Passive air over fire.

[vach50]

I have a homemade 2x6 evaporator I love. Conventional fire box with grates and a draft door under the grates. I cook at my remote cabin with no electricity, never will be.

I've been thinking about air over fire. My wood stove at the cabin has what I call passive air over fire. Air is drafted in from the back of the stove, pulled up over the fire and injected into the fire box thru a header with holes directed toward the front door. When the fire is going good, there is a lot of turbulence over the fire, burning gases and improving efficiency. It works very well.

My thought is to do something similar on my evaporator. My initial thought is drill holes thru the side walls of the firebox 6" below the top. Thought I would match the air over fire open area on the header of my wood stove using 1/4 or 3/16" ID pipe. Might be 15 or 20 pipes. Smaller diameter to increase air velocity and turbulence hopefully.

Anyone do anything similar? Any other thoughts? I've read all about blowers and auf/aof. Not gonna work for me. Thanks.

[mol1jb]

This always sounded like a cool idea but I havent heard of anyone who was successful with it.

[RileySugarbush]

I don't think it would help much. My first attempt at AOF was a similar number and size of nozzles and a low pressure blower. Did nothing significant. Without the high velocities I don't think the extra air helps combustion. May actually things down. But experiments are great teachers. Let us know what you learn.

[Super Sapper]

Experimenting is a fun part of doing this so give it a try. The only thing is reducing your nozzle size will not increase your velocity without pressure, it will only restrict your flow. If you could route the piping so it would preheat it would help.

[Waynehere]

I would think that in order to make this work, you might have to really oversize your stack to get a lot more draft? Larger diameter and taller.

[motowbrowne]

My guess is that it won't do much. The fire in your arch is a lot different than a woodstove. If you want to speed things up you could try running a leaf blower outside the shack and ducting it in underneath your grates. Should be able to get it far enough away not to be bothersome.

[CharlieVT]

...My wood stove at the cabin has what I call passive air over fire. Air is drafted in from the back of the stove, pulled up over the fire and injected into the fire box thru a header with holes directed toward the front door. When the fire is going good, there is a lot of turbulence over the fire, burning gases and improving efficiency...

Without access to electricity for blowers, I would be looking to the "low hanging fruit": Really dry firewood, fine split wood, crisscrossing the firewood for increased surface exposure to the draft, frequent and evenly spaced firings, monitoring stack temperature with a temperature probe, and managing natural draft for stack temperature. A natural draft arch should operate with a stack temperature in the range of 650-800 degrees. (Source: Boiling 101 from The Institure for Maple Education.)

I agree with others who suggest that you would probably see little gain in adding passive air over fire to your arch.

What you are describing is a type of non-air tight stove with passive air flow to increase efficiency and reduce pollution.

In the 1970s, air tight stoves had some popularity. We heated our house with a couple of Vermont Castings air tight stoves. You could "throttle" them back for a slow burn overnight. The down side of these air tight stoves is inefficient burn producing lots of creosote, carbon monoxide, etc. when the draft was reduced.

EPA regulations "encouraged" design and sale of stoves that are not air tight and have dampers that cannot be entirely closed and have perforated ducting over the fire that help increase more complete combustion. The design prevents complete shutting off of air without unauthorized modification of the stove.
While this technology does somewhat increase efficiency improving heat yield, the biggest "gains" are not in found in increased heat at maximum burn rates but rather more complete burning at slower burn rates.

We now heat our old farm house with a couple of these more modern stoves. Because the stoves can't be "throttled" back like the old air tight stoves, the amount of time a single firing will last is less; the design prevents a slow, smoldering fire. However, chimney maintenace is much easier, very little creosote buildup. But as far as maximum heat output at full, rip, roaring burn, not that big a difference.

Some reading on air tight vs non-air tight stoves:
http://mb-soft.com/juca/print/theory.html

[vach50]

That's why I love this forum. Lots of great input especially CharlieVT. Unfortunately not what I wanted to hear. Darn facts sometimes get in the way. Working on trying to improve efficiency, only been doing this 5 years and learned a ton so far. Improved efficiency last year, but still think I can do better. Will keep grinding away at it. Thanks for input!

[wishlist]

Without access to electricity for blowers, I would be looking to the "low hanging fruit": Really dry firewood, fine split wood, crisscrossing the firewood for increased surface exposure to the draft, frequent and evenly spaced firings, monitoring stack temperature with a temperature probe, and managing natural draft for stack temperature. A natural draft arch should operate with a stack temperature in the range of 650-800 degrees. (Source: Boiling 101 from The Institure for Maple Education.)

I agree with others who suggest that you would probably see little gain in adding passive air over fire to your arch.

What you are describing is a type of non-air tight stove with passive air flow to increase efficiency and reduce pollution.

In the 1970s, air tight stoves had some popularity. We heated our house with a couple of Vermont Castings air tight stoves. You could "throttle" them back for a slow burn overnight. The down side of these air tight stoves is inefficient burn producing lots of creosote, carbon monoxide, etc. when the draft was reduced.

EPA regulations "encouraged" design and sale of stoves that are not air tight and have dampers that cannot be entirely closed and have perforated ducting over the fire that help increase more complete combustion. The design prevents complete shutting off of air without unauthorized modification of the stove.
While this technology does somewhat increase efficiency improving heat yield, the biggest "gains" are not in found in increased heat at maximum burn rates but rather more complete burning at slower burn rates.

We now heat our old farm house with a couple of these more modern stoves. Because the stoves can't be "throttled" back like the old air tight stoves, the amount of time a single firing will last is less; the design prevents a slow, smoldering fire. However, chimney maintenace is much easier, very little creosote buildup. But as far as maximum heat output at full, rip, roaring burn, not that big a difference.

Some reading on air tight vs non-air tight stoves:
http://mb-soft.com/juca/print/theory.html

I have a modern air tight high efficiency soapstone stove that has " true gasification " similar as to what vach50 describes he has in has cabin. No electricity required and watching the combustion of wood gases above the actual wood is really amazing . Average about 2 -3 real cords of wood to heat a 1974 1800 square foot house 24/7. You must use properly dry and seasoned wood however. No cutting down a live oak in February and trying to burn it in October , will not work !
That link that was posted is nothing but sales literature for a JUCA stoves imo. It stated that soapstone stoves cost " thousands of dollars more" mine was under $2000 otd plus a 30 percent rebate from the federal government. Retains heat way more than " 1 hour " probably closer to 4-5 hours. Used to have a steel Blaze King from the 80's and I think a good analogy would be the amount of wood used boiling raw sap or concentrating with a ro, huge difference with the same result in the end .

With that said I don't think my stove would boil sap very well . Remember most of us load our arches every 7 minutes or so with fuel and my wood stove I load every 10-12 hours with fuel . Big difference .

[maple flats]

High pressure air over fire only works with a high pressure blower, I see no way for passive to attain even a little high pressure. The high pressure is needed to get the turbulance. You may however be able to get enough air flo in the area above the fire to increase efficiency and help but I doubt it will ever be high pressure. With some ingenuity you may be able to get high pressure air however. Look up hand crank blowers for blacksmiths coal forges, they are radial blowers and some of the Russian imports are fairly cheap, or India made blowers like this http://www.ebay.com/itm/New-Blacksmith-Forge-Blower-Hand-crank-manual-fan-like-old-champion-blowers/252864830353?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao%3D1% 26asc%3D40130%26meid%3D30e77edce94141b881b0f63369e 4fd02%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D201 778174970 Beware of Chinese blowers, they get very poor reviews. Then try to come up with a way to power the crank with something other than hand power. While hand power will give the necessary air needed, that would get real old real quick. To get the necessary air flow you will need to turn it at about 25-40 RPM on the crank. Maybe a 12V motor, a deep cycle battery and a drive belt could do it. Also consider a small solar panel or 2. While my first solar was just 2 panels and they charged a 4 battery bank, it was enough to power the sugarhouse during daylight and about 3 hrs after sunset. Then I had to fire up the generator.
Where there's a will there's there's a way.

[Mark B]

Look up hand crank blowers. Then try to come up with a way to power the crank with something other than hand power. While hand power will give the necessary air needed, that would get real old real quick. To get the necessary air flow you will need to turn it at about 25-40 RPM on the crank. Maybe a 12V motor, a deep cycle battery and a drive belt could do it. Also consider a small solar panel or 2. While my first solar was just 2 panels and they charged a 4 battery bank, it was enough to power the sugarhouse during daylight and about 3 hrs after sunset. Then I had to fire up the generator.
Where there's a will there's there's a way.

You could try a low rpm electric motor and gear it to run through an older lawn tractor transmission to get rpm reduction. Have set this up for turning a spit in a BBQ made in an old oil tank. Low gear may get you the rpm's you need.

[djblech]

Would increasing the diameter of the stack increase draft to help aof? I am planning to put in an 8" stack and reduce it just above the arch to 6". I am hoping this will act like a expansion chamber and increase draft.

[RileySugarbush]

Trying to get AOF velocities with stack draft is probably not going to happen. In my system, the AOF pressure is 8 inches of H20 across the nozzle. Thats about 0.3psi, and that is what gives the jets the 80 mph velocity that lets it mix well. If you managed to get anywhere near that much suction with your draft, which is really impossible, it would just draw up through the grate since that is a much easier path. So it would act like a normal system with great draft.

[Super Sapper]

Would increasing the diameter of the stack increase draft to help aof? I am planning to put in an 8" stack and reduce it just above the arch to 6". I am hoping this will act like a expansion chamber and increase draft.
Reducing to 6 inch would only reduce your draft as it is restricting your air flow. I would go with 8 inch all the way.