I have build a small masonry heat accumulator.
It's constituted of about 120 special firebricks,maybe 300kg total.
Maybe I'll post pics and more details someday,if people are interested.
I have quality scandinavian stove that burns with very little creosote.
The accumulator is heated withh the exhaust gasses of the stove.
It works for more than 4 years now and I'm satisfied ,sure the accumulator can last longer than the stove.
The smoke goes down for about 1,5' before going upward again.
My estimations are the stove dissipates more than half of the heat,the accumulator half of the remaining heat and the 3m iron pipe after the accumulator a significant proportion of the heat still in the gasses after the accumulator.
It can be that the efficiency is around 80%.
Probably with an heatexchanger for water in theory I can get over 90%.
But you have to be carefull not to go to far
If you get excessive condensation you know you want too much.(Maybe above 85% or90%?).
This condensation is very corrosive and will destroy any iron parts(except stainless) and penetrate in masonry.
Worse, you can create situations where co (carbonmonoxide) will be dissipated,which is extremely dangerous.
For the downward part of the flow through the masonry I have to give the same kind of warning.5' is quite a lot allready.And I would never take the risk to go down ffor more then that.The first,downwards part of the channel is the hottest and this effect in a draft in the wrong direction.There can be a point where the drafft in the upward channels is not enough to counterbalance it(of course this depends on the length and t°of the upward channel).And a bad working stove is allways dangerous,mainly for the co.I would rather say 3 or 4' is enough,keep it on the safe side and forget about the 15' downward channel.If you want too much you will have to pay it back,possibly even with your or someone elses life.
My accumulator is just an improvement of my stove,boosting the effficiency and spreading the heatdissipation.
In my case 300kg masonry is enough to handle 1/4 of the heat ffor my 8 kw rated iron cast stove.
To handle all of the heat from a 10 kw fire in masonry you need probably 1500-2000 kg masonry.
There is a drawback from the 100% masonry approach:when you light the fire it's a long time before it really starts heating.I love the idea though.
Just my thoughts,
cheers,
stonebrain