Reasons not to use a fan?

Why not remove 100% or as near to it as possible and drive the exhaust with a fan? If the mass was designed to deal with condensation is there any reason not to do it? We're talking about very low energy usage something like 50-100 watts for an efficient fan motor with a good blade design. It seems more efficient as that last 50-80 degrees needed to drive a chimney represents a greater energy loss than would be needed to drive the stove/heater.

Fans fail and electrical supply can fail. Convection works without power. Some degree of ventilation is needed in the house. A wood burner can perform this function without electricity.
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If it were really necessary to send out cold air, the final portion of the exhaust could be a cold water jacket. The riser at the beginning of the system,  should push air through the pipes, without regard to the exhaust temperature.

I realize fans and electricity fail. Everything fails. I'm looking at ways to overcome the limits of the mass run length as well as extracting all of the heat. And, I'm looking for design reasons not to use an active exhaust. The fan may fail Isn't enough of a reason not to use it. If it were no one would be using forced air furnaces. Given nearly complete combustion and the ability to deal with condensation a quality fan costing not much more than 30 dollars should give decades of service. And the additional recovered heat would rapidly make up for the purchase price and the minor use of electricity, right?

Surely the condensation becomes a substantial issue then, along with fans failing.

Also, the risk with a fan failing is not so much that your rocket no longer works, but that a failure in use leads to dangerous exhausting back into the living space. You might go to bed and wake up dead from carbon mon-oxide poisoning!

One basic principal when designing systems to to design for fail-safe rather than fail-dangerous... passive ventilation is fail-safe.

Jim Fritz wrote:I realize fans and electricity fail. Everything fails. I'm looking at ways to overcome the limits of the mass run length as well as extracting all of the heat. And, I'm looking for design reasons not to use an active exhaust. The fan may fail Isn't enough of a reason not to use it. If it were no one would be using forced air furnaces. Given nearly complete combustion and the ability to deal with condensation a quality fan costing not much more than 30 dollars should give decades of service. And the additional recovered heat would rapidly make up for the purchase price and the minor use of electricity, right?

In a forced air furnace if your fan fails the fuel supply is instantly shut off, combustion stops and there is no risk of fumes venting back into the living space. You can't do that with a wood burner - the fan may stop but the fuel load sits there burning away. And in my experience of boilers/furnaces the fans are just as likely to fail as any other component - call out the engineer, he pokes around for 10 minutes and gives you a bill for £100 parts an labour.

We all employ multiple smoke and CO detectors, no? I'm also not sure how many people who are capable of fabricating a rocket stove/heater would call a repairman to replace a furnace blower.

However, fair enough, an alarm for a failed blower.

I am currently constructing an RMH in my greenhouse, however I became too enthusiastic in creating the channel under the floor and now do not have enough power to force the exhaust through. In this case I have installed an old computer fan which runs off of my gardens solar charged system. This works amazingly. However i would not be willing to use such a system in a house. However much building regulations are an annoyance, the basic principle is to avoid integrating a possible death trap into the design. It seems to me that what you are proposing is the possibility of gassing your household in the case of failure. Regardless of an alarm (electric failure would require batteries, who will remember to check and test?) these systems when developed properly work safely with out assistance. Why complicate matters?
I would suggest following the advice of the others above rather than inviting a darwinian solution to your suggestion.

Ps I will be back to ask for advice on solving my heater problems once I have spring out of the way.

I have a 2.5" burn tunnel with a 4" exhaust. There is a 4" inline duct fan from home depot at the end of the exhaust. It probably has close to 100 hours of operation. Initially after only several hours of use the gfci would flip, but after drilling a 1/4" hole in a horizontal section of the exhaust to allow moisture to exit it never flipped the gfci again. There is also a section of wire hanging from the hole to wick moisture. I know this feature works because ice crystals formed on the wick when running the stove on a cold night. There are also dc inline bilge fans for marine use that have a completely sealed motor coil. We have one of these for a backup...

Refrigerator fans can work too.

I have a 6" system in my greenhouse and I use a fan to pull the air through. Mine is a 40 watt, 200 cfm fan on a slider, so I can adjust the speed up and down as I need. Once my system is up and running, I often turn the fan off and if I restrict the air intake on the feed tube, the system still pushes through, even without a chimney (I have no chimney, the final exhaust exit is lower than the heater and the exhaust channel).

I live in the city, so I haven't seen a power outage in 10+ years of living here, and I am doing this in a greenhouse. For an inhouse application, I get everyone's concerns and they are totally valid. Two thoughts about the potential dangers. First, I don't think many people run their heaters while they're not there. Mine is out in a greenhouse, so I leave the room and can't see it, but for most folks, if it's in a living room or family room and you're hanging around there, checking the fire, etc. it wouldn't be that big of a deal if the fan died, the system backed up some and some smoke got in the house. If you had enough wood in it to burn for an hour or so and you have left the house with it running, or you put a load of wood in before going to sleep, that would be a bummer, and probably make the idea of depending on a fan much riskier and not worth it.

But here's the second question I don't know/get. I hear people talking about winds shifting and weather patterns affecting their draft. So what if all that happened and your draft backs up, it starts to burn the wrong direction, the chimney cools, it doesn't restart back the good direction and you have the exact same problem. Now it's a question of how often does that happen for folks with rmh's in different types of settings vs how often fans fail when you're not there to catch it.

My dad used to make parts for the aerospace industry, back in the days of the $20 diode that went up in a satellite until they figured out redundancy and that making one incredibly high quality part that should never fail costs way more than having 2-3 good parts that rarely fail and will almost never fail at the same time. So what about a fan at the exhaust and a small fan at the entrance to the feed tube? Now you've pretty much taken fan failure out of the equation as a possibility for failure and you're left with power outage. In some places, that's a real concern, but in Oakland, it hasn't happened in the last 14 years. Plus, if you were really worried about it, that tiny draw on an uninterruptable power supply would leave you power for hours, more than enough time to finish a burn.

One other piece I'm curious about is the ability of a fan to adjust burn conditions, potentially to more optimal burn conditions depending on the material. I don't know if wood that is too wet or pretty oily would like different conditions or if messing with the air flow could ever improve the burn. I wonder if any of the researchers playing with these have tried this? Some folks mess with restricting air into the feed tube and others work with the path the air takes coming into the feed tube, all of this could be controlled externally with a fan. I wonder if there's a theory/actuality that the fire knows what it needs and it always pulls the right amount of air or, if one could use a fan to increase the air flow in a way that would improve the quality of the burn?

Rick,

I'm actually looking at that. I'm on my third stove in twice as many weeks and all along I've been messing with ways to collect core temperatures for a microcontroller. I've found you can tune the combustion based on the draw and completely compensate for the differences between kiln dried hardwood and damp green wood chips. The stove design is refining itself, but I've already melted through a handful of K type thermocouples. However, with each failed attempt I've learned one more way not to do it and I'm currently on day five with my current configuration. I am running an 8 inch fan on a 5 inch system. Actually there are two in an exhaust plenum, read redundancy. As for the smoke back issues, I've found by preheating the exhaust my sealed chamber with an exterior intake is working quite well.

Oh, and as for those redundant fans. It is quite scary if you turn them both on at the same time at full RPMs. I mean the sound is amazing, though realizing your core just went molten is not so cool. It is however quite scary and exciting.

Right now I'm on the lookout for a used Testo so I can create some hard data to backup my work. The first one I bought wouldn't allow me to log the data, but on the plus side I turned around and sold it to an HVAC guy at a profit.

Sounds like fun, you're a more rigorous experimentor than I am (even though I do research professionally . I can't remember who said it, but someone was talking about too much air not necessarily being a good thing. Imagine the two extremes. On one end, you have no fan, the heater doesn't get enough draw at all, the air speed is basically 0, and the fire burns inside the feed tube and smokes straight up. On the other end, you put a 10" cannister fan there, 1200 cfm, you blow out your fire (ok, not that bad) but you add so much air to the fire, that instead of supercharging it, you're introducing more room temperature air than your system can heat and your fire burns horribly. Between those extremes is an ideal amount of air that lets any load of wood burn at its best temperature and it sure would be interesting if struggling rmh's could be fixed or boosted by adding a simple fan (kind of like overclocking with a microprocessor or using nitro on a race car).

I know a lot of folks have had significant success by altering the air paths and intake dimensions around the feed tube. It would be interesting to see how similar of effects could be obtained by: a) a draw fan at the end of the system pulling air through and out of the system at variable speeds (that's what I have), or b) an input fan, that blows air into the feed tube. Heck, maybe like a turbocharger with a car, you could preheat the air you blow in by wrapping the air tube around the barrel. It makes a big difference in a turbocharger (or is it supercharger?) to use preheated air for combustion, and I know in wood gassification, they're really good at making sure everything that's going to burn is at the optimal temperature for maximum efficiency. If you could get a temperature sensor inside the burn tunnel that could communicate with a controller that would alter the speed of a fan to adjust the flow of air to an optimal rate, that would be pretty cool.

The strategy of most folks here has been on designing the system in the optimal configuration so that it heats in a way and draws in a way to achieve maximum temperature on its own. My design is non-standard in a couple of respects, but because I knew I was planning on using a fan, I wasn't worried about my deviations from proscribed practice (mainly the length of my ducting - 35' with 4 u-turns, the absence of a chimney and a small amount of space between the outside of the riser and the barrel). I haven't tried to melt copper yet but I've hit 950 on the top of my barrel and I have a 5" gap. So either the need to meet very precise, specific constraints in the design isn't that critical, or, a fan can work some pretty good magic

too much air is the enemy of efficiency.
it's worm the outdoors.

If you could get a temperature sensor inside the burn tunnel

more practical to place it on the hotspot above the barrel.