Old Farmer Science - Hay Fires


Yesterday, my wife and I went to dinner with a couple that are our neighbors. During our dinner conversation, my neighbor's wife mentioned that she had been listening to radio and had heard a story about a local barn that was full of hay catching fire. She asked me if I knew anything about how hay can catch fire. That simple question really brought back some old memories of my father and grandfather – these memories always make me smile. So I started to tell her all about hay fires ...

Barn Fires

Back in the early 1960s, my father and his four young sons spent a lot of time in a 1956 Chevy Belair driving country roads to and from the old family farmstead. Grandpa was old and sick at this time, and my uncle Harvey had taken over the family farmstead. My father liked to help his brother out with the farm work on the weekends. On the drive to the farm, it was not uncommon for us to see the remains of someone's barn smoldering. Occasionally, we even saw the barn ablaze, which is probably why some people today opt for a metal barn instead of a wooden one. Some may look to places that sell these types of barns similar to Tassie Sheds who provide Quality Sheds and barns. One day I asked my father and grandfather what caused these fires. Their answer was interesting.

Spontaneous Combustion

In many ways, a barn's haymow is a very big compost pile. When you put together organic material , moisture, and a little heat, you will get bacterial fermentation. Fermentation produces heat that can cause spontaneous combustion and a barn fire under the proper conditions.


Dad and Grandpa were pretty proud of the fact that they had never had a barn fire. They told me that to avoid barn fires, you have to follow "the rules":

  • Do not put hay into the barn wet.
  • Occasionally drive a pipe into the haymow, attach a thermometer to a string, drop the thermometer on the string down the pipe, and measure the temperature. If the temperature exceeds 180°F, the hay has got to be removed from the barn.
  • Be careful when you remove the hay – the hot hay can burst into flame when it is exposed to the air.

All I can say is that their rules must have worked. They never had a barn fire.

Modern Information

When my wife and I returned from dinner, I thought I would look up some modern information on hay fires and see if anything had changed. It turned out that not much has changed. But there are some additional details that are worth mentioning.

  • As the temperature rises, the most common bacteria die off. However, in some cases there are heat-loving bacteria (known as thermophiles) present. If they are present, an unstable situation occurs. More heat cause more bacterial growth, which results in even more heat and bacterial growth.
  • The reaction rate doubles for every 10°C rise in temperature. For those of you chemistry fans, this is a great example of the Arrhenius equation in action.
  • The exponential growth rate of the reaction with temperature means that the reaction can very quickly get out of control.

The following critical temperatures are listed in a Wikipedia reference.

  • 150 °F (65 °C) is the beginning of the danger zone. After this point, check temperature daily.
  • 160 °F (70 °C) is dangerous. Measure temperature every four hours and inspect the stack.
  • At 175 °F (80 °C), call the fire department. Meanwhile, wet hay down and remove it from the barn or dismantle the stack away from buildings and other dry hay.
  • At 185 °F (85 °C) hot spots and pockets may be expected. Flames will likely develop when heating hay comes in contact with the air.
  • 212 °F (100 °C) is critical. Temperature rises rapidly above this point. Hay will almost certainly ignite.
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