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Old 01-08-12, 08:55 AM   #21
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The number one rule for scavenging heat from flue gases is "prior complete combustion". This means burning hot with excess air. Creosote is the result of poor combustion. Heat-Booster systems now have an integrated catalytic combustor for "polishing" flue gases prior to entering heat exchanger tubes. This reduces frequency of cleaning. Any old woodstove can be operated with 85% thermal efficiency.
Water heating with woodstoves can be very dangerous if not done right. The water heating systems developed by Heat-Booster are inherently safe as they do not expose water to excessive temperatures, even during power failures. Most of the heat from a woodstove can now be transfered to water in a safe way. Patent application will be submitted in the very near future. Follow developments at Heat Recovery Systems - Solar & Wood

Another development is our low cost solar heat collector. Use as roofs, walls and fences. No liquids in collectors. Heat space or water. A version of the collector will combine solar heat with solar PV in one panel. Panels can be installed as walls, roofs and fences. These panels will be real game changers as installed cost is greatly reduced, providing a reasonable pay-back period without subsidies with $OPM.

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Old 01-17-12, 09:02 AM   #22
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Back in my days as a foundry worker, I operated a natural gas fluidized-bed calciner furnace which ran at a constant 1500F. It was equipped with cross-flow exhaust heat recuperators that heated the incoming combustion air from the exhaust being drawn out of the furnace. They were basically stainless steel boxes with tubes for the exhaust to flow through. The combustion air was introduced to the bottom half of the side of the outer box, where it was forced across the bottom half of the tubes by a divider, then up and back across the upper half of the tubes, where it exited to the insulated hi-temp combustion air pipes. Once the recuperator temp reached about 500F it had sort of a turbocharging effect on the furnace temp, making it rise very quickly from 1000F to 1500F. The combustion air exiting the recuperators was usually 450F to 700F during normal operation. The combustion air was responsible for not only providing oxygen for the natural gas to burn, but also fluidizing (bubbling) the sand inside the furnace.

I'm sure some of you can see by now how this could be implemented on a wood stove. My idea is to use an air-to-air HX to preheat the combustion air from the flue gases. The most practical method is to put metal studs in a section of 6" stovepipe, surrounding that with a section of 8" duct. A divider should be run down the pipe on both sides so that fresh intake air from outside comes in one end, runs down the length of the HX, then moves across and up the other side of the pipe to the outlet on the same end as the inlet. The flue pipe up to the HX, the HX itself, and the heated combustion air should all be insulated. I would simply remove the ash pan on the bottom of the stove and make an insert in the end of the combustion air duct to fit inside the opening, able to be moved out of the way for removal of ash. Also, a damper would be located in the combustion air duct somewhere close to the stove to control firing.

The goal is to get the combustion air entering the stove to 400F, which should be sufficient to activate pyrolysis of the fuel on contact, creating the "turbocharging" effect that I'm looking for.

I like the aforementioned expansion chamber idea too. Maybe that could be a use for the extra 55 gal. drum that's been sitting in the garage . The HX could be mounted between the stove and expansion chamber. I'm thinking that after that the exhaust should be pretty well cooled down. Any creosote buildup due to low temps should happen in the expansion chamber and not in the flue beyond that point, hopefully.
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Old 01-22-12, 06:22 AM   #23
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EPA type wood stoves introduce pre-heated secondary air to facilitate combustion of combustible gases and particulate matter that otherwise would become smoke and "creosote". Heat is taken from firebox, not flue gases. High temperatures and excess oxygen are required for complete combustion. It is probably easier to use the EPA method for wood stoves.
Myself, I would like to use some forced air to create some turbulence in the firebox. This air could be pre-heated using flue gases.
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Old 01-22-12, 08:15 AM   #24
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I have been toying with the idea of adding secondary air to this woodstove, it has no damper in the firebox door like most do. There seems to be more than enough area from the open bottom grate to provide both primary and secondary air for the fire. What about putting the expansion barrel right behind the stove so that the firebox exhausts directly into the expansion barrel? The primary air pipe could run right through the barrel, preheating it directly from the exhaust gases. A secondary air pipe could be run through the barrel to discharge directly into the exhaust from the stove. That way, if the exhaust is hot enough, secondary burn could take place inside the expansion barrel. Instead of damping down the flue, secondary air could just be opened up more when the flue temperature heats up. Hotter flue temp = more secondary air = hotter primary air. Primary air could be used solely to control the primary burn in the stove.
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Old 02-25-12, 07:49 PM   #25
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Are there any ways to improve efficiency of zero-clearance fireplaces? I have one in the house I bought and would rather improve its efficiency than replace with a wood stove. It has a fan for the heat box and I have connected it to outside air, including an inline fan. I have also made the glass doors more airtight by applying high temp caulk to the folding edges of the glass so they seal when closed, and added door gasket material to the outer edges of the door. I still don't always get good combustion and would like to improve it.
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Old 02-27-12, 06:36 AM   #26
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Quote:
Originally Posted by Plantman View Post
Are there any ways to improve efficiency of zero-clearance fireplaces? I have one in the house I bought and would rather improve its efficiency than replace with a wood stove. It has a fan for the heat box and I have connected it to outside air, including an inline fan. I have also made the glass doors more airtight by applying high temp caulk to the folding edges of the glass so they seal when closed, and added door gasket material to the outer edges of the door. I still don't always get good combustion and would like to improve it.
What is the problem with the combustion? Are all the flames going up the flue, or is there a lot of smoke coming from the chimney?

We need specifics!
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Old 02-27-12, 08:01 AM   #27
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Mostly smoke going up the chimney
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Old 02-27-12, 11:46 AM   #28
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I picked up a newer EPA Certified wood stove with the secondary burn for cheap. I also got one for free that was made in 1985 and plan on converting it to a secondary burn stove. It looks very straight forward to do after you look at one that has been done already.

I have some pictures of the secondary burn tubes, but I am unable to resize the pictures without distorting the images.

At the back of my stove and on the bottom, there is a hole on each corner where it goes into a upside down "L" shaped tube inside the stove. That supplies the fresh and highly heated air to the secondary burn chamber on the top of the stove. Thats where you see the tubes with holes in them. There is a really good Youtube video explaining it.

On the front of the stove there is a fresh air intake and it controls the fresh air that goes in and down across the glass. You can see that in the picture I uploaded below.
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Old 02-27-12, 03:04 PM   #29
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Quote:
Originally Posted by Plantman View Post
Mostly smoke going up the chimney
That's usually a sign that the fire is burning too hot with not enough air, OR that too much air is being fed to a cool fire. You said that it had a combustion air fan, correct? Is there a damper or speed control so that you can control the amount of fresh air that is let in? If so, try playing around with it, making small adjustments and waiting a few minutes for the fire to adjust, to see if you can get it burning better. If the fire is burning hot with too little air, you'll see slow rolling reddish flames, and some black smoke coming from the chimney that smells like carbon/soot (an acquired smell). That's your cue to hold off on adding any wood for a while and give it more air. If there are short yellow flames in only parts of the wood, that is usually a sign that air is being forced into the stove faster than the fire can maintain the flames, basically blowing it out. You'll see a lot of grey smoke in this case, which should smell like whatever kind of wood (or fuel) you put in, and creosote (also an acquired smell) which is what you DON'T want in your flue. Back off on the air and maybe add more wood if there isn't a whole lot in it.

Is there a flue pipe on it or is the fireplace built right into the chimney? One thing that I feel is absolutely necessary for a wood stove/fireplace is a flue thermometer. I used one from an old Coleman grill/smoker on mine. It doesn't give any temperatures except warm, ideal, and hot; but after using it over a few burns I have a good idea of about where the needle should be. It is also a good indication of whether the fire is getting hotter or colder. I do plan to get a real one sometime though (with actual numbers on it ). They are about $15.

Also, most wood stoves use draft fans instead of combustion fans. They have a better tendency to create a strong draft instead of forcing cold air into a fire the may not be hot enough to create a strong enough draft to handle all that air. Adding one if possible might definitely help out if you have draft issues.

I have come to realize that firing a stove is more of an art than anything, kind of an old-world technique that not many take the time or have the opportunity to learn these days.

Hopefully this helps. Let us know what you find out, more details (flame color, size, how much fuel, what kind of fuel, damper settings, etc.) would definitely help us help you out as well.
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Old 02-27-12, 03:11 PM   #30
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Quote:
Originally Posted by Geo NR Gee View Post
There is a really good Youtube video explaining it.
Excellent video! I have been looking for something like this because I also plan on modifying my woodstove for secondary combustion. So it's basically perforated air tubes that run up the back and along the top of the stove? Sounds simply enough.

Keep working on those pictures, we're very curious to see how these things work!

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