of the burn tunnel/riser base with perlite refractory mix - didn’t go well did NOT want to stick even to itself, let alone anything else.

So anyway .. it’s back together now and I’m burning it gently now to cook off some of the wetness before I crank it harder later.

Today is flat calm and it’s noticeably less rockety, I even had a puff or two of smoke into the room

More radical experiments, with bigger barrels and/or viewing windows and mild steel top plates await, I will keep you posted.

Forgive me if I haven't read all the suggested fixes in detail, but what often comes up in these troubleshooting discussions is a question about the size of the plenum going from the barrel to the ductwork.  That transition is an important area and subject to creating a choke point.  Again, please excuse me if this has already been mentioned.  I also am of a mind that a full size oil drum is probably moving in the right direction as well.

Your build seems to be on the edge of drawing well enough... reliable starting draft but not able to run very hard sounds like too many constrictions in the flow path. A larger barrel may make a considerable difference. I have found that my 2" thick 8" i.d. perlite-clay riser rockets great; I know redoing yours to be 2" thick would be a considerable job, but it would drastically increase flow area with your existing barrel. A 5-minute riser would give even more space at the expense of new material cost.

The manifold transition to ductwork is important as mentioned, but if riser/barrel improvements don't help enough, I would suggest the major change of digging out the ductwork leaving the mass walls 6" or so thick, and turning your design into a bell which would eliminate a ton of friction. Going down almost to the floor with the cavity and making it 12" high and 2' wide would give a nice bell space.

Thank you chaps .. time zones delay my reply

Converting from pipes to a bell isn’t on the cards - not for this year any way, The stove is indoors, and I don’t really trust my masonry skills to produce an airtight chamber, the pipes have a good airtight seal silicon gaskets at all joints, plus foil tape, plus cobbed in - I am comfortable they are gas tight.

I am trying to find a steel 55 gallon drum with a clip top (or two) , not too battered and within striking distance - but actually, for the ‘experiment’ anything would do

I will keep you all posted

Nick, I have  lots of videos on my channel but this one might interest you …..

Thanks Fox, watched with great interest

I would say I have triple your fuel consumption and half of the heat output … perhaps not comparing apples with apples as much of my heat will be going into my mass

Half the (immediate) heat output is actually fine for me .. there is enough immediate heat for the room (which is well insulated)

Today I have noticed (although it might be my imagination) that the (concrete) floor is warming up …
Actually … no ..  I just measured it and it’s not my imagination .. under the carpet, within 1” of the stove it’s 20c … fully 5c warmer than under the same Carpet on the far side of the room

So I may have unwittingly created an underfloor heating system

I like your cook plate p, Are you saying that the hole is a necessary feature for the dishing deformation ?

One theory for the lacklustre performance of my stove is that the relatively non-conductive barrel is not ‘dumping’ enough heat to ‘drive’ the downdraft, perhaps cutting the top out of my  beautiful barrel and replacing it with a (much more thermally conductive) mild steel cook plate would help.

Steel cook tops are notorious for deforming and breaking seals, the hole seems to address this very nicely!
Yes my stove has been developed over several years to suit my exact needs that are large multi temperature cook top, loads of quick radiant heat and enough mass (about 300kg of vibrated concrete) too keep the room warm for a few hours after the fire is shut down.
You might of noticed how my barrel is very easily removable if not I do have a video!
I did say your concrete floor might act as a giant heat sink….