In recent years there has been many stories about dryers catching on fire. Should we be concerned? Yes of course. We should take seriously anything that may put our family at risk.
Was the problem the dryer? Rarely. After investigating it is usually determined to have been the venting within the Home catching on fire, and not the dryer.
Obviously appliance manufacturers are concerned about the possibility of any dryer related fires. They have made it a policy to advise both service companies and consumers that the use of plastic venting is prohibited. They have also begun issuing cautions not to exceed suggested maximums for venting length. Let me try to explain the details of this problem.
The drying process
When clothes are being dried inside your family dryer there are two processes happening. Firstly, heat is applied to the air inside the dryer drum as it turns. This raises its internal temperature to approximately 175 Fahrenheit causing moisture to be driven out of the clothes by evaporation. Secondly, large amounts of air is passed through the clothes. Surprisingly, the real trick to efficiently dry clothes is not the heat, but rather this vast volume of air.
Ever wonder why the clothes on the clothesline dry so fast on a windy day? The hero is the wind. Well, the same process takes place inside your family dryer.
To make them dry faster air is constantly blown through the clothes during the drying cycle. The tumbling action of the drum further exposes the clothing to the hot air flow. While they tumble the air picks up moisture from the clothes, carries it down the venting, and dumps it outside the home. Most people think the venting is to push the lint outside. Actually, its primary purpose is to dump the moisture outside the Home.
It is a process that works efficiently. That is, as long as nothing is allowed to interfere with it. Impede, slow down, or stop the airflow and the process quickly fails.
In the past Homeowners who wanted to vent their dryers did it using rigid sections of venting. The sections were secured together (using screws or duct tape), and elbows were added if necessary, to connect the dryer and venting to the wall outlet. Although time consuming to install, straight venting sections were durable and would often outlive the dryer. This was in the era when laundry equipment always sat in the basement, against an outside wall.
Then along came flexible plastic venting. It made installations easier. It turned an hour installation into a ten minute job. The flex though tended to become brittle and break easily. Also it was prone to blockage and needed to be replaced every few years. But since plastic venting was more convenient we continued with its use.
Then came a change in lifestyle. As both parents went off to work the household dryer was moved to accommodate our faster paced lifestyle. To save us time it was moved from the basement to a ground floor laundry room. Although moved to the working level of the Home, it was still close to an outside wall.
So you are saying, "I know all this, but what does it have to do with venting fires".
I answer, "Have patience, we are almost there".
Taking this desire for easy access still further the dryer was moved again.
The laundry room is now often located near the centre of the home, close to the family room or kitchen. If located upstairs it is often centrally located between the bedrooms, allowing faster access to where most dirty laundry is produced. Easier for the homeowner that is, but no longer near an outside wall. The distance from the dryer to an outside wall of the Home is now substantially farther than it used to be.
Presto, we have come to the crux of our problem. The venting is too darned long.
Physics and the venting pipe
It is a lot more difficult to push air down a long venting pipe than a short one. This is because air inside the pipe has weight and volume. Obviously, the air inside a longer pipe would weigh more than a shorter one.
After about twenty feet of venting pipe the dryer begins having difficulty pushing against all this weight. The average dryer motor does not have enough strength to overcome the weight of the air inside the pipe. The result is that the air in the pipe begins to slow down.
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