Here is an update how the winter is going here in New Hampshire with this Jotul 118 converted to rocket mass heater
. Overall it is going well, and my estimate is that it is three times more efficient with wood than my old outdoor 5036 central boiler.
I have learned a few things in the first few months of this first winter. For example, I burn mostly pine, either bunches of 12 foot long 2" x 2" pieces of pine bundled into a 3' x 4' x 12' bundle for nearly free or free lumber cutoffs from a local
truss company. But, because the vast majority of my burn is burning the creosote in the gases with the second combustion as well, when I inspected my stove a couple of weeks ago there was only a bit of soot built up on the inside of the system. Not enough to worry about until cleaning next fall.
I've also learned that the extra oxygen provided by the P-channel is quite important. My P-channel runs under my firebrick inside the cast iron
stove entering from a hole I cut on the bottom of the stove. But because my P-channel hole comes out from the bottom of the riser, it can fill with ash
and get clogged. I have to clean it out every week or so just to make sure I'm getting enough oxygen in the riser for a proper second burn.
I want to give an accurate picture of how much fuel I use and how much attending this fire needs. I am burning indoor air right now, so my door is left open at about a 45° angle. My firebox is effectively about 16 inches long, but if I fill it only at the back near the riser with 12 inch material, then no smoke gets into the house even with the door open. The burns do not last for about an hour as I listed in my last post. If you stuff it completely and just start the fire it might last for an hour, but I end up putting more wood in about every 15 to 20 minutes in order to maintain a good clean secondary burn in the riser. I set a timer on my phone and come back to put more wood in on this 15 to 20 minutes schedule. My house is decently tight and reasonably well insulated, but it is a large house with a 64' x 32' footprint
and two stories with a flat roof, somewhat like a brick. But on a cold day like today where the high was 15°F in the low will be about 0°F tonight, I need to keep up a burn for about four or five hours total in order to keep the house at an average of 63°F (56 degrees in the mornings, 68 degrees when I go to bed). This ends up being about 60 to 75 pounds of wood (15 pounds per hour), and if I assume I'm getting about 5000 BTU per pound, then this is about 350,000 BTUs from this stove for the day.
There is still too much heat going up the chimney for me however. I do have maybe 600 pounds of rocks in the bottom left barrel, but they can only hold so much heat. I have put three box fans on high pointing at different parts of the system, and that helps quite a bit to transfer heat into the room. But the chimney above the barrels is still hotter than I want to be. I'd like to add another barrel on each side another fan as well. Also would like to burn outside air through an intake in the floor.
The duct fan didn't last too long, but I found I didn't need it once it got a little colder and the system had a better draw.