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Fire Hazard Alert - Is Your Dryer Ready to Ignite

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.

Since the air slows down the moisture accumulates in the venting rather than being carried outside. This causes the venting interior to become wet and lint traveling through the pipe will cling to this wetness.

This starts a vicious cycle within the venting pipe. It goes something like this: The more lint in the venting, the more blockage; More blockage means slower air flow; Slower air flow means more moisture in the venting; More moisture in the venting means more lint.

I think you understand the scenario now.

Taken to extremes the lint can block the venting closed. When this happens it can cause the dryer to overheat. The normal drum temperature of 175 Fahrenheit can quickly shoot up to 300 Fahrenheit or higher. It may even get hot enough to allow lint in the venting to ignite. If a fire of this type starts within flexible plastic venting it can quickly burn through the venting and allow the fire to spread.

Therefore, remove any flexible plastic venting and replace with rigid, straight sections. If the total length is less than fifteen feet, flexible "metal venting" is acceptable.

Calculating true venting length

So lets look at how we can determine if a venting problem is in our future.

Manufacturers generally suggest a venting length of 15 feet (and two elbows) to be the maximum. But the true venting length can be deceptive.

"So how do I know if my venting is too long"?

The true length of your venting is determined as follows:

1. Measure all the straight lengths and add them together

2. Count all the turns or elbows and multiply this number by 4

3. Add up the totals

Example Since an elbow or turn is equivalent to an additional 4 feet of pipe 20 feet of venting with 4 turns would actually be:

20 feet + 4X4 feet = 36 feet

Don't be surprised by the true equivalent length of your venting. In modern homes it can be substantially longer than the manufacturers suggested maximum.

If the blockage becomes critical the dryer will stop doing its job properly. As a homeowner watch for the following signs that the venting may be starting to block.

Clothes coming out wet
Excess lint left on clothes at cycle end
Inside of dryer feels wet
Taking too long to dry a load
Clothes very hot at end of cycle
Electrical consumption greatly increased

Summing up

"So who is to blame for this problem"?

I say there is no culprit in this scenario. If you want to blame anything, blame our fast paced lifestyle. Gone are the times when laundry day was a full days work. We all want instant gratification and instantaneous results - even with our laundry chores.

So as a consumer what can you do to alleviate this problem? Well you certainly can't move the laundry room. The best thing you can do is to be aware that the problem exists. Consider taking down the venting and cleaning out the lint buildup during your annual spring cleaning. Also, regularly walk outside and check the vent cap where it exits your house. Remove any lint buildup and make sure the flap moves freely. If you see a lot of activity from birds in you backyard check it immediately. They love to build nests inside the vent cap.

Other than the previous suggestions a little common sense might avert a tragedy. Do not operate your dryer while asleep, out of the house, or next door at the neighbours. And always remember the old saying that states "better safe than sorry".

About the Author
Copyright 2004 by Donald Grummett - All right reserved.
Donald Grummett is an appliance service manager in Ottawa, Canada. In the trade over 30 years as both a technician, business owner, and technical trainer. For more information about appliances including FAQ, Stain guide, Recycling, and Newsletter visit

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