Slant Vs Flat Grate Stokers

A Coal stoker furnace or stove controls most operations including automatically feeding the coal. They are quite similar to any conventional oil and gas units and easily operated for extended periods of time. They commonly use rice coal but may use larger sizes like buckwheat. They can be used as primary heat, supplementary heat or have a dual set up with your existing oil/gas furnace.
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stoker ace
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Post Sat. Mar. 15, 2008 8:38 pm

Hi to all,

I have been heating a 2500 sq/ft cape cod with an oil boiler since the days of 0.75 heating oil. The cost is out of control and getting worse every day. In my search for alternatives I stumbled upon this forum and have been reading all I can to educate myself on heating with anthracite. I have a lot to learn but it is clear to me there is a wealth of knowledge and experience here on the forum. I was fortunate to find you guys. Over the summer I intend to install a stoker boiler as my primary heat source while keeping the oil unit for backup. Since I work long hours I need the heating system to be at least semi automatic - so a stoker of some type would be required. I have been looking at the various feeder designs of the different boiler makers. While I have not ruled out any options at this point the simplicity of the Keystoker design appeals to me. I was able to look at a KAA-2 and a KA-6 at a local dealer. The KAA-2 had a flat grate stoker and the KA-6 the slant grate stoker. Based on the size of my house and the current oil useage the dealer said the KA-6 would best meet my needs. He said however that he liked the flat grate design better than the slant grate - that the flat grate was easier to light and had less problems with clinkers. He said that all the KA-6's he ordered from now on would have the flat grate stoker and a six inch flue pipe instead of the standard slant grate stoker with an eight inch flue pipe. I have no experience with these but the slant grate design looked a lot more industrial and heavy duty to me. Can someone give me some insight on the flat vs slant stoker and advantages/disadvantages of each ? If a KA-6 can be ordered with either type - which one would you spec if you were buying one?

Last edited by stoker ace on Mon. Mar. 17, 2008 12:42 am, edited 1 time in total.

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Post Sat. Mar. 15, 2008 11:20 pm

Thats a very interesting question

I would direct it to Jerry at Leisure Line he is a stove manufacturer and has been around a few different designs and I'm sure has had to consider it when designing the product he's building and selling.

Lets see if he notices it here, if he doesn't you could re-ask over in the Dealer / Vendor area .
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Post Sun. Mar. 16, 2008 9:03 am

I believe there is a patent out there from Keystoker on the flat grate design, I seem to recall running across it. It appeared to be based on the stove-style stokers (which the KAA-2 uses). The KA-6 is a totally different animal, and I don't know that they would go through the trouble to totally redesign it because it's basically bullet-proof.

I'll see if I can dig up that patent.
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stoker ace
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Post Sun. Mar. 16, 2008 8:04 pm

Thanks for the feedback. The feed mechanism of the slant grate stoker I saw in the KA-6 certainly appears more robust (bullet proof comes to my mind as well) than the feeder in the flat grate in the KAA-2 so I was wondering why the dealer would be "pushing" the flat grate unit. It was not a question of price point as we never discussed the difference in cost - he was so convinced the flat grate was a better performer based on units he has sold that he intended to order all future KA-6's with the flat grate design (adjusted for the correct BTU output of the KA-6). I was hoping to get opinions from "real life users" of each type of stoker to see if your experience backs up the dealer's position (or not) so I could make the best "informed" decision if I decide to go with a KA-6.


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Post Sun. Mar. 16, 2008 8:16 pm

I'd be very surprised if the KA6 boiler can make it's rated BTU with the much smaller flat grate stoker. I don't think he CAN order a KA6 with a flat grate stoker unit..

I think the difference in the grate performance is that the flat grate stoker will have a deeper firebed.. so the coal should burn more completely.. Very similar to an underfeed stoker burner or the Harman style 'push up' style stoker..

I did an experiment with a Leisure Line stoker... I put a 'speed bump' at the end of the grate, it's effect was to slow down the ash as it was pushed down the slanted grate.. The coal bed became about twice as deep, maybe 2.5" instead of 1 or 1.5" deep before.. It seemed that I had a more complete burn of my coal,,, and had less unburnt pieces.. but my experiment was short, I changed to a different coal, so my 'before and after' data was no longer valid.

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Burning Pea/Buckwheat through an antique stoker [semi retired SSboiler],
Running an Axeman-Anderson 260M boiler burning Pea, About 150-250#per day
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stoker ace
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Post Mon. Mar. 17, 2008 12:41 am

Thanks Greg L.

The dealer was not going to spec the same exact flat grate stoker as it comes in the KAA-2 but the same design with a larger "bed". He called it the 138K unit which is close to the rated 144K of the slant grate unit that is listed for the KA-6. The 138K flat grate unit would have the same outside feeder system parts as the smaller flate grate unit in the KAA-2. I do not know if this is something Keystoker is building specifically for him. Because of my limited knowledge I asked numerous questions and told him the flat grate feeder system looked like a step backwards in durability compared to the much heavier duty feeder on the slant grate unit. He said the flat grate unit just performed better - easier to light, less issues with clinkers, and more complete burn with less unburnt pieces ending up in the ash pan. He said the slope on the slant grate often causes individual coal chunks to roll over as they go down hill which can stifle the flame and inhibit complete burning. If I understand correctly your speed bump experiment kind of supports this. The speed bump keeps the coal on the burn zone of the bed long enough to insure a complete burn - right ? The dealer may well be correct in what he said. I have no reason to doubt him just trying to gain a better understanding of how these work and potential advantages/disadvantages of different designs before I buy. The only other similar "simple" stoker boiler I have read about (simple appeals to me because I can fix it if it breaks) is the Harman VF-3000 but it is only rated at 95K BTU and I am thinking it might not quite be big enough for my house. I have not seen one in person but plan on looking at one as soon as I can find a dealer that has one. I would value any opinions on the KA-6 or the VF3000 or any insight out there on the topic of flat vs slant grate so I can make an educated "procurement" decision. Thanks!

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Post Mon. Mar. 17, 2008 1:19 am

For me I would want the original KA-6 Keystoker design. They are very simple with few moving parts, and the parts that move do so very slowly. I installed a used one manufactured and first put in service in the early 80's. I put 7 tons a year through it. I don't know how much it burned previously but nothing even looks close to being worn. The gear boxes, from what I hear and mine is no exception, do drip a little oil eventually. The KA-6 stoker uses an oil burner motor to run the stoker and the main fan. These motors last a long time and if it dies a replacement should be available with little problem. They use a small Fasco blower that runs 24/7 to help the fire stay burning during times of low demand. If this blower were to die during the heating season it would not be a big issue because the timer could be readjusted to make it work until a new blower were in place. If you want to keep fire all year long for hot water, and I would recommend this if coal is cheap enough in you area, the Fasco blower is necessary for summertime operation or the fire may go out.
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stoker ace
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Post Tue. Mar. 18, 2008 9:54 pm

Thanks gaw. I have the same opinion of the original KA-6 stoker design. I am very familiar with oil burner parts as that is how I have heated my home for some years. My first impression of the KA-6 was that it was dead simple, very heavily built and easy to source parts for. It looked like I could buy some of the stuff right from Graingers or our local oil burner supplier. That is why I reached out to you guys for some education as I have no experience with coal burning - my intuition did not agree with what the dealer was telling me when he was describing the superiority of the flat grate unit. He wasn't blowing smoke he is totally convinced in his own mind the flat grate is better - so much better he plans on ordering all future units that way. I do not know if he has established that Keystoker will actually build them his way but he seemed very convincing. He also wanted to reduce the flue collar on the KA-6 to six inch as he said not too many people had an eight inch thimble on their chimney and that compicated installations. From what I have learned since starting this post I think the dealer may be right - the flat grate may have an advantage in overall burning performance - but the original slant grate design has to be more durable long term. I am still considering options at this point and plan to change over to coal burning over the summer. Thanks to input from the forum members here I am convinced my "gut" feeling was correct and if I go with a KA-6 I will insist on the original slant grate stoker. Thanks to all for the replies. This is a great resource for "newbees" like me.

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Stove/Furnace Make: Keystoker furnace
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Post Thu. Mar. 20, 2008 10:01 pm

Hi Stoker Ace,

I heat my house with a Keystoker 150 furnace which I installed a year ago so this is my first winter. Before I bought the furnace I also looked into the difference between the slanted and flat stoker and finally called Keystoker. I found out that the patented flat stoker is generally used in the stoves but in the larger furnaces the slant stoker was used. Apparently it works better where you are burning tons of coal. I must say the Keystoker stoker certainly does work well and is extremely well made. The only problem I have had is chunks of coal getting jammed in the stoker. In 6 ton of bagged rice coal this happened 3 times resulting in the fire going out but no damage to the stoker because of its design. By the way, Keystoker makes oil burner backup for this furnace. Maybe it is also available for the boiler version. This oil burner also works well for taking the chill off the house in early fall and late spring.

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