Anti puff back device for AA.... isn't working

mwcougar wrote:hi all

from some experiance with my ahs 130.. wet or damp or really wet coal just adds to the problem. just from my experiance of having finding parts of my baro on the floor and on the other side of the ATV. just 2 cents......

Maybe Fred can put the coal in the dryer before loading the boiler.

traderfjp wrote:put the coal in the dryer before loading

HA! My coal is dryer than a popcorn fart.

LsFarm wrote:Steady draft venting the combustion chamber of flamable gases in burnable concentrations is what is needed,, the burnable concentration is easier to create and maintain in the 130 size boiler, and the 5" pipe.. the AA 260's larger heat exchanger size and 6" flue pipe quickly dilute the gas to a non burnable mixture. This is providing you have a steady draft..

Greg L

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That sounds reasonable. This gets us back to the draft issue . Perhaps a barometric dampener contributes to the problem when the flue doesn't have the strongest draft.

Fred what is the draft guage reading. It sounds like a strong draft can keep the boom from happening. Maybe you can increase the draft until the fire gets going. I don't know-just thinking out loud.

I'm not having a puff back issue. I haven't had a puff back since day two. The whole idea of the anti puff back device is to gain a tiny bit of efficiency while guaranteeing no puff backs. If I could close the 5/8" hole that air would go through the coal instead of over the coal. My draft stays about .02 but gets a bit higher while it's stoking. The AA fan makes its own draft.

When Greg wrote "Seady draft venting the combustion chamber of flamable gases in burnable concentrations is what is needed" I think it would be better understood if it were worded "Seady air venting the combustion chamber of flamable gases in burnable concentrations is what is needed". Using the word draft we automatically think of chimney draft. For the burnable gasses to be diluted it doesn't require chimney draft, it requires air to be drawn in over the fire while it's stoking. During stoking chimney draft has little to do with the AA's burn as the blower fan is providing the air movement.

If I got this device to work and it were cheap to build, then perhaps every AA and AHS would come with one from the factory and puff backs would be a thing of the past. Wouldn't it be cool if they could advertise "absolutely no puff backs, guaranteed."

A similar but more overall value would be a device that allow an A-A or AHS boiler to vent without a chimney stack. They already have power venting during stoking. What's needed is a small power vent device to provide idle fire venting. I envision a blower providing over the fire combustion air through the flapper door access port. It would need a reliable battery backup that would last long enough for the idle fire to burn out in case of a power failure. Get that product approved by a safety agency and you would have a real winner. A whole new product category, power vented boilers! I've got the design skills to do it but not the motivation. Go to it entrepreneurs.

Actually it IS the draft that clears the area above the fire, and in the heat exchanger once the inspection door has opened..

We had an AHS owner,, maybe it was an AA, that had real problems with puff backs last year,, they installed a draft inducer for use in warmer weather when thier chimney wasn't pulling a very strong draft.. it seemed to cure the puff backs

I think a light draft, just when the inspection door opens, pulls just the right amount of oxygen into the firechamber, mixes and is ignited.. If the draft is strong, the combustibles are quickly pulled out of the firechamber.. it's a tightrope walk.. just the right combination causes the puffbacks..

The 5/8" hole adds fresh air over the fire while the fan is running, diluting the gases above the fire,, this is all that is needed to make the combustionable mixture too 'lean' to ignite..

More theories..

Greg L

Greg has the explanations that fit my observations. I suspect the source of the gas is the unburnt but heated coal, the transfer head coal in the A-A and the hopper funnel neck coal in the AHS. My experience also indicates it better to have the fresh flushing air enter near the top of the inspection port, i.e. an area closer to the combustionable gas. On the AHS it's accomplished by paper clips on the flapper near the top or in my case by a stronger spring. Both make an air inlet opening at the top of the flapper when the combustion blower is running. In the AHS I can see how the outgassing coal gases could get trapped near the the hopper funnel entrance even when the combustion blower is running. Then at the end of the combustion blower cycle, the inspection port suddenly opens, inrushes extra combustion air and "puff". The conditions are all there, a good flame from the just well stoked fire, outgassing heated coal and finally fresh combustion air.

Got a phone call from my buddy that's been running his AHS flawlessly for four weeks now. Today, BOOM, five or six seconds later, another! Figure that out! He said the first one was like a shotgun going off, the second one like someone slammed a door. He heard the first one, went running to the cellar and witnessed the second one. He assumes it was the first one that bent his damper.

I'm wondering if it's the inrush of air that causes it, or a flame licking up. During stoking the heat & flame is drawn sideways, when it stops stoking the flame now pops up. I vote for flame ignition rather than air inrush ignition. Doesn't really matter.... the cure is the same....more air over fire during stoking. (or an anti-boom purge device)

What was his draft at the time of the "incident"?

Matthaus wrote:What was his draft at the time of the "incident"?

Low. Today is unseasonably warm with no wind & damp. He's had .02 right along, but today was close to, if not at zero. His chimney is about 30 feet, mason, interior. He's added a paper clip & will stay in touch.

When he first called his first words were "Got that anti-boom device working yet?".

The "draft" was him, running out of the house.

Fred, Maybe you could use the Automatic draft control off of a Harman SF260?

I don't know if these items have been looked at but here are a few of my thoughts coming from running a AA260m with no problems like this.

#1 Size of site tube, location, and height in the coal burn chamber.
#2 length of time fan runs after site cover opens if at all.

I’m posting a video of a timer run total video length is about 2mins, items covered are draft reading, flue gas temp, and damper opening before and after run. Also the site cover opening and fan run time after it opens (motor has stopped but centrifugal force keeps it moving even after site cover opens for a bit)


http://www.engtechs.com/temp/coal/aa260m/aa260m_timer_run.avi

The secret is simply draft, not enough draft or fire in wrong location in fire pot (as in when boiler is first started). The design has no inherent problems as long as there is sufficient draft and correct fire location. Of course with the denser load of buckwheat then more air is needed to help the draft.

Of course we all love a good science project so Freddy has our attention.