First: The distribution of oxygen to the hottest spot is better in a “top down” start-up. Complete combustion requires adequate oxygen, and when the fire is top down, adequate oxygen is always available. In a bottom up fire, there is likely to be less O2 at the hottest point, and there is a chance the fire can be suffocated as wood is loaded on the top of the fire.
Second: Less heat is lost in a “top down” start-up. Clean combustion requires high heat. In a top down fire, as the bigger pieces at the bottom are heated up, the gasses they give off are consumed by the small hot fire at the top. This is good combustion design! In a bottom up fire, the gasses from the bottom pieces are cooled by coming in contact with the wood above them.
Third: No need to add wood or re-stoke the fire in a “top down” start-up. Not only does this mean no work for the operator, it also means that the wood at the bottom is heated more consistently, the air-to-fuel ratio is not subjected to sudden changes, and exhaust gasses are heated, rather than cooled. In a bottom up fire, when you add wood, the airflow is changed, the fire is cooled off (by opening the door and adding unheated wood), and more unburned gasses are created.
Fourth: Having hot coals at the top in a “top down” start-up is a great thing! The radiant energy from those hot coals heats the wood beneath them. Gasses from the wood beneath them are burned as they pass through the hot coals. In a bottom up fire, the hot coals are at the bottom of the fuel load, so they can’t contribute to burning gasses from the wood above them.
|Four Stages of a Top Down Fire|
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Why Woodstock Soapstone Stoves are totally clean burning almost immediately with “top down” start-up fires.
Remember that when you build a “top down” start-up fire, you are maintaining the heat of the fire to the maximum extent. You are not quenching the fire by opening the door, adding wood, or limiting the supply of oxygen. There are two big reasons why we are able to clean up emissions almost immediately after light-off.
1. Our catalytic combustors are made of a very thin, high tech stainless steel material called Durafoil®. The Durafoil is just 50 microns thick; about the same thickness as a human hair. This ultra-thin material heats up very quickly, enables the catalyst to react with the exhaust gasses, and ignite very quickly. We have been able to get our catalysts to ignite easily in five minutes or less using a “top down” fire and dry kindling.
2. Our hybrid stoves supply secondary air through a flat, perforated, stainless steel fireback, and are able to achieve secondary combustion within minutes of lighting a “top down” start-up fire. A “top down” start-up fire creates the maximum amount of start-up heat. The kindling flames high in the firebox are more than adequate to initiate secondary combustion.
To sum up our story, if you have a Spring Chill in your home, cool at night but warm enough during the day, try using the "top down" start-up method to start your fire. You should find your stove will heat up faster to ward off that chill, and your fire will burn longer and cleaner. And, you can astound your friends and family with your pyrotechnic expertise.