Burning up grates on my Harmon stoker

I will post photos of the old grates when I take them out and replace them. It will probably be saturday till they come into the stove dealer. Also, I measured out how much coal I burned in a 24 hr period. It was 84 lbs. and the ash from that burn was 7 lb. 6 oz.

Jerry

Hi Jerry, that is only around 45,000 btu per hour. Certainly not out of line with the design of the unit. What was the run time during this period, was it continuous out as you had described? It has been suggested that per haps something is wrong with the grate support or that there is a leak. The only area that I seal with furnace cement is at the back lip by the pusher block and around the edges where the gasket material is under the grate holder. Based on the picture you posted there are not any big leaks in any of those areas.

At 9% ash that is some nice coal you are burning, so no issue there. I am still thinking that the manual damper is hurting rather than helping. If you truly have too much draft the only way to prevent is is provide more air up through the baro and thus reduce the draw on the stove. A bigger baro to better match the chimney would do this.

Pictures of the grate support structure are eagerly awaited.

I am surprised at only getting 45,000 BTU per hr. and that is running continuously. I will do another test when I get in the new grates. I can open the manual damper all the way and let the baraometric open all the way if needed. However, isn't that sucking alot of room air up the chinmey??

Jerry

jerry wrote:I am surprised at only getting 45,000 BTU per hr. and that is running continuously. I will do another test when I get in the new grates. I can open the manual damper all the way and let the baraometric open all the way if needed. However, isn't that sucking alot of room air up the chinmey??

Jerry

If the draft at the stove is higher then specified for your stove then your heat and money are going up the chimney. The fast flow of air through your stove causes the fire to burn hotter then it's supposed to.
I'll have to ask Matthaus this: Is it possible to have such a strong draft that it overcomes the combustion air and accelerates the burn, esp. if you have leaks around the burn grates?
With the manual damper wide open and a full burn going, is the baro open all the way?
You said the draft was checked, what was the draft reading? If the baro is open all the way and the draft is over specs then a larger baro would be in order.
Yes, the baro introduces room air into the chimney. An important thing to remember is that the hotter the chimney gets the more the draft increases. So an important factor in controling draft is controlling the chimney temperature as well controlling the flow of gases from the stove into the chimney. Anyone who burns wood and has had a chimney fire can attest to this, a chimney fire creates a great draft!

Yes... with the maual damper unrestricted (full open) the barometric damper is wide open. That is with the barometric weight set at .04. If I increase the weight, the the flapper will be partially open. The draft in the stove is -.04. I had borrowed a draft meter from the dealer and checked it with numerous setups. I now have my own meter so I can take readings any time. I can change the draft either higher or lower using the restrictor plate on the blower motor and the manual damper.


What should the chimney temp be and what is the proper way to take a reading. should it be taken before or after the barometric damper???

jerry wrote:isn't that sucking alot of room air up the chinmey??

Yeah, that is the drawback. I don't recall, did you ever say how big the flue in the chimney is?

flue gas temp should be taken before the baro to get a true reading. I use a small 50 to 550*F dial type with a 6" probe that I drilled a hole in the stack and just leave it there. To answer John's question about accelerating the combustion air, the draft is the draft if it reads .04 before the baro that is what it is. The more we talk about this I think my idea of a bigger baro and pipe might be bullshit. I just can't make up my mind about what effect that manual damper is really having. If the baro is wide open with the manual damper open and then is gradually closed and you can watch the baro close it seems to be having the desired effect.......... I dunno.

The chimney is a 6" out of the stove for about 6 feet total and the it goes into an 8" masonry that goes two stories plus the attic.

jerry wrote:The chimney is a 6" out of the stove for about 6 feet total and the it goes into an 8" masonry that goes two stories plus the attic.

That chimney can probably pull quite a draft. Mine's a 6"X6", 2 1/2 stories, with the baro held closed it was pulling 1.5" WC and rising.

Matthaus wrote: To answer John's question about accelerating the combustion air, the draft is the draft if it reads .04 before the baro that is what it is. The more we talk about this I think my idea of a bigger baro and pipe might be bullshit. I just can't make up my mind about what effect that manual damper is really having. If the baro is wide open with the manual damper open and then is gradually closed and you can watch the baro close it seems to be having the desired effect.......... I dunno.

What I mean is say the baro's open all the way, the draft goes above the set point, but the baro can't open any more, in theory can the draft increase to the point where it will overcome the combustion fan and actually increase the amount of air being drawn into the stove, above what the combustion fan is providing?

Also, I think a larger baro is the way to go, or 2 of the current size. I suppose you can slow down the flow with the manual damper, the baro will close somewhat, but the gases going into the chimney be at a higher temperature. I would think at some point the baro will begin to open again due to the higher temperatures in the chimney causing a higher draft. Why not use a larger baro and let the draft stay at the set point automatically?

Sounds like we need to run a carefully controlled test, complete with a huge amount of alcohol.

I am quoting the Barometric Draft Control explanation from the Field Controls website:

"Static pressure of the cool air (1) Illustration A exerts pressure on the outside of the furnace or boiler, the breaching, and stack. The pressure difference between the room air and heated gas (air) causes products of combustion (2) to flow (draft) through the unit and rise through the breaching and chimney.
Room temperature air (3) enters through the barometric draft control (4) in the precise amount needed to overcome the excess drafts caused by temperature variations, wind fluctuations, and barometric pressure changes.
Combustion of fuel is complete and the process is stabilized. The velocity of combustion gases through the heat exchanger is slowed so more heat is extracted. The unit operates more efficiently, reliably, and requires less maintenance."

well, I pullled out the grates this morning and to be honest they were worse than I thought. The grate holder looks fine and the gasket seems to be in place. See pictures below...

Jerry

Looks like the damage is being done at the very beginning of the grates, it seems that you have the bulk of the combustion taking place right off the feed at a point that the grates are least able to dissipate the heat.
On my unit at that point of the grate the coal is just igniting and still even has a shiny black top I think you need to increase the stroke to get that fire farther out onto the grate I find that using the DOTs on the feeder 2 dots is a nice setting, if you are pushing 2 dots my next thought is how often, it could be that even tough you have a 2 dot stroke the RPMs of the motor could be way slow never advancing the coal down the grate .

Lets see what some of the other folks think .

Wow

They really are burned pretty badly. I think Coalstoves is right, you have combustion all the way back into the throat of the stoker. There is probably a lot of air leaking past where the back of the grates sit on the grate holder. I know the current reccomendation is to just lay the grates in, but I used furnace cement around all the edges of the grates. This will make sure that the air only comes up through the holes, and not around the edges. The castings are not machined in this area, perhaps, yours has a high spot or two that letting a lot of air leak past.

Good Luck

Bill

Is there an accumulation of fines or ash under the grate, keeping the grate insulated from the cooling effects of the combution air??? In hand fed stoves, if the ashes get deep enough to touch the bottom of the grates, the grates overheat and warp...

Do you regularly clean out under the grates, and keep the ashpan clean so that the fire isn't backed up on the grate??

It's the only thing I can think of that hasn't been mentioned yet [unless I missed it]

Greg L

I also think Greg and Highlander are on the right track. Those grates look like they've been subjected to sustained intense heat with improper or uneven airflow underneath.

There is a cleanout trap under the grates- looking down from the inside of the glass, there should be a lever-looking thing pointing to one side, underneath the grate holder. Can you swivel it forward to open the dump? Even with my combustion fan running full time, I kill the power for a short time every month(ish), cool the stove enough to reach in from the front with a fireplace poker, and open that slide to clean out some fine ash into the pan. If allowed to accumulate, I could picture that fly ash getting compacted and messing up the airflow.

If you still have the grates out, you should be able to look right down through the grate holder and see the ashpan if the slide is open.


Just to cover more bases, any idea what the stack temps were before the baro damper and/or surface temps of the stove when running? I have to re-read this thread, but only 45,000 btu's, an uncontrollable draft, and grates burned out like that don't add up (I'm no math whiz either, by the way... ) Unless the draft is pulling air through partially blocked grates, making for hotspots at wherever the fire can suck air, but the combustion blower is somehow not feeding air?? Maybe??