Monday, January 12, 2015

Air-Jet Re-burn Drafts

After my last post about Marjorie singing, a reader used the comments section to ask me what Margin Stoves' "Air-Jet Re-burn" drafts are, so I wanted to take a quick minute to explain them.  From what I have read, studies that were conducted in order to find out how to improve the efficiency of woodburning appliances proved that the highest efficiency was achieved when hot air entered the firebox from above the fire.  In most of the new-style heating stoves that I have seen, the hot air enters through rows of small holes along the ceiling of the firebox.  Our Jotul heating stove is designed this way.  The reason that this design results in high efficiency is because the hot air coming in the "jets" often ignites unburned gases that are the result of combustion (hence the "re-burn" part of the name).  In fact, in our Jotul, you can watch the flames shooting out of the little air jets as this happens. 

Margin stoves are similarly equipped, but because the roof of the firebox is also your cooking surface, the entry point of the hot air is a little different.  First, room air enters the stove from the bell draft on the firebox side of the stove.  It begins to be heated right away.

The "Air-Jet Re-burn" draft is the silver bell draft
between the louvers on the firebox side of the stove.

The air then travels down the side of the stove into the area where the ash drawer is, entering through two holes in the sidewall.

The two holes where the air enters the area where the ash drawer
 sits beneath the firebox.

This picture shows what it looks like when the ash drawer is in place.
Now, in many cookstoves the air would be able to travel up through the grate to the bottom of the fire.  If you have completely cleaned the ashes out of the firebox, this will happen when you initially start a fire in the Margin Gem, too.  However, once the fire has been going for awhile, a sufficient layer of coals and ashes will prevent much air coming up through the bottom of the fire.  Instead, air travels into the firebox through the air-jets in the corners and in the middle of the side of the firebox which is against the oven.  These holes are evident in the picture below.

A view of the Margin Gem's firebox from above.
Note the air-jets in the corners and on the middle
Frequently, when I lift one of the lids to stir, stoke, or view the fire, I can see rivulets of flame coming out of the air-jets as the hot air entering the firebox from them ignites combustion gases before they exit the firebox.  Unfortunately, I haven't figured out a way to photograph this because they don't last very long since opening the lid spoils the draft of the stove.

At any rate, I hope this answers and clarifies what "air-jet re-burn" drafts are and how they function.


  1. Hi Jim. I am really enjoying catching up to all you have posted here.
    I am looking at buying my first stove. A Findlay Oval, early 1920's I think. The colour is Jade green with a tileback. Includes original matching cookware.
    My question is what the heck the dial is in the middle of the stove back below the warming oven. The stove pipe is behind the stove. It does not go through the warming oven. The dial would open 3 small ports, looks like a nuclear trefoil. It is not a flu lever. I have seen them on other ovens as well.

    1. Hey, Geoff!
      If the stovepipe would travel up the back side of the backsplash, it sounds to me like the dial you are talking about would have opened into the stovepipe. Drafts in this location were common in both heating and cooking stoves years ago, and I think they were most effective for hard coal fires. The idea was that once you had a well-established fire, you could reduce the oxygen pulled through the fire by letting room air enter the flue at this spot. Basically, it was designed to spoil the chimney draft a little in order to slow your fire.

      If it doesn't appear that it could be connected to the stovepipe, you've got me stumped, but I'd just about put money on the above being its function.

      Your stove sounds like a beauty! Be sure to get in touch with me so that I can do a "Readers' Cookstove" post about it when you get it up and running.