Bonus question. When you do start a fire and it’s raging, why is it that sometimes the wood doesn’t even completely burn up?
In: Chemistry
Construction lumber is dry…really dry. Often also made of pine, which burns hot, easily, and fast. The wood in your walls is extremely dry and will catch on fire, and spread much more quickly than the wet/green wood you may be using in your fireplace. Take a small piece of 2×4, put it in your fireplace and you will see what I mean.
Fire need a lot of heat to spread properly. A small fireplace fire loses the necessary heat to the inverse square law after a couple of centimeters, but a house on fire causes enough heat to ignite other things with just a spark even meters away.
You don’t *expect* the house to burn so you don’t pay attention to it. Meaning your perception of time from ignition to noticing a big fire is different.
It’s like heating milk. When waiting for it it takes ages. If you walk away and are distracted you can clean up the mess.
Im no expert but I think this is selection bias. Sometimes that fireplace fire lights really quick. Sometimes it doesnt. Sometimes the house catches quick. Sometimes it doesnt. The one doesnt affect the other, and you dont hear about “small fire outside house, lightly singed three boards”
Multifuel stove owner here.
To get your fire started you wants dry tinder with lots of surface area. I used fur from our dogs (my wife collects it when she’s brushing them) and wood shaved from dry kindling. I few strikes using firesteel and a knife is enough to set it alight. It’s just a small flame like a candle at this stage. Slivers of kindling go on top with plenty of air gaps so as not to smother the flame. Increasingly larger sticks are added. I will feather the stick with a knife to make it easier to catch. I only add logs once the fire has established itself. Once the stove is really hot – it can take an hour or two because it’s heavy – logs burst into flames as soon as they’re added.
The stuff you put in a fireplace to burn generally is stuff that is going to burn rather slowly, houses on the other hand are made of, and full of, stuff that can burn very quickly, stuff like carpet, furniture, cloth of any kind, etc all burns quickly.
Another compounding factor is oxygen, the rate at which something burns is dependent on how much oxygen it can get. The fireplace is relatively small and confined, so oxygen has to flow into it, a house is more spread out so it’s already got lots of oxygen available.
To add to what others have said, fireplace logs are generally meant to catch and then slowly burn, not erupt in flame. Wood is also usually cut to a size and shape for a fireplace so that it will burn a certain way and not all the way down. A fireplace usually won’t go above a certain size. Things like curtains and rugs will catch fire and burn brightly as well as grow out of control, leaping to other objects. The greater heat will make even wood used in houses burn stronger and hotter.
Materials. If you have a house made of stone, it probably won’t burn and if you put dry straws in the fireplace, they will burn up very quickly. Most plastics burn very well and a typical house has a lot of plastics all over.
Another issue is when you start measuring. In case of fireplace, you know exactly when you light the match. In case of a house, the fire there has probably burned for a while before anyone has noticed anything.
If I were to take a guess, its probably because our houses usually are ventilated and have more oxygen supply than say a semi closed fireplace. Moreover thick and moist logs (due to humidity absorption) can be harder to burn due to increased ignition temperature.
Also, there are things in our houses which are easier to burn due to lower ignition temperature than wood, like cotton clothes.
The physics is complicated but basically fire likes to burn a whole lot better when you don’t want it to
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