Post by invention1 on Mar 7, 2019 8:32:25 GMT -8
Rocket Stove Postmortem:
Here are some of the issues I faced with the rocket stove. This was not supposed to be a prototype.
1. Mortar failure. This was the killer and caused me to decide to tear everything down but the bench. The MSDS for purchased refractory mortar lists the ingredients - get a mortar that lists clay or Kaolinite as the main ingredient. If it lists Sodium Silicate and Silica sand or quartz, but no clay, it will fail in your rocket stove.
[EDIT] It may also be the case that all refractory mortars will fail in a rocket stove due to repeated expansion and contraction of dissimilar materials (bricks/mortar). Should we be casting cores exclusively?
2. Creosote buildup. This is very worrisome. I don't believe we were getting good combustion, especially on pellets. Lots of smoke in the pellet mode. Smoke means creosote, and also poor combustion efficiency. Creosote can mean chimney fires, a serious issue.
Although there was no black buildup on the walls of the cordwood firebox and the riser, there was a point inside the pellet horizontal fire tube that was blackened, where there was a change in direction and diameter. This probably explains the combustion issue - the firetube (like a horizontal riser) should be a smooth path so as not to disturb the combustion of the smoke. The pellet system was designed like a J-rocket, with a slot for secondary combustion air, but after that it should have a smooth, constant diameter fire tube (whether a vertical riser or a horizontal fire tube, the fire doesn't care). However it had design flaws that unexpectedly caused a serious issue.
[EDIT] I now believe that the pellet fire tube or riser should be made of insulating firebrick. I used hard firebrick, important for a cordwood firebox to resist abuse, but pellets are gentle beasts.
Most people haven't reported creosote problems in things like benches. This is a serious issue that could be dangerous. I noticed the most deposits on the inside of the metal bell, which is a steel tank 20" diameter 54" tall. Most bells and benches are built so that they are not easy to clean. I'm thinking that the bell might need to be removable for inspection and cleaning. If the pellet system can't be made to work properly, it must be abandoned.
Even on cordwood, I don't think I was getting good draw therefore poor combustion. This resulted in anemic fires, which is why I was adding fans, but still didn't get the airflow up to speed. Trying to blow cordwood combustion air through the pellet firebox, through the pellet grate, through other obstructions and turns was too much resistance. I have great draw at the chimney, and saw signs inside the masonry bell that that part of the system was always under negative pressure. I have measured the draw at the chimney to be on the high end of what is recommended. It isn't a chimney problem, it is a primary air inlet problem. The air inlet is sized right but such a system can't stand any twists and turns. The idea of combining the primary air inlet for the cordwood and the pellet system into one inlet was flawed.
3. Kapow! On pellets, there were a few places where pellets could fall through my pellet basket system, into the ash drawer. Add a few coals, and the ash rawer turned into a source of smoudlering smoke. Smoke is fuel, and eventually it would fill up the ash drawer volume, and find a source of ignition. It would then explode, not dangerously but it was quite exciting. A good four foot flame shot out of the primary air inlet one time. Yow! I solved this problem by adding the fan, and a baffle that redirected primary air into the ash drawer, purging it of this explosive smoke. However that baffle was one more thing to limit airflow, another thing making anemic fires. There needs to be a secondary grate under the pellet grate, and the ash drawer, if used, needs to get purged by any incoming primary air. The ash drawer is probably only a pellet stove feature - not really needed and normally not used on a cordwood stove. The same potential exists in a cordwood stove with an ash drawer although I did not observe it. The ash drawer won't be on the cordwood section.
4. Pellet Feeding Problems I had great luck with prototype pellet feeding systems about 2" square and straight 45 degrees down. I had poor luck with a prototype pellet feeder with a 2" tube with a 45 degree bend in it. Pellets got stuck. I discovered that a gentle 45 degree 2" Metal Conduit fitting was just right, would feed pellets pretty well. I needed this bend to get the pellets to feed right. However in the real stove, whenever this metal tube got up to temperature, it would make the pellets sticky, and they'd adhere when they were stopped, after filling the firebox. On in other words, the gravity feed system would fill up the pellet firebox, then the pellets would back up, and stick in the tube when hot. The result was a hard, almost impossible clog that stopped up the whole works after the fire load of pellets had finished burning. This happened several times, it was not a fluke.
What is needed for gravity feed is a larger tube (many are using 4" square) that is absolutely vertical. If the tube has a thermal break so it doesn't get so hot, that is better. The tube should be accessible for cleaning. The pellet hopper, that contains the fuel that will eventually gravity feed, has to be absolutely airtight to prevent backburning. This is a lesson from a commercial stove manufacturer, that had to recall stoves and replace the pellet hopper.
I've messed about with auger pellet feed. It is noisy, pellets are gritty and tend to bind up easily, cloggin up the works, it takes a large amount of torque - pellets are not liquid, they are more like little rocks. I broke the last pellet auger I was prototyping, which was quite stout, and was darn tired of listening to it grind away. A timer control would be required to make a feeder work. Gravity feed is so simple if I can make it work.
Part of the reason I needed the bend was because the pellet section of the stove was on the bottom, could not put the feed tube in the middle of the cordwood firebox. Duh, the pellet stove needs to go on top, with the pellet hopper directly above for a straight vertical path, the cordwood section must go on the bottom. Insulate between them with ceramic fiber insulation to keep the pellet system happy.
5. Fans
No more howling, fans are almost required to make a pellet stove work despite some DIY counterexamples. Pellets create resistance to airflow, baffles create resistance to airflow, small firetubes and risers create resistance to airflow, buildup of ashes in the pellet grate create resistance to airflow, all of these factors make natural draft iffy. I'm planning on a combustion air fan for the pellet system and allowing a space in case the cordwood system requires one. Hopefully I will design out the problems in the cordwood section to make it unnecessary.
People are worried that a fan will cause smoke to blow out of the masonry parts of the bell, but I did not experience this issue, even after discovering some pinhole leaks in the bell. Inside, there was no creosote by the pinhole leak, showing that air was indeed traveling into, not out of, the bell, and the bell was under negative pressure. The port in a cordwood rocket is a significant pressure drop, and the inside of the riser and bell are always be under negative pressure. The fan was an afterthought, so the wires were laying all over the floor, not tidy. The fans are 12 V, because 120V would be dangerous if there was any problem with overheated wires. Everything stayed nice and cool, but I still would not use 120VAC near a home-made strove. The next iteration will have conduit or wireways built in.
7. Minor issues I switched from refractory cement to regular Type S masonry cement too soon - there was always a crack where I switched to the portland cement based product. Either use the refractory cement all the way, or plan on sealing those joints with something like silicon rubber, which will handle elevated temperatures within reason. Dow 100% Silicone Window and Door caulk, for example, lists a service temperature of 400F after full cure, the outside of your stove masonry parts should never be this hot. I used silicone to adhere the bell to the masonry, it was fine.
[EDIT] A post below recommends gasketting between the firebox masonry and any normal masonry or bell.
I built a 6" stove. It was tiny, and hard to fit wood into. This stove has a firebox a little over 8" wide. That's just a teeny space. I'm considering scaling up, not only because I think I can use the heat, but it's a lot easier to stoke if I don't have to whittle every piece of wood into kindling. I believe I could build an 8" stove in the same spot without too many issues, I've figured out how to scale up the bench and bell some to get the proper area, and the existing flue is already 8". If I build the cordwood section to Peter's dimensions exactly, I shouldn't have the airflow issues. My house could use the additional heat in the coldest weather, which we had this winter.
8. Successes One of the issues people complain about is that the doors on rocket stoves smoke. I put the top of the door even with the top of the port, and the stove did not smoke excessively. The smoke, when there was any, would just balance at the door when it was opened.
9. Other than that it was a hit Ah, laying on the bench and getting thoroughly warm was just a treat. Finding the stove still warm when I get up in the morning was amazing. Quick heat off the metal bell, plus slow heat from all the masonry hours later was incredible.
Here are some of the issues I faced with the rocket stove. This was not supposed to be a prototype.
1. Mortar failure. This was the killer and caused me to decide to tear everything down but the bench. The MSDS for purchased refractory mortar lists the ingredients - get a mortar that lists clay or Kaolinite as the main ingredient. If it lists Sodium Silicate and Silica sand or quartz, but no clay, it will fail in your rocket stove.
[EDIT] It may also be the case that all refractory mortars will fail in a rocket stove due to repeated expansion and contraction of dissimilar materials (bricks/mortar). Should we be casting cores exclusively?
2. Creosote buildup. This is very worrisome. I don't believe we were getting good combustion, especially on pellets. Lots of smoke in the pellet mode. Smoke means creosote, and also poor combustion efficiency. Creosote can mean chimney fires, a serious issue.
Although there was no black buildup on the walls of the cordwood firebox and the riser, there was a point inside the pellet horizontal fire tube that was blackened, where there was a change in direction and diameter. This probably explains the combustion issue - the firetube (like a horizontal riser) should be a smooth path so as not to disturb the combustion of the smoke. The pellet system was designed like a J-rocket, with a slot for secondary combustion air, but after that it should have a smooth, constant diameter fire tube (whether a vertical riser or a horizontal fire tube, the fire doesn't care). However it had design flaws that unexpectedly caused a serious issue.
[EDIT] I now believe that the pellet fire tube or riser should be made of insulating firebrick. I used hard firebrick, important for a cordwood firebox to resist abuse, but pellets are gentle beasts.
Most people haven't reported creosote problems in things like benches. This is a serious issue that could be dangerous. I noticed the most deposits on the inside of the metal bell, which is a steel tank 20" diameter 54" tall. Most bells and benches are built so that they are not easy to clean. I'm thinking that the bell might need to be removable for inspection and cleaning. If the pellet system can't be made to work properly, it must be abandoned.
Even on cordwood, I don't think I was getting good draw therefore poor combustion. This resulted in anemic fires, which is why I was adding fans, but still didn't get the airflow up to speed. Trying to blow cordwood combustion air through the pellet firebox, through the pellet grate, through other obstructions and turns was too much resistance. I have great draw at the chimney, and saw signs inside the masonry bell that that part of the system was always under negative pressure. I have measured the draw at the chimney to be on the high end of what is recommended. It isn't a chimney problem, it is a primary air inlet problem. The air inlet is sized right but such a system can't stand any twists and turns. The idea of combining the primary air inlet for the cordwood and the pellet system into one inlet was flawed.
3. Kapow! On pellets, there were a few places where pellets could fall through my pellet basket system, into the ash drawer. Add a few coals, and the ash rawer turned into a source of smoudlering smoke. Smoke is fuel, and eventually it would fill up the ash drawer volume, and find a source of ignition. It would then explode, not dangerously but it was quite exciting. A good four foot flame shot out of the primary air inlet one time. Yow! I solved this problem by adding the fan, and a baffle that redirected primary air into the ash drawer, purging it of this explosive smoke. However that baffle was one more thing to limit airflow, another thing making anemic fires. There needs to be a secondary grate under the pellet grate, and the ash drawer, if used, needs to get purged by any incoming primary air. The ash drawer is probably only a pellet stove feature - not really needed and normally not used on a cordwood stove. The same potential exists in a cordwood stove with an ash drawer although I did not observe it. The ash drawer won't be on the cordwood section.
4. Pellet Feeding Problems I had great luck with prototype pellet feeding systems about 2" square and straight 45 degrees down. I had poor luck with a prototype pellet feeder with a 2" tube with a 45 degree bend in it. Pellets got stuck. I discovered that a gentle 45 degree 2" Metal Conduit fitting was just right, would feed pellets pretty well. I needed this bend to get the pellets to feed right. However in the real stove, whenever this metal tube got up to temperature, it would make the pellets sticky, and they'd adhere when they were stopped, after filling the firebox. On in other words, the gravity feed system would fill up the pellet firebox, then the pellets would back up, and stick in the tube when hot. The result was a hard, almost impossible clog that stopped up the whole works after the fire load of pellets had finished burning. This happened several times, it was not a fluke.
What is needed for gravity feed is a larger tube (many are using 4" square) that is absolutely vertical. If the tube has a thermal break so it doesn't get so hot, that is better. The tube should be accessible for cleaning. The pellet hopper, that contains the fuel that will eventually gravity feed, has to be absolutely airtight to prevent backburning. This is a lesson from a commercial stove manufacturer, that had to recall stoves and replace the pellet hopper.
I've messed about with auger pellet feed. It is noisy, pellets are gritty and tend to bind up easily, cloggin up the works, it takes a large amount of torque - pellets are not liquid, they are more like little rocks. I broke the last pellet auger I was prototyping, which was quite stout, and was darn tired of listening to it grind away. A timer control would be required to make a feeder work. Gravity feed is so simple if I can make it work.
Part of the reason I needed the bend was because the pellet section of the stove was on the bottom, could not put the feed tube in the middle of the cordwood firebox. Duh, the pellet stove needs to go on top, with the pellet hopper directly above for a straight vertical path, the cordwood section must go on the bottom. Insulate between them with ceramic fiber insulation to keep the pellet system happy.
5. Fans
No more howling, fans are almost required to make a pellet stove work despite some DIY counterexamples. Pellets create resistance to airflow, baffles create resistance to airflow, small firetubes and risers create resistance to airflow, buildup of ashes in the pellet grate create resistance to airflow, all of these factors make natural draft iffy. I'm planning on a combustion air fan for the pellet system and allowing a space in case the cordwood system requires one. Hopefully I will design out the problems in the cordwood section to make it unnecessary.
People are worried that a fan will cause smoke to blow out of the masonry parts of the bell, but I did not experience this issue, even after discovering some pinhole leaks in the bell. Inside, there was no creosote by the pinhole leak, showing that air was indeed traveling into, not out of, the bell, and the bell was under negative pressure. The port in a cordwood rocket is a significant pressure drop, and the inside of the riser and bell are always be under negative pressure. The fan was an afterthought, so the wires were laying all over the floor, not tidy. The fans are 12 V, because 120V would be dangerous if there was any problem with overheated wires. Everything stayed nice and cool, but I still would not use 120VAC near a home-made strove. The next iteration will have conduit or wireways built in.
7. Minor issues I switched from refractory cement to regular Type S masonry cement too soon - there was always a crack where I switched to the portland cement based product. Either use the refractory cement all the way, or plan on sealing those joints with something like silicon rubber, which will handle elevated temperatures within reason. Dow 100% Silicone Window and Door caulk, for example, lists a service temperature of 400F after full cure, the outside of your stove masonry parts should never be this hot. I used silicone to adhere the bell to the masonry, it was fine.
[EDIT] A post below recommends gasketting between the firebox masonry and any normal masonry or bell.
I built a 6" stove. It was tiny, and hard to fit wood into. This stove has a firebox a little over 8" wide. That's just a teeny space. I'm considering scaling up, not only because I think I can use the heat, but it's a lot easier to stoke if I don't have to whittle every piece of wood into kindling. I believe I could build an 8" stove in the same spot without too many issues, I've figured out how to scale up the bench and bell some to get the proper area, and the existing flue is already 8". If I build the cordwood section to Peter's dimensions exactly, I shouldn't have the airflow issues. My house could use the additional heat in the coldest weather, which we had this winter.
8. Successes One of the issues people complain about is that the doors on rocket stoves smoke. I put the top of the door even with the top of the port, and the stove did not smoke excessively. The smoke, when there was any, would just balance at the door when it was opened.
9. Other than that it was a hit Ah, laying on the bench and getting thoroughly warm was just a treat. Finding the stove still warm when I get up in the morning was amazing. Quick heat off the metal bell, plus slow heat from all the masonry hours later was incredible.