Coalgun- Puff backs & Explosions

Mac, I'll try to help you get to a solution to your problems. First the AHS Coalgun is a good boiler and properly installed and operated will give good long term use. It along withe the A-A boiler is unique in the way it burns coal. Coal is burned in a tube, hence the term "Anthratube". When this method of burning coal was researched in the early 1940's there was nothing like it, in fact some of the experts at the time were surprised by the test results. A report was published in 1945 on the Anthratube. I've uploaded a copy. Much of the development work was done by Anthracite Industries, Inc. A company long gone. The report is somewhat technical but what I want you to understand is the concept of "fire height". By this I mean where the coal actually burns. This is important because if the "fire height" is in the wrong place problems result. The only way to control "fire height" is with the ashing grate. Stop the grate operating and the fire will burn higher. Stop it too long and you will have the fire burn it's way up the feed tube and you will have a hopper fire. Operate the ashing grate more and the "fire height" will be lower. Operate it too much and you will put burning coal in the ash bucket. Followed by raw coal.

So between these two extremes is where the "Coalgun" needs to operate. If you read the referenced report carefully you will see they determined the fundamental bed burning properties for Anthracite.

Anthratube_Design_History.pdf

I'll quote from page 59 of the referenced report.

"As a result of the investigation of the fundamental properties of beds of burning Anthracite, it was shown that the thickness of the oxidation or intense burning zone in the bed depends depends upon the size of the fuel used. In general, it was found that Anthracite burns approximately 4 to 5 pieces deep. In other works, the oxygen in the air for combustion will pass a given amount of fuel surface before it is completely consumed."

Read and re-read this until you understand what it's saying and then think about the implications. First the burning coal is only 4 to 5 pieces deep, not much using Anthracite pea coal. Second all the oxygen in the combustion air sucked up by the combustion blower gets all used up. This is key to understanding "puff backs".

Let's think what happens when the "fire height" is just right, whatever that is. When the blower is running hot combustion gases are drawn into the boiler tubes heating the boiler water. Sufficient air flows thru the coal bed to support coal combustion. Next the blower stops. The inspection port opens. Fresh air with new oxygen enters, drawn in by both chimney draft and hot air rises principles. IF, the "fire height" was such that all the oxygen was used up AND combustion-able gases exist a "puff back" will occur. The oxygen for the "puff back" came in via the inspection port door. This is why minor "puff backs" are often heard when the combustion blower turns off.

So, what's the correct "fire height"? Seems to me it needs to be no lower than a few thickness of coal below the bottom of the inspection port. You should be able to see the burning coal when you look in thru the inspection port. If you do it means the coal combustion gases that didn't have enough oxygen to burn are on their way thru the boiler tubes. Sucked there by the combustion blower. Now when the blower stops and fresh air enters these unburnt gases are past a source of a flame which would ignite them. Adding fresh air has no effect and the open inspection port door just allows the natural chimney draft to work.

How to adjust "fire height"? It's changed by operating or stopping the ashing crank motor. It has nothing to do with the thermocouple controller temperature settings. The controller settings are used to maintain the position of the "fire height" oncee you get it to where it needs to be. Raising the "fire height" can be achieved by breaking the electrical connection to the grate motor. Clearly if it's not ashing the "fire height" will rise up in the "Anthratube". Conversely, applying power to the ashing motor will lower it. How you break or make the connections to the motor is up to you. I chose to put the motor in series with a switch. Normally the switch is closed. When I want to raise the "fire height" I open it. Since coal responds slowly you need to watch it, because if you forget it, you will have a hopper fire. Use caution.

You could use your thermocouple controller as an on-off switch. Just use extreme set points that either turn on or off the grate motor. Remember these are temporary set points only. Used solely for setting initial "fire height". Again pay attention. A wrong setting for too long could cause a hopper fire or an out of fire condition.

I hope this helps.

I just want to supplement Yanche's post.

On my thermograte installation there is a separate switch for the grate operation that will stop grate operation but otherwise allow the unit to operate normally. This switch is normally ON but it can be used to stop grate operation at any time. The AHS instructions actually tell the operator to switch the grate off during unit start up to allow the fire to fully develop.

In my experience the appearance of where the fire level is when looking through the port varies depending on whether the unit has just operated on forced draft or whether a period of time has lapsed since the last forced draft operation.

I have had best operating experience when the burning coal level appears to be above the bottom of the port--WHEN VIEWED IMMEDIATELY AFTER A FORCED DRAFT BURN. On the other hand if the fire is viewed a considerable period after a forced draft burn very few ir no glowing coals will be visible.

In the EFM world we judge the coal to air ratio after a 30+ minute burn cycle, with the stoker still running. It sounds like the Coal Gun needs a similar burn cycle before you judge the height of the fire.

I appreciate all the feedback. I have lowered the settings below to 113 to increase the burn height in the sight tube and everything looks great . The problem is everything looked good the day it blew the stack and baro apart! If I had all day to make sure the fire was perfect they should put a glass window on it instead of the tombstone cover and call it a handfed unit instead of a stoker. :lol

I have to joke a bit , everybody breaks my nuts about burning coal when I have acres of fallen oak trees and a big fancy fireplace I never use in my house. The boiler works great and I am burning a lot of coal and heating my house /shop cheaper than using heating oil or chopping wood all weekend long. The reason I went with an AHS was not to be fussing all day with a fire like a hand fed unit.

Mac

I would add that it seems best if the ash grate motor is wired to run only when the boiler fan is operating. AHS can assist with this minor alteration, which is how their UL listed boilers are now wired.

Macdabs, what temperatures are you nominally maintaining within your AHS S-260 boiler. What is your L4006A Aquastat set at? I wonder if this could have some impact. Just curious.

I got a private message from a forum member saying the "Anthratube_Design_History.pdf" link in my posting above doesn't work. I just tried and it works fine. Anyone else download the file successfully?

The link really points to: "http://nepacrossroads.com/download/file.php?id=37073". It's a file I originally posted in a private message and I just linked it here.

lsayre wrote:Macdabs, what temperatures are you nominally maintaining within your AHS S-260 boiler. What is your L4006A Aquastat set at? I wonder if this could have some impact. Just curious.

The boiler is at the factory setting at 180* and the low side I think is around 160-165* . I have two Wilo large pumps feeding my oil boiler to my house at 245' with insulated pex and one circulator pump for the boiler . I have one smaller wilo pump feeding my in floor heat at the shop all on one zone/ 3200 sq ft .

The only time I have seen the boiler drop to 150-155* would be on a very cold day in the teens with a large heat demand for my house and the garage at the same time. And it would only be for a few minutes and the boiler would get above 160*-170* in no time. I am very impressed with the heat output of the boiler and the ability it has to maintain a constant temp 0f 170-180* the majority of the time. Now it uses a fair amount of coal during the low temp days that the house has a large demand.
I am heating the house warmer than I ever did with oil . I heat probably at least 7,000 sq ft the majority of the time with the home at 70* and the shop at 63*. The shop is a 6-10" concrete slab on top of foam insulation. I can't say enough about the in floor heat in the shop . Demand for heat is very rare and for maybe for only 10 min. throughout the day. With the thermostat at 63* room temp the slab is probably around 70* and is the most comfortable shop I have ever worked in. I have the domestic coil for hot water in the shop since the location of the boiler is in the shop and have more than I would ever need . The big mistake I made was running the water supply line from my house in the same pipe as my heat runs. What happens is the water supply line becomes very warm so I end up with more hot water than I need . I probably will end up placing a 40 gallon well tank so the water can cool .

Mac

lsayre wrote:Macdabs, what temperatures are you nominally maintaining within your AHS S-260 boiler. What is your L4006A Aquastat set at? I wonder if this could have some impact. Just curious.

I'm no longer just curious of this. I dropped my boilers temperature (via the L4006A) by only 5 degrees yesterday evening at about the time I typed the above (with no other changes), and I have experienced a couple robust whomphing puffs since, whereas I had none before. The digital CO meter in my boiler room was reading zero when I just checked it, but the history (which had been day in and day out consistently also reading reading zero through yesterday evening) was showing a 12, meaning that sometime during the night it went up a bit, then it came back down to zero again. I just set my L4006A back up to 180 degrees.

Put one of these on your DHW line to moderate the temperature. There are several good brands.

http://www.taco-hvac.com/en/products/50 ... tegory=116

lsayre wrote:Put one of these on your DHW line to moderate the temperature. There are several good brands.

http://www.taco-hvac.com/en/products/50 ... tegory=116

I have mixing valves on the whole system, the problem is the main water supply line is wrapped with my feed lines to my oil boiler. Without demand for water the 245' supply line becomes almost 180* from sitting in the same pipe as the heat lines. The supply line is 3/4 pex @245' so it takes some time before fresh cool water flushes through. Washing your hands in the sink is not enough use. I think adding a uninsulated well tank should take care of it .

Mac

The owners of the Coalgun who are members of this board should realize that thousands of people own a Coalgun and don't have the benefit of a forum (like this one) to discuss issues and concerns with other owners. What I'm trying to say is, how many others exist who have no idea what might be wrong with their Coalgun or even a basic concept of where to go for help.

Again, not being an owner of a Coalgun myself, I can only rant from my keyboard soapbox. I would not recommend a Coalgun to anyone at this point until a satisfactory resolution from the manufacturer is formally put into place. The Axeman-Anderson is the father of all Anthratubes and is built to last a century of continuous, trouble-free use.

If the principals that currently own AHS would realize the gravity of the situation, they would surely understand the need to respond to customer complaints, analyze and engineer a solution or remedy, write a proper operating manual and ensure the future success of the company by rethinking the culture of apathy toward it's customers.

EarthWindandFire wrote:The owners of the Coalgun who are members of this board should realize that thousands of people own a Coalgun and don't have the benefit of a forum (like this one) to discuss issues and concerns with other owners. What I'm trying to say is, how many others exist who have no idea what might be wrong with their Coalgun or even a basic concept of where to go for help.

Again, not being an owner of a Coalgun myself, I can only rant from my keyboard soapbox. I would not recommend a Coalgun to anyone at this point until a satisfactory resolution from the manufacturer is formally put into place. The Axeman-Anderson is the father of all Anthratubes and is built to last a century of continuous, trouble-free use.

If the principals that currently own AHS would realize the gravity of the situation, they would surely understand the need to respond to customer complaints, analyze and engineer a solution or remedy, write a proper operating manual and ensure the future success of the company by rethinking the culture of apathy toward it's customers.

I questioned AHS about the puff back issues before purchasing my S 260 and I was told it was draft issue and if I had good draft I wouldn't have any issues.. I never really have had any problems or even heard a wimper from the boiler at all . The explosions always occured when I was not in the shop and I would arrive to see pieces on the floor . My wife says the anti -coal gremlins come out when I Ieave. I was searching on the net and found that the AHS wood guns have a puff back issue and one of the users claim it has something to do with the cyclone funnel in the exhaust and ash. Nice to know we are not alone .

At this point in the game. I changed my settings as instructed , verified draft , and replaced my last Baro on this boiler. If it blows things apart I will make a visit to the factory with the boiler and a bill and all the other AHS owners are more than welcome to come with me.

Mac
Mac

I just watched my S130 blow flame out of the barometric damper during a puff-back. It was a Doozy. Happened within scant seconds of the fan cutting off. Manometer was reading 0.03" before and after the event. I just checked my zero on the manometer and it zeros just fine when disconnected.

Considering that I had no puff-backs all last season, and the only change I made is Harmony vs. Blaschak, I have to believe that the Harmony pea itself is at least somewhat suspicious at this point.

I just dropped my SV to 117 degrees. Very leery of going below 120 degrees, and the manual says to not go below 120....