Kent Tile Fire (and Sherwood) stoves

Non-Kent-related discussion of the Shelton study is continuing in this thread:
https://www.hearth.com/econtent/index.php/forums/viewthread/67756/

OK, back to Kent-related things. As I weigh which direction to go for the next mod, here's where things stand. Adding the upper row of insulation raised firebox temps and clean up combustion inside of it, but it didn't reduce smoke production in the early burn. So I'm going to concentrate on two things: adding some above-fire air, and increasing turbulence before it exits into the secondary baffle.

First experiment is with the air. Where should it be introduced - at the front, rear, middle, or all three? I'd prefer to keep it simple and add just one source. It just so happens that my Quad, which has the same size firebox, has a "startup air" source. It consists of one hole at the bottom rear, and a row of holes across the back just under the baffle. There's even a separate control so I can very the air amount. So I can use the Quad to emulate rear-only secondary air in the Kent. I plugged the bottom hole and the doghouse air, and now I can compare how it burns with one secondary air source at the back, to the three burn tubes above. With separate controls for both, I can switch back and forth in a few seconds.

The result can easily be summed up as: the fire follows the air. Compared to the standard 3 burn tubes, introducing all of the secondary air from the back caused the fire to burn more from the back. And turbulence (mixing) also occurred more in the back and disappeared toward the front. (I had seen these same things with the Morso 2110 and Jotul F602, so it wasn't totally unexpected.)

Since the primary aim is to improve combustion in the first part of the load, adding air at the back is not going to do that all by itself. The good thing about the rear air is, there is no excess - it all gets used before it exits. So it may help some, but it's not sufficient. More overfire air at the front is needed.

I see three possible ways to get this. The obvious way is to add a burn tube there, with separate air inlet. A second way would be to add some holes in the airwash manifold and "bleed off" some of the primary air into the exhaust stream. The old Jotul 602 (pre-EPA) did this. And the Quad does it massively. A third way, and the least invasive, would be to alter the shape of the airwash manifold's front edge, using the diffraction to "tune" how much air gets pulled away from the airwash main stream. I'm not sure how much of a difference one can make doing this, it will take some study. But I know this principle works in acoustics.

Another thing I want to play with is the alleged "swirl" pattern which is shown in the promo drawings for the Kent firebox. The simplified drawing below was taken from the Shelton paper. It shows that gasses make two trips through the fire before exiting. Well, I have never seen this happen with this stove or the Sherwood I used to have. (I do see it all the time with the X33...). The Kent behaves just like any other stove with a flat horizontal baffle, with the classic "S"-shaped flow. So I'm going to add a lip at the front of the baffle to try to create this double swirl flow there. I have some 1" Kaowool M board that should work fine. Should be interesting.

Here is a photo of the "lip" at the front of the baffle, with some side insulation added, using 1" Kaowool M board. My camera didn't have enough wide angle at this distance so the pic only shows one side. Vermiculite would have worked fine too, I just don't have any on hand that is 17" wide. This idea is based on details that Resiburner, a former Sherwood owner, remembered about what Kent's emissions kit looked like. Besides creating turbulence near the mixing point, by moving the effective draft point forward and down a little, this piece will pull a little more air up from the airwash.

If this works, then adding some bleed air from the airwash manifold will probably be the next step.

This stove continues to impress. At 6pm this evening I relit from coals from a medium-size load started at 8pm last nite. That's 22 hours.

I replaced the small insulation pieces shown in the above pic with a single piece cut to fit the whole area on the sides in front of the baffle. That definitely helped raise the temps there and clean up the early-stage burn. If there's interest, I'll remove one and post a pic of it. I also experimented with the placement of the piece at the lip. The results are inconclusive. So I've kept it pretty much flush with the front of the baffle. My preference would be to cover the whole baffle with 1/2" fiber board but this is working well.

I can see there are definite advantages to having all of the air come from one source at the front. Besides giving a true front-to-back burn and the glass staying amazingly clean, all of the air washes over the coals from the prior load, burning them down. There is no coals buildup. And as it does so, the air gets under the wood, maintaining a very lively fire without a doghouse to stimulate it. And as I explained in another thread, with no "always on" secondary air, live coals last MUCH longer. All this leads to a stove that is very pleasant to use and very user-friendly. And very efficient, too. I can't prove it empirically, but it's heating this space better than the Quad did, with smaller loads. And the Quad had handily trumped every other stove I've had in here before. Very impressive.

It's now to a point where it's in the same class as a modern EPA stove as far as visible smoke emissions. All you have to do is burn it hot, as it was designed to be burned, and leave some spacing between the logs so the air can penetrate to the rear of the stove. We don't need no stinkin' secondary air to burn clean.

madrone said:

The double-circulating flow drawing is amusing. Magical.

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Truly. One wonders how they got away with it.

I'm assuming your burn times are shorter than those of us with EPA II stoves are used to.

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It depends how you define "burn time". Some like to use a certain value of stovetop temperature. That really measures heat storage, not burn time. The two that make sense to me are a) the length of the flaming period of the burn cycle, or b) how long the stove can maintain sufficient live coals for a 'warm startup'. I prefer the latter. And clearly the Kents are exceptional at that. But most EPA stoves can be easily modded to do very well. It's all about air control.

What I'm getting from this experiment is that most stoves are using multiple rows of secondary air as a way to meet emissions with reduced air intake? i.e. cleanest possible longer burns?

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Yes, they essentially reduce the primary air and move it over the fire. Total air throughput is likely higher on most EPA stoves. But doing so doesn't necessarily equate to longer flame periods. Many other variables at play there.

How do those first 20-30 minutes compare with the Quad? I'm guessing they're both dirtier on start up?

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Like other stoves with lots of lightweight insulating material in the firebox, the Quad burns clean almost immediately. With a top-down startup, visible smoke time is measured in seconds, not minutes.

The Kent can't do that. Too much exposed metal in the firebox. But the mods I've made help a great deal. Before adding the upper-front insulation, the first 20 minutes or so had plenty of visible smoke - not belching/disgusting, but enough to make me uncomfortable. Now, it's thinned out and a shorter period. Getting that area warmed up more quickly is the key.

In light of the trend in Euro stoves, is it inevitable that we’ll be reinventing the masonry heater?

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Some stoves are going in that direction. But the Kent doesn't fall in that category. The ceramic tiles aren't great heat storage.

What I do see in some recent Euro stove entries that mimic the masonry heater approach is minimal use of secondary air. I'll find some videos later and post the links. But it's hard for me to get excited about a stove that can only store and dribble out a couple thousand btus/hr...

For the same burn rate they're in the same ballpark, though I discount that some because the Kent convects nearly all of its heat out the top, and my thermometer is only mounted six inches above the stove top due to the short vertical section on the chimney.

Well the Kent has a shortcoming. It doesn't handle sappy pieces of pinon as good as the Quad does. That's one situation where an excess of secondary air comes in handy. I'll have to make sure to burn those pieces one at a time.

Round 2 of the mods begins. The added insulation around the secondary baffle holes is definitely beneficial. Getting the temps up in this little area in front of those holes is essential to getting a cleaner burn. Next step is to give it a little more air for more complete combustion.

It's important to understand going in that, broadly speaking, non-cat EPA stoves burn cleaner by providing an excess of air to the fire at all times. Only at the highest burn rates do they make good use of all that air. At other times, the excess air just dilutes the exhaust and cools the stove. Knowing this, and since the Tile Fire works so well as is, I'm not going to add enough air to cover the highest firing conditions. I want to optimize the "cruise" phase, where the stove settles in and spends the most time with flames present. I'm guessing it won't take much air to do that. So I'm not going down the road of adding a burn tube. I want simple, strategically-placed mods that can be easily reversed if they don't work.

So the first thing I'll try is bleeding off some of the primary air directly into the area that was just insulated. The air will get mixed with the flames as they get pulled through the baffle holes, and should clean up the burn.

To do this, as you can see in the pic below, I drilled seven 13/64" holes symmetrically across the air manifold. These may need to be bigger, but this seemed like a reasonable starting point. Air pulled through these holes will get dispersed some by the lip on the underside of the manifold, so it shouldn't appear as discrete tongues of flame.

I'll fire it up this evening and report the results.

The short version is, another successful mod. This is working pretty much as hoped/expected. For details, read on.

In the first fire, the only difference I saw was that the flames seemed to fill the area on front of the baffle holes a little more. But nothing decisive. So for the next fire I enlarged the bleed holes up to 1/4". Now I could definitely see that there was in fact a small amount of air coming through these holes, and that flames in front of the baffle were making good use of it. Smoke emissions from the chimney were definitely improved for the first, say, 3/4 of the flame cycle, and for a good chunk of it were totally transparent, as good as the best EPA stoves I've seen. Stack thermometer is reading a good 50 degrees higher than before. All this points to more complete combustion in the secondary chamber. I've burned four loads in it like this, and these observations hold true in all of them.

If I could do it over again, I'd move the bleed holes up by 1/2" or so. That would allow the air to come out in more coherent "tongues" like we often see from secondary burn tubes. That would increase turbulence and mixing. As it is, it's like a gentle "sheet" of air that hugs the primary air chamber and "pulls" nearby flames into it.

Where this modification makes no difference is toward the end of the burn. Remember, this is a true front-to-back burner, and there is no air source in the rear of the stove at all. So once the burn gets to the point where flames are not actively rolling up into the secondary baffle, this added air is useless, and may even be counterproductive. A small amount of over-fire air at the very rear would feed those end-of-cycle flames and may be in order.

If I had to pick one spot, I think the optimum location for introducing secondary air into this stove would be a burn tube located approx. 3" behind the baffle holes. That would have the highest chance of being effective in all burn phases. But that would require preheating channels on both sides, etc. and defeat some of the benefits of this simple yet very effective heater.

Sorry, my bad. Mistake edited. Sometimes the mind slips into reverse while moving forward!

reality, you need to clean the chamber through the chimney connector. Vac'ing through the front holes will only clear the first inch or two.

Toby, can you post a pic of the ID plate on the back of the stove? I don't think this version was ever brought to the US.

Thanks Toby. That is definitely not a version that was brought into the USA. Some thoughts:

Your baffle is installed backwards. The purpose of the "V" is to stop the exhaust from going straight up the chimney, and distribute it across the whole top surface. So the point of the "V" should be toward the front of the stove, and the baffle plate should be flush with the rear.

That said, it sounds like this is too much stove for your space, and you're forced into burning it slower (and dirtier) than it should be. You can try burning smaller, hotter fires less frequently. Or consider replacing it with a smaller stove.

EDIT: I just increased the contrast of your pics so I could see details more clearly, and it doesn't look like the baffle can be reversed. You are probably going to have to live with it as it is. This is definitely a counter-intuitive geometry.