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Uneven burn, dirty glass - troubleshooting and solving

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by precaud, Jan 4, 2009.

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  1. precaud

    precaud Minister of Fire

    Joined:
    Jan 20, 2006
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    2,293
    Loc:
    Sunny New Mexico
    This is actually the last in a series that I started a little over a year ago as "Taming the wild Quadrafire 2100." You can find the previous installments by searching for those words. I decided to title this finale more generically because many stoves (from many brands) have the same symptoms from the same problem.

    The "taming" part (getting better control of the firing rate) was actually solved in those previous posts - the small firebed-level air feeds were the culprit, and making them a bit smaller was the easy fix. What remained was the pattern of uneven burning. No matter how you loaded the wood, there was a right-to-left "swirl" burn pattern, lively flames on the right, wrapping around and exiting on the left side, sometimes leaving unburned wood in the left rear corner. Now, obviously, the stove was not designed to burn this way - we don't pay good money for stoves to do this! None of my previous tweaks solved it. The likely cause is a stronger airfeed on the right side. So we'll closely examine the primary air feed (aka the airwash) system for things that could possibly cause such an imbalance.

    For details, see the first photo, showing the top half of the stove door opening. The airwash air enters the stove at the bottom front (above the control lever,) gets preheated while it passes up through the channels on either side, and enters into the airwash manifold via slots in the top corners.

    The first imbalance to notice is the cutout in the right vertical channel (the blue arrow) for the door latch mechanism. This reduces the area of the right channel by 50% at that point - a definite imbalance. Problem is, that restriction should weaken the air feed on the right, but that's not what we're seeing. So that's not the cause.

    Quad tech support's response: Adjust the airwash gap screws, located inside the stove (the two yellow spots on either side) for a 3/16" gap at the screws. Well, the gap on both sides is larger than that with the screws all the way out, and there's no adjustment to make it smaller. So that's no help. But... the gap does measure larger on the right than the left. That makes sense. So I decided to measure the gap as it is and compare it to the way it is supposed to be.

    (See the photo again) Quad says the gap should be 3/16" at the left and right yellow dots, tapering down to the dot in the center. Using feeler guages, I found the spots where the gap actually measures 3/16" (marked in red) and then found the point where the gap is smallest (marked red C.)

    Seeing it mapped out visually, it becomes clear just how lopsided the airwash on this stove is, and the symptoms start making sense. The gap area is largest on the far right, over a very small section of the gap, which means that the strongest stream of air is being fed right into that corner of the stove. And that's exactly what I see when the fire is burning.

    Now see the second photo, which looks up into the gap on the right side. The yellow arrow points to the adjustment screw. See the bulge in the gap around it? It doesn't look like much, does it? Is this small bulge enough to cause such a big problem? We shall see.

    So what caused this bulge in the first place? I think it was two things: poor design, and sloppy assembly of the airwash manifold at the factory. This manifold, the thinner metal plate in the photo, was mounted a little bit out of kilter, lower on the right side. The factory then adjusts the gap screws to bring it into "spec." Well they obviously had to torque the bejeezuz on the right side screw to get the gap up to 3/16". All was fine until the stove was fired up, and the intense heat in this critical combustion zone warps the manifold and makes the right side bulge a permanent feature.

    Design-wise, this is caused by the manifold being an essentially flat plate. Had it been designed with more curvature, it would be more warp-resistant.

    OK, back to fixing it. I first tried putting spacers on both sides but that didn't move the low point, and the object is to somehow move the low point back to the center, and then fill in the bulge on the right. So I focussed on first making it symmetrical about the center (the way it is supposed to be.) This was best achieved by wedging a small steel rod (made from a large nail ground to length) near the minimum point (the red arrow.) The exact height and location that gave the best symmetry was found by measuring the gap symmetry.

    The bulge on the right was reduced with a thin steel clip (third photo.) These are actually called Tinnerman nut retainers, which in the past were frequently used to secure adjoining panels of electronic chassis. Made from spring steel, they have good dimensional stability, so they won't slip off the panel when subjected to heat. Happily, I just happened to have one lying around that was about the right thickness and shape. The fourth photo shows the clip in place on the stove.

    So, how does it work? In a word, superb. The right-to-left "swirl" burn pattern is gone, and the flames burn evenly across the whole width, both at the firebed level and in the secondary combustion zone. And the glass tells the tale: it stays much cleaner, and what buildup there is, is the same on both sides. This is how it should be.

    Lessons learned:
    1. The Quad ACT airwash system is fraught with design and QC problems. Anecdotal evidence on this site shows that the new ACC system doesn't improve on it, and is worse in some respects. I bet they can do better.

    2. Small deviations in the airwash gap of ANY stove WILL have a big impact on the way the stove burns. In retrospect, this makes complete sense, given the importance of air in the combustion process...

    3. If your stove has persistent uneven burn patterns, and the gaskets aren't leaking, take a close look at the airwash system. It's the proverbial elephant in the room.

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