Combination stove for yurt heating

[fuzzy]

Hey all, I've been researching this stuff for years now, and I finally have a reason to use it. My friend is building a 26' diameter yurt, and he's left it to me for designing a cooking/heating apparatus for him which would burn efficiently, and keep the house from freezing overnight and during the day while he's in class or at work. So here's what I've got so far:

Aprovecho-esque cooktop with a modified "j" elbow for the fuel intake exhausting into a 3-flue mass heater constructed of three sections of precast concrete (stacked to make a 16"W X 32"L X 24" H mass) lined with thin walled steel pipe. The cooktop will be able to take the majority of combustion heat, with the heat riser made of clay/sand/perlite/sawdust refractory which has shown to take the heat without cracking. I know from testing with foundries that concrete can take high heat, though it must be heated gradually, which is why I elected to remove the actual combustion chamber from the casting (it made for a right nice cooktop anyway...). Concrete can also crumble, so lining the flues with pipe seems like a reasonable thing to avoid that issue.

SO... I have no data to know how long 850lbs of concrete will take to heat, and consequently if it can store enough energy to heat 530 square feet for 10 hours at a time.
Also, I've calculated this using a 4" primary flue, 4" with a 1.5" tube inside(as a water heater) for the secondary, and a 3" for the final exhaust(or put another way, 100%, 85%, 65% for flue area). Is this enough, or should it be bigger? Thanks,
-Fuzzy

[Donkey]

Hmm..
Well, for space heating I usually start with six inch flue sizes. Smaller than that is somewhat less than truly useful. Also, six inch pipe fittings are common, easy to scavenge and carry a low cost (probably due to production volumes). I've found that 4 inch systems are kind of a bother to prep wood for and require constant fiddling as the smaller wood burns away faster.
The risk being the possibility that it could heat your concrete parts too quickly, six inch systems can be quite energetic.

I really can't speak to the 850 lbs heating question.. Just too many variables. Perhaps the mass stove folks have a rule of thumb for that.. ?? The cob benches I build tend to be several tons and I've never attempted a tight nail-down of heat storage capability per volume for it. My general rule, more mass is better up until the point where you've cooled the flue too much and created a condensation problem.

One piece of SOLID advice I can give is this (and you may already know this): place your water pipes and/or anything you wish to dump heat into AFTER the heat riser. It's obvious when you think about it, though I can't tell you how many times I've seen folks (who should know better) design heat exchangers (water coils, etc.) into the heat riser itself. The riser NEEDS to stay as hot as possible inside and anything in there stealing that heat away is counter productive.

Oh, and.... Welcome to the boards!

[fuzzy]

Thanks, I assumed that having the water heating downstream was just common sense, I suppose assuming will not get you anywhere... The heat riser and everything will actually be in the cookstove part, actually an ecofogon or ecohorno in design, depending on what he needs. The input for that I do believe will be a 6" system, so that the cutting of wood, etc as you mentioned will not be a problem, as well as having ample heat to cook with. The exhaust from that is what is getting plumbed into the 4" flue system in the mass heater. I'm working on some drawings, and will post them later.

[Donkey]

...
So, you would be splitting the flow through multiple 4" pipes??
You probably already know this, though I'll point out the obvious again.. It's a general rule to keep (piping) volume the same (or greater, never smaller) throughout the system (end to end). Also (more of what SHOULD be obvious but isn't), when you double pipe size you quadruple it's volume (cross-sectional area, actually). Meaning that a single 8 inch pipe would need four 4 inch pipes to match areas.
SO.. A six inch heat riser, firebox has a cross sectional area of about 28.26 inches square. A single 4 inch pipe has about 12.56 inches square of area.. It means you will need 3, four inch pipes with a combined area of about 37.68 inches square. 2 four inchers are just a bit too small coming in at around 25.12 sq in..

Of course, as the flue gasses cool they contract. Pipe sizes can be reduced, downstream, along the flow-path somewhat.. I imagine that doing this properly would increase heat exchange efficiency somewhat. I also imagine that getting the numbers right could be quite challenging and getting it wrong can lead to a stove that's more trouble than it's worth!
I have been told that (again) mass stove pro's have a rule of thumb for this, though I've been less than successful at dredging the info up.

Also, I think I've deduced (correct me if I'm wrong) that you intend to use an Aprovecho style, horizontal feed box. I'd like to recommend a down-feed style box instead as they are a bit less fiddly. The wood works itself down into the fire, tending to keep it in the optimal burn zone automatically.
I'd be glad to elaborate, if you wish.

Sounds to me you have a fun project ahead. Please keep us abreast as you progress!!