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.

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! :)

Mostly smoke going up the chimney

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. Secondary Combustion Explained.wmv - YouTube

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.

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 :thumbup:). 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.

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! :thumbup:

One thing I want in that wood stove would be an outdoor air intake piped into it. That way I'm not pulling my warm indoor air out of the house by going inside the stove and out that stack.

My thought on that is that is how leaky most houses are already. I recently pulled out the sliding window to clean it. There is a track at the bottom that has vents in it to the outside. Its not adjustable either. The vinyl windows are not an airtight unit as one might think.

Fresh air also gets pulled in when you turn on the bathroom exhaust fan, clothes dryer, stove fan, etc. They even installed a fresh air fan on the house furnace with a timer coming on every hour!

When is the house too airtight? When the stove is smoking out the front? One of my first houses I built it was so tight that you had to open a window to start the fire and keep it going. I always felt tired in that house with or without the fire. Lack of fresh air to the brain..........:confused:

You have a good point. I was reading this morning about stove pipe that is triple wall with fresh air coming down one of the sections and pipes into the stove. I remember seeing a mobile home with fresh air coming from under the house through the floor and into the stove......

Here are a couple of photos of the latest wood stove secondary burn modification. The first picture is the overhang that needed to be cut to make room for the secondary burn fresh air supply.


My other wood stove, which is EPA Certified, does not have a control for the fresh air going in the upside down "L". I was thinking of adding some kind of control to adjust the fresh air, but will wait until after I fire it up to see how well it works first.