Tuesday, January 6, 2009
Paen to rocket stove furnace
Our rocket stove furnace space heater water heater storage unit heat exchanger works!
Well, lets start on What works, and put off what doesn't.
1. We can roast hotdogs inside in the winter. And there is NO smoke inside the house. The burning is efficient, and it all goes down, across into the firebox, up into the heat riser, down inside the stove surface to the floor, and out the vent. It's comparable to a pellet stove (see the Rocket Mass Heater book). Having the combustion surrounded by maximum air with turbulence creates a higher combustion temp and more complete combustion. The result is primarily CO2 and H2O in the vent. And a hot fire. It's difficult to avoid burning the hot dogs before they are even warm in the middle. And, if the open fire is needed for ambience with less monitoring of little fingers and toes, we have
a spark screen installed.
2. The vent temperature upon exit from the chimney is cool. The 28 feet of 8" schedule 40 junk salvage steel gas pipe welded together and sealed has transferred the heat of combustion to the inside of the living room area. It's as good as the high-tech heat pump used in the vent of super-efficient furnaces to do the same thing.
3. The CO alarm works. I tested it by deliberately trying to make the stove burn backwards. It did, smoked up the place, and the alarm went. I know the burner stack, stove body, and interior vent pipe is tight.
4. The firebox/heat riser stack inverts the flame. The initial combustion gases move downward, then across two feet into the base of the heat riser. The combustion is complete by the time the gases reach the top of the heat riser, 4 feet up from the bottom of the firebox, the heat driving the gases upward. It takes 15 minutes of strong burning to heat up the inner core heat riser--well casing. 8" schedule 80 steel, to make this drive upward really strong, and getting the stove to roar (softly, not an objectionable sound). This is surrounded by some 200lbs of sand and a layer of vermiculate against the inner core. Holding the sand and vermiculite in place is a discarded 55 gal drum cylinder. All this is surrounded by a 30" diameter tank, formerly an industrial water pressure tank discarded because the rubber diaphragm broke. Only this tank is visible inside the house, beside the firebox. Between the barrel and this surrounding tank is a 2"
gap, through which the vent gases descend and cool, down through the entire circumference, giving off the heat to the outside steel tank. The pressure of this "rocket pump" pulls the flame in the firebox down and across, and up into the inner core.
5. With short pieces of wood, the fire box can be left open and no smoke come into the house. But if the firebox, kept open, is over-stacked full with small pieces of wood, especially longer than 1 foot, part of the flame will begin creeping up, eventually pulling the fire backwards, and the flame
into the house. The fire in the box goes out because the vent gases contain too little oxygen, and the fire turns into s smoldering smoke bomb. This scenario is prevented by:
6. A door. See the picture of the three-sided door, which completes the feeder tube so that nearly all the combustion air must pass onward past the unburned wood into the tube, to the fire-supporting coals on the ash pit, and follow the planned burn path. I have not been able to make the stove burn backward with -the feeder tube enclosed by the door (see pict with it beside the box) -waiting for fifteen minutes of strong burning with short pieces of fuel before inserting long pieces -making sure windows in the upper part of the house are closed, especially on a cold day Any one of the above conditions neglected CAN reverse the burn path. Not a good thing.
7. Stacking the feeder tube and leaving. I have no problem doing this when I make sure of the above three conditions. Three possible conclusions are observed the next morning:
-All the fuel has burned
-The wood jammed in the feeder tube, and combustion stopped without reversing the burn path.
-The wood was not adequately cured, and combustion stopped without the support of other smaller, better-cured wood pieces.
So the stove is easier to manage by checking it every hour or so, or staying right with it and roasting hot dogs/marshmellows in the open fire inside the house. What fun.
But it is manageable for longer term burning, unattended.
But (repeating those conditions!):
-close the door, ensuring that air must descend in the feeder tube to the firebox
-Use one piece of well-cured wood, large enough to maintain hot coals in the ash pit. Or two smooth straight pieces of equal size. No crooked, snaggy, or thin pieces, and not more than two.
-Wait to stack the feeder and close the door until the sound of the stove indicates the inner core has heated. 15 minutes minimum, for the roar to increase and stablize.
8. Ash removal. Our firebox is built down into the floor of our barnhouse with the vertical tube ash pit 12 inches below the floor surface. Early on we removed the 2" foam eps, and the bottom of the pit is on undisturbed clay subsoil about 6" below the original grade.
The ashpit is easy to access to remove ashes. And the small quantity of ashes is surprising. In a full week of burning nearly every day, the ashes barely filled a large
9. The thermo-siphon from the firebox heat exchange loop up 14 feet to the 250 gallon storage tank easily opens the check valve and circulates, making a complete changing of the water in the tank adding about 6 to 10 degrees F in four hours of strong burning.
10. The loop to the floor pex radiant heat pipes, with a 1/25 HP pump, takes ALL the heat from the thermo-siphon loop and pushes it back down into one or more of the four zones in the floor. The storage tank stays cold. The floor DOES store the heat.
WHAT DOESN'T WORK:
1. See number 10 above. The tank is very slow to build up the amount of heat needed to really warm the floor. The floor space is 30'X40', pretty large, and 12" thick, clay, gravel, sand and concrete. There is not enough heat transferred to the 6 foot loop in the firebox to heat the big tank and the floor both at any reasonable speed. A 12' loop in the firebox would be better.
2. See number 4 above. The firebox is too small. It's only two bricks deep, about 6", and should be three bricks deep. The fire gets very hot, but it's also confined to a small space, so lacks the BTU's needed to meet the demands of this large area, even with super insulation (40X60). We do have the auxiliary stove on the east side of the barn, which can easily burn with about 8X more than the rocket furnace of the quantity of fuel on fire at one time. This of course heats up the place much faster, maybe 3 or 4X faster, but not 8X. It also smokes a lot more.
Posted by David Mark at 6:32 PM