The Rocket Gets One More (Final!) Tweak

It was one year ago that I figured that the lacklustre performance of the rocket was due to the chimney, or lack there of. Boy, did adding a good, insulated chimney make all the difference! But now that I’d made it through the second heating season with the thing, I was still convinced that it could be better. There wasn’t anything cataclysmic going on, save for the odd blow-back of smoke into the room from time to time, especially during windy weather, but I could live with that. What convinced me that things could be improved was when I started taking temperature readings at the various inspection/priming ports, especially the one in the chimney. Let me explain.

To recall, this little rocket is a 6” configuration, snaking through a big wide bench in a 100 sq ft office. When I say snaking I’m not kidding. By the time I added on the chimney, I had seven (7) 90s added to another 25 feet of straight stove pipe. The 90s add a lot of friction and if you account that each one adds the equivalent of 3 feet to the overall run, I was pushing the flue gases through 46 feet of ducting!!! That’s a lot for a 6” system, as I’ve read and since measured.

It never fired up all that quickly. It might take a good hour before the room was 10 degrees C warmer. The magnetic temperature gauge would take 1 to 2 hours to climb to 400 degrees F (sorry for the change in units; I live in Canada). The flue gases were moving, they just weren’t roaring.

The first inspection port is at the manifold and after about an hour I plunged a temperature probe into the stream and took a reading. It read 220 degrees F. Moving downsteam, I took another reading at the second port, at the deep bend in the dog leg, and read 180 degrees F. The third port, which is the last one in the building directly behind the stove just before things exit into the chimney was hovering at a cool 95 degrees F. In the chimney, it was reading about 90 degrees F. You might say, wow, the cob is really sucking lots of the heat into the bench, which is a good thing, isn’t it?

Well, according to a friend who builds masonry heaters for a living, if you’re flue temperatures are 90 degrees F or below, you’re in trouble. And we also know, all other factors equal, chimneys with warmer flues collect less creosote. Given that I’m pushing 45 feet (lots for a 6″ system) and have exhaust temperatures hovering around that uncomfortable 90 degrees F threshold, I thought we’d see if we couldn’t speed this thing up.

As I’ve done before, and what’s makes cob such a joy to work with, is that you can chip it away, rehydrate it, and reuse it. We sought to remove the dog-leg in our system, taking out three (3) 90s and an additional 8 feet of stove pipe, effectively removing 17 feet and reducing the total run to 29 feet from the 46.

Piecing things back together we fired it up and wow!!! did that make a difference. The startup was noticeably quicker, the temperature gauge climbed to 400 degrees F in 10 minutes (vs. an hour before), and the chimney was pumping out the flue gases skyward (not the lackadaisical downward tumble that used to be the case). I’ve yet to take new readings because summer has quickly settled upon us and it has been hot out, but things are running faster, hotter, and I’m getting the performance that I wished for when I first endeavoured to build this thing. I can tell you I’ve finally settled on a configuration that I’m delighted to live with and that’s a good, good thing. Oh, and rather than making more cob, we made a cubby to store the tea and kettle. I must say, it looks pretty darn sweet and it makes use of an otherwise useless space.

So don’t give up on a lackluster system. Just remember to mind the gap between the heat riser and the barrel, give yourself plenty of space in the manifold (more than you think you need), don’t overlook the chimney, and don’t lay more stove pipe than the system can handle. If you do, you’re bound to get flue temperatures that create problems.

Thanks for reading!


2 thoughts on “The Rocket Gets One More (Final!) Tweak

  1. Charles Martin

    Have been seeing, reading, watching hints that the Steel combustion chamber is not neccesary and possibly has negative consequences. One might make a combustion chamber using plywood forms that would burn thus slowly heat and solidify clay that would then be used as combustion chamber. What is your opinion on the Steel combustion chamber? Thank you Charles


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