Competing Air-Tight Fireplace InsertsAll the competing Fireplace Inserts on the market work on the so-called Air-Tight principle of operation. In the late 1970s and early 1980s, many were built and sold by hundreds of manufacturers. People liked them!
But there was a problem. We consider it a basic design flaw in ALL wood-burners which use the Air-Tight principle of operation. The problem is that, in order to control the heat output of the fire (to even it out and make it last longer), the air supply to the fire is restricted.
The effect of this was to cause incomplete combustion to occur. Imagine trying to run a Marathon race with a cork up one of your nostrils or with a handkerchief over your nose. Your body would NOT have enough oxygen to work efficiently and effectively.
This is similar to the situation in an Air-Tight wood-burner when the draft control is closed way back. SOME of the chemical reactions of burning could still occur (the various oxidation reactions) but many would NOT occur because there was not enough oxygen available to react with.
Under some operating conditions, this has the effect of sending a LOT of unburned fuel up in the smoke, as partially burned chemicals (as hot gases) collectively called CREOSOTE. Under some (Air-Tight) conditions, fully 1/3 of the energy available from a wood fire can be carried away into the chimney as creosote. This is clearly very wasteful as regarding COMBUSTION EFFICIENCY.
It gets worse. This creosote (actually an assortment of about 160 different complex hydrocarbon chemicals) will condense into solid form when cooled below about 350°F. This often happened inside the chimney as the smoke cooled on its way up and out of the house. This caused the creosote to condense and solidify on the walls of the chimney. Often very rapidly. In 1981, we were at a trade show, talking to a representative of a factory-built chimney manufacturer. He described some experiments they did with a then-popular (but now out-of-business) Air-Tight woodburner, where the entire 8" chimney sealed itself SHUT with creosote in THREE WEEKS of use! WOW!
OK! So why talk about all this stuff here? Well, Air-Tight Fireplace Inserts became particularly notorious for creating such problems. It turns out that the smallish one-size-fits-all body of Air-Tight Fireplace Inserts allows a LOT of empty space BEHIND it in the original firebox of the fireplace. And it turns out that a LOT of the creosote that was condensing on the walls of the chimney walls was dripping back down into that space. And it turns out that there was no easy way to even know that this large stock of dangerous fuel existed back there. So when it eventually caught fire, the resultant chimney fire amounted to the "Mother of all chimney fires." Many houses (and some lives) were lost. Insurance companies, and eventually the government got involved.
ALL such Air-Tight Fireplace Inserts are now REQUIRED to have a provision for a DIRECT-CONNECT kit (which usually costs about $200 more and is a real "pain" to install. This kit is designed to carry ALL of the smoke leaving the Air-Tight Fireplace Insert, through a flexible hose, up well into the chimney itself, and it has a metal shelf closure which seals the chimney from the firebox. This whole set-up was mandated for Air-Tight Fireplace Inserts exclusively to eliminate the problem described above, about the accumulation of large amounts of creosote behind a small insert body in a fireplace. (Many people don't know that this is the reason for the existence of direct-connect kits.)
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