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Post by sksshel on Feb 6, 2019 9:52:27 GMT -8
In general, I am very satisfied with the performance so far of my Double Shoebox Rocket Mass Heater. However, I don't think it is yet performing as well as it should. THE PROBLEM: The surface temperature on the metal chimney is only reaching 100 oF. As I recall, this should be somewhere around 120 - 130 oF. When designing the Bell, I tried to stay under the maximum Internal Surface Area (ISA) requirement. (57.1 sq ft for 6" system) Any suggestions?
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Post by Vortex on Feb 6, 2019 10:13:19 GMT -8
Internal chimney temperature and surface chimney temperature can be very different, how are you measuring it? Usually 150F to 190F/ 60C to 80C seems to be suggested as optimum internal chimney temp, measured at about 3 feet above the top of the stove. Do you have a bypass damper for startup?
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Post by Deleted on Feb 6, 2019 10:24:56 GMT -8
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Post by DCish on Feb 6, 2019 14:01:52 GMT -8
Ditto on the internal vs external temp measure
Also, is your stove well dried? If not, that might be it.
On the other hand, if it is dry and burning well and giving you the heat you need, you may want to just use it for a while and keep an eye on the chimney for accumulated deposits.
If you decide you want higher flue temps in the end, one way is to insulate part of an internal surface if you have access (using ceramic blanket or similar). Another way, if you are drawing flue gasses from the bottom of a bell, is to open an entrance to the flue slightly higher up until you get the temp you want.
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Post by sksshel on Feb 7, 2019 9:23:05 GMT -8
Vortex and @karl: I'm measuring with an IR Thermometer. The temperature at the initial exit from the bell is 74 oF. It is old-galvanized. So if I adjust that by .88, the result is 88 oF. That seems too low. If I adjust it by new-galvanized .23, that makes it 303 oF. I can put my hand on it comfortably, so that seems too high. I do not have a bypass damper. I have not had any trouble getting the initial firing each day to draw properly. The temp at the black "L" before exiting through the wall (see pic) is 101 oF. Matte black requires no adjustment. That is about 3.5 feet above the exit from the bell. The point 3 feet about the top of the bell is triple wall chimney pipe. I don't detect any surface temperature change there. I would not want to drill through it to take a measurement. I could put a temperature probe through a drilled hole in the first 2 spots to get an accurate internal temp. @dcish: my stove has been used every day for about 2 1/2 months. 1 - 3 firings per day for an hour or two each time. The cement blocks were filled with fairly wet cob. The blocks were lined on the inside with troweled on geopolymer. I have considered raising the exit port from the bell in order to raise the flue temps.
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Post by sksshel on Feb 7, 2019 9:26:26 GMT -8
OBTW: I will replace many of the flue pieces in this picture with black stove pipe once I determine the stove is working properly. I intend to encase the lower portion in cob as well.
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Post by sksshel on Feb 7, 2019 9:27:27 GMT -8
@karl - the geopolymer has been performing fabulously, thanks again for your help.
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Post by Deleted on Feb 7, 2019 10:25:55 GMT -8
@karl - the geopolymer has been performing fabulously, thanks again for your help. I always knew that geopolymers are great refractory. Some people here have a deep faith in the products of the refractory industry, as the only blissful way into the heaven.
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Post by Vortex on Feb 7, 2019 11:57:07 GMT -8
I thought you might be using an IR meter on stainless, I didn't want to get into explaining about emissivity until I knew that was what you were doing, but Karl explained it anyway. I recommend you get yourself a good probe type thermometer to measure the temp inside the stove pipe. The BBQ/grill ones that just screw into a hole in the stove pipe are the easiest.
If your stove has really good gas flow its possible you don't need a startup damper, but then I would expect it to be more likely to suffer from thermal runaway. I can also light my stove fine without using the damper, but if I open the damper at the start until the topbox heats upto at least 160C / 350F the difference in overall heat output is about twice as much. Also If I light it without opening the damper, the internal chimney temp stays around 40*C / 110F. Whereas using the damper once I close it the temp drops back to about 80C/176F and stays there.
Can you post a video of it running mid burn, showing the combustion in the top chamber - that would speak a thousand words.
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Post by sksshel on Feb 7, 2019 15:04:19 GMT -8
I used a temperature probe inside the chimney at 2 places. I then timed the burn and noted the temps at both places. The first place was immediately outside the Bell. The second place was at the "L" prior to exiting through the wall.
At 5 minutes, both places jumped 35oF. At 10 mins, position 1 was over 150oF and the "L" was around 140oF. At 15 mins, position 1 was around 160oF and the "L" was hovering around 145oF. The IR thermometer was between 90oF and 100oF. Clearly, this was a mistake using the IR.
I intentionally used Kiln-Dried wood to perform this test.
This video was taken around the 15 minute mark.
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Post by Dan (Upstate NY, USA) on Feb 7, 2019 18:43:49 GMT -8
If you are getting complete combustion, have enough draw to prevent smokeback, and its heating your house with minimal firewood use then mission complete.
You could modify your stove to run with 130F exhaust but WITHOUT a Testo or similar device you might be making your stove run worse just trying to get to 130F.
I used to get 100F exit temps all the time (contact pyrometer), then I modified my stove to heat the clean air coming into my house and now my exit temps are 130F all the time. (counter-intuitive exit bell changed the draw somehow) Did my efficiency go up? Who knows since I won't spend money on an expensive Testo to compare (just yet, priorities).
Bottom line, my exhaust is steam and CO2 and my wood consumption has not gone down or up.
Without a Testo you have to use qualitative comparison, with a Testo you can do quantitative comparison...
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Post by peterberg on Feb 8, 2019 1:22:18 GMT -8
At 5 minutes, both places jumped 35 oF.& At 10 mins, position 1 was over 150 oF and the "L" was around 140 oF. At 15 mins, position 1 was around 160 oF and the "L" was hovering around 145 oF. As a rule of thumb, the temperature should be above 60 ºC (140 ºF) within 20 minutes after starting from cold. In most circumstances, this is enough to avoid a chimney stall according to what I've experienced over the years. Your bell csa is very close to the recommended values and it runs well, heat well, chimney tempearature is within the above limit and it doesn't appear to smoke. It runs like that without a bypass damper which can be seen as confirmation the recommended values are correct. After all those values are based on estimation and extrapolation. The more confirmation we've got the better. Thanks!
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Post by Vortex on Feb 8, 2019 2:32:40 GMT -8
well to me all that creosote on the glass tells me something is not right. if you're normally burning unseasoned wood that could explain it, but even then by mid burn of a batch of kiln dried timber the temperature in the top box aught to be high enough to have burned that glass clean.
I take it you're using the normal primary and secondary air sizes and setup that Peter recommends for the DSR?
At a guess I'd say that without a bypass you're combustion is never getting to a high enough temperature to start and sustain an efficient burn. As I said above, I could also run my stove without using the bypass, and I would get a sluggish fire that produced a lot of creosote and about half the overall heat output.
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Post by sksshel on Feb 8, 2019 4:27:41 GMT -8
well to me all that creosote on the glass tells me something is not right. if you're normally burning unseasoned wood that could explain it, but even then by mid burn of a batch of kiln dried timber the temperature in the top box aught to be high enough to have burned that glass clean. I am burning a lot of "Questionably" seasoned wood, cut too large, that I purchased locally. Next year, I plan to cut and split my own wood so that variable can be eliminated.
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Post by DCish on Feb 8, 2019 7:13:01 GMT -8
Drying: Sounds like your stove has had plenty of time to dry! Geopolymers: Kudos on taking the lead on the geopolymer angle. I started down that path at one point, but just couldn't wrap my head around it. I'll look forward to hearing how it performs over time! Flue temps: Sounds like the probe thermometer got you the data you needed, and gave you good news. With the significant drop between position 1 and position 2 (160 to 145), you may want to consider insulating rather than cobbing that run from the stove to the insulated chimney. Or if the cob is for aesthetics, insulate it, then cob it. It looks like you are getting all the heat harvest you can support already, and may want to preserve any remaining heat to run the chimney. Burn temp: As Vortex pointed out, depending on where in the burn that pic is taken, the flame does look pretty tame and dark. I routinely see my afterburner area glowing dark to bright cherry red ( www.paragonweb.com/Kiln_Pointer.cfm?PID=464), putting brick temps in the 1200F to 1500F range. At that temperature, any soot that may have accumulated earlier in the burn is consumed and both the firebox and the afterburner are clean of soot, except perhaps in the odd corner here and there at times. Looking at photos of Peter's burns, it seems he always uses thin wood scraps, or wood that is split relatively small. I have followed that practice by loading a couple of ~2" square lengths on the bottom, and getting progressively smaller toward the top, with lots of 'twig'-size stiff at the top to jump-start a quick flame. As Vortex describes, letting it breathe early on until it gets really roaring seems to be an important part of getting it to reach a high burn, only buttoning it up and turning it down after it really takes off. If your firebox is more on the dense side rather than insulative, high burn will likely take a bit longer to establish. Wood quality and size is also an amazingly potent variable. If I light a load as described above in my dense firebrick testbed, I can usually add a 3" x 3" or 3" x 4" log to a rip-roaring fire and have it keep the afterburner bricks glowing, but if I wait too long and put a big chunk on just embers, it's a no go. And that's with a well-seasoned chunk of wood. If the wood has enough moisture content that you can hear it sizzling or see liquid boiling out the end of a piece of wood during the early part of the burn, then you know you really have a damp load and will struggle to get a hot fire even if your wood is split relatively small. Bypass: If you try burning smaller splits of wood, leaving the door ajar longer to get it roaring better, and making sure you are using dry wood, and you still aren't getting a good bright glow in your afterburner, it looks like it wouldn't be that terribly hard to install a bypass by knocking a hole in the upper part of your bell and putting a Tee there with a full-closure damper in it. My direct experience with this is only with my commercial wood stove that is hooked to a bell bench. While I can start it without the bypass, it is a royal pain. While I have full respect for Peter's focus on simplicity and use of designs that do not require a damper (I do think this is the best way to go for any professional installation), I do plan to include a bypass on my own core when I bring it inside since I am not confident I will calculate all the parameters just right, and the bypass is relatively easy to include. Still, since your build is already in place, hopefully just cracking the fire door for longer will get your burn temps established better.
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