Coalgun- Puff backs & Explosions

: McGiever On: Sun Dec 30, 2012 8:06 pm


Is that not what the hysteresis does?

fwiw
Generally once the ash temps start going up they can go up quickly so the difference between a 5dg and a 10dg hysteresis may be only couple of strokes (more or less) on the grate. So the hysteresis can control ashing time to a degree.

Just an observation:
Often after a long idle time the ash layer becomes a little thicker and cooler, not yet quite cool enough to call for ashing, the idle fire is smaller as well. (note that I have never measured ash thickness but long ashing cycles generally follow long idle times)

Then there is a call for heat and the fan comes on, the cool air goes thru the now thicker ash layer and can drop it easy 30+- dg below SV, the boiler gets up to temp and the fan turns off but the ash has not reached SV so it keeps putting more new coal on a now hot but idle fire (insert puff back here?) until SV is reached.

The ashing scenario during a more active period seams to not drop ash temps as drastically as after a long idle and will return to SV much faster.
ymmv

lsayre wrote:Yanche, by what principle and mechanism does your older S130 ash? Is it as a percentage of fan running time? Are you experiencing any puff-backs?

I just had another puff-back. Ash level in the pan is substantially more now than it was earlier today, so my assumption is that sometime during the day it must have ashed long and hard and dropped the fire down low again. It seems as if ashing more predictably and more evenly in quantity over time (as opposed to temperature) would be of potential benefit.

My boiler ashing is controlled by a ten minute cycle timer. It is powered only when the combustion blower is running. I have it set to 1.5 minutes. So the ashing grate moves 1.5/10 % of the time the combustion blower is running. My fire height is near the top of the "Anthratube". I'm burning UAE pea coal and I have no puff backs.

I had the thermocouple only, installed by AHS when the boiler was back for modifications. My intent was to use my own thermocouple controller and convert to the thermo ash style control. After trying to understand the logic of the thermo control (I couldn't) I stayed with the timer control.

Yanche, thanks for sharing that! I may consider going to a timer system.

This thread has been quiet for several weeks.

Lsayre--have you resolved your problems? Please post your current experience and, if the problems have been resolved, what steps finally resolved the problems.

I'm not currently experiencing puff-backs. SV = 100 degrees, and hysteresis = 5 degrees seems (so far at least) to have been the answer here. Still burning 100% Harmony pea.

Disclaimer: Do not use these radical settings based upon my experience. Consult with AHS.

Update that's a couple days late: I guess I jinxed myself be posting the above post. We had a warm spell a few days back and the boiler wasn't firing, and the ash grate temperature slowly drifted down to about 85 degrees (which is well below the 95 degree ashing point that my settings of SV=100 and 5 degrees of hysteresis bring to the table), but alas the boiler could not ash at or below 95 degrees because it is wired to ash only when the fan is running, and with no heat calls there simply was no fan running.

Well, when it turned colder again the boiler finally fired up the fan, it began ashing immediately, and it ashed 100% throughout the entire firing cycle (which was long because the boiler had been sleeping). Mano was reading 0.03" since it still wasn't all that cold outside. 2 blasts occurred in rapid succession well into the firing session, while it was still firing, and then a third blast occurred about 30 seconds later when the fan cut off as the boiler finally reached its 180 degree set temperature. Ash grate temp reached 100 degrees right at fan cut off.

No damage to anything that I could see. Just a whiff of the telltale sulfur smell and some soot that had gathered on the floor below the tombstone cover. The digital CO monitor in the boiler room remained at zero then and thereafter (as did my other detectors throughout the house). I would therefore rate the blasts as not very extreme.

My opinion now is that the innovative thermal ash grate monitoring system is a very huge mistake. What these boilers need is to ash just like Yanche's older AHS does, via a cycle timer, with ashing occurring during every firing cycle and for only about perhaps 20%-25% of fan run time. If they ever ash for 100% of a long fan cycle (and they do) they can drop the fire precipitously low and the potential for blasts goes way up. As it stands now, the thermal ash grate monitoring system only ashes the boiler on about every 4th heat call and fan cycle (give or take a cycle) on average. If instead it ashed for about 20-25% of every single fan cycle the fire level would conceivably never drop more than a quarter of the distance it will drop in one long and hard ashing event. I have emailed AHS with my thoughts on this.

Larry,

I have been on the sidelines this heating season because I've been watching other AHS posts, and my puff backs have been less since making all the changes to my boiler last season. Well, I had my largest, loudest and scarriest explosion ever last week. The house shook! After a thorough system check I found out that my reostat fan control failed and the ash dump motor was running at full speed. This fan control switch was installed last season to reduce the dump motor speed to lesson the amount of fresh coal entering the burn chamber. I located a more industrial control (used to adjust the speed of hand routers and able to handle 15 amps) and installed it today. The speed of the dump motor is less than 50% of the original factory speed. I will monitor the boiler for the next week and report my findings. To summarize my actions to date:

1. Rewired the ash dump motor to run only when fan is running
2. Set thermo ash dump temp to 100, 5 histeric, 110 shut off
3. Installed reostat to reduce the ash dump motor speed to 50%

I agree with Larry that the root of the explosion problem can be traced to the thermo ash control system. The timer was less efficient but safer. My experience tells me that having the domestic hot water coil in service contributes greatly to increased explosions. AHS is unwilling to get involved with the explosion problem discussed on this forum and are probably watching for us to solve the problem via trial and error. I'm thankfull that we have this forum to assist us.

Does ashing 50% slower (or more) mean that it ashes less, but that it also ashes on more fan cycles? That seems like a valid approach.

Lsayre/rychw,

As you may know, I had lots of puffback problems but have had no puffbacks since the fall of 2011 when I finally lowered SV to 118 to 120. I still have the differential at 10 and my ashing motor will run even after the fan shuts off.

I note that both of you have the setup that limits ashing to when the blower is running. I am wondering if that may be a contributing factor for a stove that is otherwise correctly set up with the thermo ash control.

I think the is widespread understanding that the cause of puffbacks is a fire height that is too low and/or the introduction of too much fresh coal too fast which generates an explosive gas that accumulates rather than being burned as it is generated.

It seems to me that limiting ashing to when the blower is running could, under some circumstances, lead to the introduction of large amounts of fresh coal. Specifically if there is a blower run where the thermograte is activated immediately before the thermostat shuts the blower down and then there is an extended period with no call for the blower to run. When the blower is finally activated the sensed temperature could be very low (as reported by Lsayre) with the result of a very long ashing run introducing large amounts of fresh coal.

Bob wrote:Lsayre/rychw,

I note that both of you have the setup that limits ashing to when the blower is running. I am wondering if that may be a contributing factor for a stove that is otherwise correctly set up with the thermo ash control.

I think the is widespread understanding that the cause of puffbacks is a fire height that is too low and/or the introduction of too much fresh coal too fast which generates an explosive gas that accumulates rather than being burned as it is generated.

It seems to me that limiting ashing to when the blower is running could, under some circumstances, lead to the introduction of large amounts of fresh coal. Specifically if there is a blower run where the thermograte is activated immediately before the thermostat shuts the blower down and then there is an extended period with no call for the blower to run. When the blower is finally activated the sensed temperature could be very low (as reported by Lsayre) with the result of a very long ashing run introducing large amounts of fresh coal.

I have operated the boiler for 5 seasons on the factory settings and have had explosions (too many to count) during that time. That led me to make the changes to prevent the explosions. Yes, the explosions are a result of too much new coal being allowed to enter the burn chamber too fast. That is why slowing the rate of ash dumping will slow the rate of new coal allowed to enter the chamber. It is important to have the fire as high up the burn chamber, tube, as possible. I needed to lower the thermo ash dump temperature to accive that. The factory settings set by AHS are not correct, especially if you have a domestic hot water coil and thermo grate combo. Again, this is trial by error with no help from AHS.

What is the perceived link between using the DHW coil and the puff-backs?

lsayre wrote:What is the perceived link between using the DHW coil and the puff-backs?

My observation is that the DHW demand increases the chance of explosions because the boiler water temp is marginally lowered instead of greatly lowered when circulator pump call. There is a greater chance of too much new coal being added that will not ignite fast enough to prevent coal gas build up and then, boom. I hope the slower ash dump motor speed will prevent too much coal added too fast and the resulting booms.

hate to bust in on a nice party, and I know nothing about the AHS. Just what I have read here in 41 pages....
However, how do you push enough coal to satisfy a call for heat at 50%??? If I cut my VF3000 back 50%, it wouldn't heat my garage let alone the house.

Kevin

KLook wrote:hate to bust in on a nice party, and I know nothing about the AHS. Just what I have read here in 41 pages....
However, how do you push enough coal to satisfy a call for heat at 50%??? If I cut my VF3000 back 50%, it wouldn't heat my garage let alone the house.

Kevin

I believe that AHS has not properly tested the boiler under all aspects with the thermo ash controller. The ash dump motor dumps too much new coal into the boiler and the 50% rate is a test to prevent the build up of coal gas. All coal boilers and stoves are different so reducing the coal feed amount on one boiler may not work on another boiler.

KLook wrote:hate to bust in on a nice party, and I know nothing about the AHS. Just what I have read here in 41 pages....
However, how do you push enough coal to satisfy a call for heat at 50%??? If I cut my VF3000 back 50%, it wouldn't heat my garage let alone the house.

Kevin

Because the coal is always there, held within the column of the 6" tube (Anthratube for AA's). It's merely a matter of where within the tube it is burning. High is safe (to a point, unless it goes high enough to burn in the hopper) and low (heading down toward the ash grate) causes puff-backs.

AA and AHS boilers are not your typical stokers. For the AHS, picture a round hand fired stove that is a scant 6" in diameter, and with a huge 250 lb. capacity gravity hopper on top of the 6" round stove, and which has periodic automatic ashing via a grate 'shaker' (sort of) that is driven by an electric motor. Surround this 6" tube stove in water, and now you have a boiler. The ashing causes the hopper coal to enter the tube and eventually it gets down to where the fire is. These boilers stoke by gravity from above via ashing from below. Everything takes place within the 6" diameter tube. Only about a 4 nuggets high fire exists, and the remaining 250 lbs. of coal don't ignite due to oxygen starvation.

Ashing more slowly (the ash grate has a knife edge that cuts the ash off the bottom of the tube as it moves back and forth) doesn't deprive the tube of coal, but rather deprives the ashing knife from cutting off too much ash over one time interval (an interval of fan induced firing of the tube via air drawn in from below via the fan). Cutting away the ash at half the speed just means that instead of ashing roughly every 4th fan running cycle, it will probably ash every 2nd fan cycle, but only cutting off half as much ash each cycle, so the net effect is the same amount of ash cut away for every 4 fan cycles.

The net effect is that the 4 nugget high fire zone within the tube also can only fall half as far down toward the ashing grate with each ashing cycle, but a full load of coal from above the tube is always there. Nothing separates or divides or cuts off the gravity hopper from the fire containing anthratube. Everything is always wide open.