Mad About Science

By Brenden Bobby
Reader Columnist

Wood stoves are common in North Idaho. Throwing combustibles into a metal bin and setting them on fire for heat seems like a straightforward idea, but there’s a surprising amount of science and optimization that goes into heating your home.

The premise of how a wood stove works is straightforward. You ignite flammable material inside a metal compartment, which then radiates heat as the metal cools over time. However, there are some hidden science facts tucked away in this simple design. 

Mmm… heat. Courtesy photo.

The basis of the wood stove is that warm air rises and cold air sinks. This is because of the properties of thermal expansion. Warm air expands and takes up more space, which makes it lighter than the cold air that is closer together. A wood stove heats your home by offering the warm air a place to go: up your stovepipe and into the upper levels of your home. This is why most two-level homes with a wood stove will have it somewhere near the center of the dwelling, so that it can heat the area equally.

This is less important in a single-level home, and popping the stove in the corner will achieve the desired goal, though some rooms in your house may be a bit chillier than others, depending on air flow through the abode.

The interior of your stove’s design is also important. A wood stove is calibrated to achieve a near-perfect equilibrium. If your stove is too large, it will require a much greater amount of fuel in order to heat your home. If it’s too small, you won’t be able to fit very much in the stove and the fire will burn slowly and it won’t put off enough heat to warm its surroundings.

A good fire needs three components in order to burn: fuel, air and heat. In a wood stove, your fuel is wood, obviously. While it seems logical to cram as much wood into a stove as you possibly can in order to start a fire, you’re depriving the fire of a crucial component: air. Ever wonder why wood shavings catch fire a lot faster than a log does? It’s all about the air.

Unless you’re blasting it with a high-powered laser, wood only burns on the surface. As the surface is burned away, lower layers of wood fibers are exposed and begin to burn. One large, solid log will burn slowly over a long period of time; but, if you break that log up into several pieces and create much more surface area, it will burn faster.

If you want a fire to burn overnight, put smaller pieces of wood in the middle with two larger logs on opposite sides of your pile. The middle will burn out quickly while the two exterior logs will burn slower, keeping enough heat in the morning to easily rekindle the flame.

Most stoves are lined with bricks along the bottom, with more bricks keeping the stove off your floor. The bricks act as insulation, keeping heat where it needs to go and minimizing heat leak. Dirt works similarly for campfires, though it’s recommended to avoid using river rock for insulating your fire, as moisture trapped inside the rocks becomes steam and escapes the stones with violent cracking.

You might be wondering how air gets into an enclosed space like a woodstove, and why smoke doesn’t just billow out and throttle your house. This occurs primarily through the damper, which is a mechanism that’s usually located at the top of the stove or on the stovepipe itself. When open, the damper allows air to flow in to fuel the fire. The trick is allowing enough airflow in to ignite the fire and keep it burning without causing a rapid burn and wasting all of your fuel. Generally, you’ll keep the damper open while you’re starting the fire to maximize airflow, and once the flame catches, you’ll want to only have it about one-quarter of the way open. Check the literature your stove came with, as this varies by model. Hopefully you didn’t use those instructions to kindle your first flame.

The color of your stove will also determine its ability to retain and radiate heat. The color black absorbs light and converts it to heat, while the color white reflects most light and reduces the amount of heat retained in an object. This is why black rubber water barrels are often placed in a greenhouse — they absorb light, convert it to heat and transfer that heat to the water, which keeps the ambient temperature in the greenhouse stable after the sun has gone down and the heat slowly radiates off while the water cools.

The most critical part of your stove is the stovepipe. Its length and placement is dictated by a range of factors, including which parts of your house you’re trying to heat, which direction the wind blows over your roof and the size of your house. 

As wind blows over your roof, it creates eddies, which can cause a disruptive flow into the chimney that pushes smoke back down into your home. Even worse, smoke and ash will cause carbonate buildup in your chimney over time as the hot air and smoke makes rapid contact with cool air that causes carbon material to cake onto surfaces and water to condense. This material is practically pure carbon and extremely flammable, so make sure you get some help to clean your chimney regularly.

A fun bonus for the firebugs: If you’re camping and you can’t find any suitable kindling to start a fire, corn chips make for a potent firestarter. Corn chips are essentially dehydrated hydrocarbons dipped in oil, and while it may smell awful while they burn, you’ll be able to start a fire with ease and even have a snack to fuel your belly while you wait.

Stay curious, 7B. 

This week’s article was suggested by Reader News Editor Lyndsie Kiebert-Carey. Thanks, Lyndsie!