I've been gathering information for a 8 inch batch RMH for my home and along the way I came upon this video that features a batch rocket/masonry stove hybrid that has a grate in the bottom of the combustion chamber as a means of letting the ashes fall to a removable tray. This seems to be almost standard equipment in the masonry stove world but I have never seen it combined with a BBRMH.
Would some knowledgeable person care to comment on what sort of expectations to have for the life expectancy of a grate such as this. My thinking is that perhaps if the supplied air inlet were to be the standard Peter style, and not coming up through the grate, as the masonry stoves have it, then the grate might last much longer. Having an ash dump like this seems like such a design advantage, I felt it was worthwhile bringing this question to the forum.
I would say: nay! For more than one reason and also one exception.
A grate implemented like this won't live long when air is entering through the ash drawer. It also influences the combustion in a negative way because off-gassing is commencing to a rate the afterburner can't cope with resulting in black smoke.
Even when the ash drawer is closing really tight a grate like in the video will let cold air under the grate in the front part and exhausting it heated up out the rear part. Ramping up the pyrolysis rate to a level which will be too much for the riser' function. This isn't just a spooky story, I tried to implement something like that once. Just one narrow slit in the middle of the firebox' floor and clean combustion was gone during half of the burn.
The single exception is this: if the grate is restricted to the rear half of the firebox floor and closed off fully or near so with a flat piece of fuel during most of the burn it wouldn't have the above mentioned disadvantage.
Lately some questions were asked about this and I always tell the same story.
First, ash in a bog standard stove consists of about half of small pieces of charcoal. On a closed floor as in the recommended configuration the ash bed will get much, much hotter so nearly all charcoal will be burned.
Second, my preferred method since a year is this: I close the air intake when the glow phase is well on its way. The fire dies because it is starved of oxygen and leaves some charcoal on top of the ash bed. The next run I build the fuel batch on top of that and after the burn the remaining charcoal is the same volume as when I started. The reason for this is that the charcoal under the new batch will catch fire very easily, caused by the much lower self combustion temperature as compared to wood. This, together with the insulation properties of the (shallow) ash bed, will help to burn the wood more completely in a very even way.
Of course the ash bed is building up although very much slower than you would expect. Once a week in midwinter I scoop some ash out and leave the rest in situ to insulate the underlying floor channel. In the shoulder seasons there's no need to do that more frequently than once a month. Of course this could be different in your circumstances, it's just an example.
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