Chimney Filter

Hey everyone!

So I just got back from testing, and I can finally show the project I’ve been working on for 8 months!
There’s been some recent (around December) studies out that indicate that the effects of tiny soot particles are much more serious for global warming than previously thought.

There already exist ways to replace your fireplace with a more efficient one, so I was aiming for a solution that would be a quick and relatively inexpensive fix.

As I mentioned elsewhere, it may not look like much for an 8 month project, but there was a ton of math and research involved.

I started with the initial concepts in parallel with my research. The point of this was to make incredibly stupid concepts first, then slowly improve them with knowledge, but still have the first few unbiased by the existing trends in the field. This took about 3 months because of the research in parallel.

Quite a few calculations were done here to figure out exactly what materials were needed and how much. The excel file I used is huge, but here’s an extremely compressed version if you’re interested:

Taking cues from these initial concepts, and my full knowledge of the field, I created some refined concepts:

Noticing some patterns in these refined concepts, I broke them down into the two required elements: collection and cleaning

The collection method was a pretty easy pick. The biggest challenge was already to ensure the pressure drop was low enough to avoid sending smoke back into the house, so the external collection was selected.
For the cleaning method, I wasn’t really sure if cleaning was even really needed, so I built the prototype with no mechanism to see how much it accumulated.

Some build photos:

And finally, a quick shot of testing a few hours ago from my phone (I’ve got some nice ones on a camera, I’ll post them up soon)

Conclusions… well I honestly haven’t had enough time to think it through yet. In general, it worked almost exactly how I thought it would, but the carbon stains on the inside are showing a few strange flow patterns which I’ll have to consider the reasons for.

Would love to hear your comments, criticism, advice and suggestions for improvement!

Some pictures from testing

As a wood stove user myself I find the project interesting. Do you think the results would be any different if the path and distance between source and filter were more representative of a typical wood stove and chimney? i.e. elbow and x-length flu.


It would likely be better actually.

As long as the fire was hot enough, the draft was going well. There were a few draft issues in the test when the fire was too cold (you can see some of the sparks going on the outside), but a larger insulated chimney would hold the heat better over time. Also, the opening on the top of the fire pit was smaller than the chimney pipe, so there was an expansion loss there.

I designed it to only restrict the flow about the same amount as an elbow would.

How old is your wood stove? The nice thing about wood stoves is that most ones have to be EPA rated and produce a really efficient burn. Fireplaces on the other hand are completely unrated so that’s more where it would be needed :slight_smile:

I suspect it would work better too. You should also consider trying ‘normal’ cord wood as it would perform differently than kiln dried lumber.

Our Vermont Castings Defiant is probably 20 years old (don’t tell the EPA) and burns fine, I just replace the seals every few years. We could certainly get a more efficient one but wood stoves are expensive and heavy!


You’re in the target market then :wink:

I agree, I would have liked to use cordwood for a more realistic amount of smoke, but since this was as part of an academic thesis, we had to follow the ASTM standard for wood-burning. I also would really like to see how it works over time, we were only able to do around 5 hours of burning, so there wasn’t really much accumulation.

I’m curious, since you’re a regular user, do you think a yearly filter replacement would be sufficient, or would one of the cleaning mechanisms be needed? The image above was the accumulation after 2 hours of burning.

I’m curious… Why did you choose this as a project or how did you come to this problem? Seems very practical and useful, yet out of the realm of most major projects I see (usual smartphone, tablet, ces type stuff,etc. ) so just wondering.

Refreshing to see such a real world type problem tackled that first have the obvious s3x appeal of “blog me” going on.


Thanks R!

Actually, the first set of ideas I was pitching to profs were definitely more along those lines. I was originally going to do a wind-powered window air conditioning unit, but after running some initial numbers on it, it looks like current technology isn’t advanced enough.

At that point, my prof mentioned that he had seen a news report about this new problem. I had the same reaction initially, but two things convinced me to take it on:

First, it’s the type of project that I would like to work on professionally. I know some design engineers who just take ID concepts and implement them. I’d like to be at the front end with the designers, working on the functional aspects of the concepts. If you don’t know what vortex shedding is, or how it works, how are you going to think of using it as a solution?

Second, it was so far outside of my comfort zone that I knew I had to do it. I know my stuff when it comes to mechanical design, and my initial reason for doing a thesis was to prove it. However, this covered areas of engineering that aren’t taught in classes, and I thought it was important to learn the ability to research into those areas, then apply them effectively without having a teacher.

Do you have any ideas on how to make it more appealing? Chimney Filter probably isn’t the best title, but I don’t want to go with something pretentious like “Carbon Black Extraction System”. I’d also have liked to add some styling to it, like a slanted rain cap or something, but my manufacturing options were limited to what we had in the school shop.

I’m actually not in the target market mirk, because my wood stove works fine and I’m not concerned about ash particles.

How did you get turned on to this? Is the goal to replace fireplaces (or stoves?) with more efficient ones or to reduce soot? I realize that you are a student but a convincing problem/needs statement backed up with research, market assessment, and implementation plans will be useful.

Neat project nonetheless.


Here’s an article covering the study that spurred the project:

The key line in the article:
Once considered negligible … BC (soot) is the second-most important agent of climate warming, after CO2.

Now the study was only published in 2013, so the knowledge hasn’t really spread yet. Not that it’s actually going to production, but the plan was that as people discover the problem (through marketing efforts or otherwise) we would be first to market.

Municipalities are also starting to ban inefficient wood stoves as a primary heating source, so I’m expecting people will be looking for ways to reduce their environmental impact without a full expensive replacement.

I guess I should make an image that explains this in more detail… Thanks :slight_smile: Do you think that’s enough justification or should there be more?

I would argue more references to support the argument. I could see your project making a lot of sense in densely populated areas that burn wood or coal as a primary energy or heat source, there just aren’t that many of them in developed nations and those in emerging economies could probably care less.

To me, the bottom line is this: Does burning stuff cause smoke/carbon release? The answer would be yes. How much of an impact would filtering wood stove exhaust and particulates have on climate change? My bet is not very much, compliance and regulations only go so far. I live in rural NH and burn wood in the winter so that I do not need to burn as much oil, it is a hassle but also offers a level of independence and localized energy security should things go south over a long winter (extended power loss). The last thing I am concerned about is locally generated, global smog.

Good luck with the project mirk!


agree, I would have liked to use cordwood for a more realistic amount of smoke, but since this was as part of an academic thesis, we had to follow the ASTM standard for wood-burning.

I don’t know if you are aware of it, but it is generally suggested, by manufacturers, that kiln-dried “construction” wood not be used in wood-burning fireplace and stoves due to the increased temperatures it generates (due to less moisture content (<12%)). Freshly cut wood may have a moisture content of as much as 50%, while “seasoned” (air dried) wood generally is sold when it hits 20% or so. And it does create significantly more smoke due to that moisture contributing to incomplete combustion.

The danger presented, when burnt in a fireplace that has previously been burning “firewood”, is that the increased flue temperature may ignite built-up creosote and tar left by incomplete combustion.

Given that “firewood” contains tars and oils, how long do you expect your filter to function without service, or replacement?
A little historical note: In years past, when burning at the stake was considered an appropriate form of execution for heretics and other political prisoners, it was considered more humane to use dry fire wood so that the condemned would suffered “less” than when wet wood was used. If ones crime was say sedition against the King, one might expect a prolonged execution when wet hay was used as kindling for unseasoned firewood.

p.s. What did your neighbors think of your experiment “short stack”? Ours consistently build a fire in a “fire pit” on their front deck leaving us to deal with smoke in the house or closing all the windows; I’d add that they have three fireplaces in their house.

How did you address filter replacement (or inspection) due to clogging? That seems like a huge obstacle given how inaccessible it is.

Thanks for the replies! I just handed in the report (hence the late reply), and they helped a lot while forming my discussion. It’s also helping me to get in the right headspace for my defense in mid-Sept.


Do you know a specific manufacturer that says that? The ones I found for reference actually recommended to keep the flue gases hot (between 300 and 500 F) to limit creosote buildup in the first place. I don’t think this is a realistic recommendation however, as we had trouble keeping the fire hot enough (I designed the prototype to have appropriate draft at this temperature, and backdrafting occurred in cooler periods).

Near the start of the project, I got contact info for a guy who specializes in predicting tar output from combustion, but he was on sabbatical, so that was a problem that I was hoping the testing would shed some light on. Now that the project’s over and I was restricted to using lumber, I’m kind of at a loss for an answer. If the accumulation doesn’t restrict operation over the course of 2 or 3 years of use (recommended chimney cleaning is once a year, adding in a safety factor), I think it’ll be fine. If it would, then something has to be done to prevent or clean it, but for that I need to know a lot more about the tar properties (does it stick to fibreglass? what’s the average viscosity as it hits the filter? is there anything that can be done to force it to become brittle? etc.), too many questions to answer with more pressing projects in the pipeline unfortunately… :frowning:


I’m not sure if you mean the disassembly, or reaching the roof, so I’ll answer both:

The filter “cartridge” is just the rippled fibreglass, sealed on either side to an “O” cut out of stainless steel. Both the chimney attachment and the cap have an angled edge, and they all just press fit together. It’ll definitely be a problem with wind, but with access to more manufacturing methods, I’d like to make the attachment a quick release.

For reaching the roof, chimney cleaners have to access both the top and bottom of the chimney, so they’re up on the roof anyway. There would have to be some kind of partnership to ensure that chimney cleaners start offering filter replacement and cleaning services as an add-on. Assuming that is possible, the intent was to keep the life to about 2 or 3 years, to account for people not getting them cleaned often enough (recommended is once a year). The prototype was designed with no self-cleaning mechanism to get an idea of how much soot accumulates per burn, and determine if self-cleaning mechanisms are necessary, and to what level.