Dryer vents

Problems with clothes dryer exhaust vents are very common defects found during a home inspection.  I see dryer vent problems of some type on about 75% of the inspections I do.

And this isn’t just a theoretical problem.  Dryer fires are very common.  The National Fire Protection Association says that there are an average of 4,550 clothes dryer fires each year due just to a failure to clean and perform other routine maintenance on a dryer, and there are many more dryer fires from other causes.  These fires result in at least several deaths every year, hundreds of injuries, and around $240 million in property damage.  This is a very real problem.

As with most problems, just following the rules will go a very long way towards alleviating much of the risk.

The rules I’ll highlight in this blog post come from the International Residential Code (IRC), which is enforced in most of the United States.  And literally the very first thing that the IRC has to say about venting a dryer is this: “Clothes dryers shall be exhausted in accordance with the manufacturer’s instructions.”  So if the dryer manufacturer requires it then you have to do it.  No arguments, no complaints, no excuses.

You’re allowed a single piece of flexible “transition” vent for your dryer.  This flexible piece allows you to make the connections and then slide the dryer into its final position.  This transition vent needs to be listed and labeled in accordance with UL 2158A – the Underwriter’s Laboratory specification for this type of flexible dryer vent.

A very common problem is that the flexible transition duct is the wrong type and doesn’t meet the UL 2158A specification.  Typical foil-type dryer vents aren’t allowed by code because they aren’t UL 2158A approved.  And every  dryer manufacturer’s instructions I’ve ever seen (which you have to follow) says don’t use them.  These foil dryer vents clog easily, and that restricts air flow and causes heat to build up.  And that’s a fire hazard.  These vents can be crushed easily, again restricting air flow.  They also can be punctured easily which can allow combustion gases from natural gas dryers to leak into the house.  And any kind of white plastic flexible vent is a complete disaster.  That material is flammable!  You should replace it before using the dryer again.

Thin foil vent punctures easily. This is unsafe — fumes from combustion are leaking into the house.

The kind you want usually looks like this below.  It’s fairly lightweight but it’s still self-supporting — you can hold it at one end and it will remain rigid.  It has a kind of corrugated appearance.  It bends and extends very easily.  But most important: it’s listed as UL 2158A.  That’s what you need to look for.

Here’s a kit with several additional pieces.  We know this is good because the package says it’s UL 2158A listed.

UL 2158A listed. This is a good product to use.

Here are some problematic products you might find at the home center.  Stay away from these.

This kit shows a foil vent, which isn’t allowed. The package says it’s “UL listed” but it doesn’t say which UL specification it’s approved for. It must say UL 2158A or you shouldn’t use it. Also there shouldn’t be a screen. More on that below.

Also, this flexible transition vent can’t be any longer than 8 feet.  Anything conforming to UL 2158A will follow this rule, but some flexible vents are much longer than this, as long as 25 feet or more.  These are usually not intended for venting clothes dryers – many of them are for things like venting a bathroom exhaust fan.  So any flexible vent longer than 8 feet is automatically no good for a clothes dryer.

This package says it’s dryer vent, but it’s 20 feet long. That’s way too long. This product can’t possibly be approved. Don’t use this.

You’re allowed only one piece of flexible transition vent.  I sometimes see two or more flexible pieces connected together.  This isn’t allowed.  And none of this flexible transition piece can be concealed behind any type of wall or ceiling, including a dropped ceiling.  The flexible transition piece needs to be exposed and accessible.

Often times this single transition piece is enough to connect to the terminal on the outside of the house.  If not, then the rest of the dryer vent – all of it – needs to be metal, at least No. 28 gage, with a smooth interior surface.  The vent needs to be 4 inches diameter.  Multiple pieces of vent need to connect together so that one piece funnels into the next piece in the direction of airflow.

Just to be clear, you’re allowed one piece of flexible transition vent and then everything after that has to be rigid metal.  You can’t use two flexible pieces.  You can’t use one flexible piece and then some rigid metal and then another flexible piece.

And here’s another common problem: There shouldn’t be any screws at the joints of a dryer vent.  The IRC specifies that screws can’t protrude any more than 1/8 inch into the vent, but in practice that’s mostly impossible to control and after installation there’s no way to confirm how far the screw protrudes.  So the reality is that no screws should be used.

But you must secure the joints.  The typical way to secure the joints is to use tape that conforms to UL 181B.  This type of tape has a shiny foil face and you have to peel off the protective backing after you unroll the tape from its spool.

And there is a maximum allowable length for the dryer vent.  Absent any manufacturer’s instructions (but hey, there are always manufacturer’s instructions) the IRC allows up to 35 feet of vent minus a bit for each bend.  As an example, a 90 degree elbow can reduce the allowed overall length by 5 feet depending on the bend radius.

Once you get the vent outside the termination must not have any kind of screen on it.  A screen will catch lint and restrict the flow of warm air.  Once again, this causes heat to build up and that’s a fire hazard.  The terminal should just have a backdraft damper that easily opens when the dryer is running and pushing air out of the house, but then closes when the dryer is off to keep out cold air as well as pests.  I see a lot of backdraft dampers that are quite old and are stuck because of lint build-up.  When this happens the whole terminal should be replaced.

After installing your dryer vent according to the rules you should clean it occasionally – around once a year or so.  Lint is extremely flammable, and so the best way to prevent a dryer fire is to eliminate the lint.  You can disconnect the flexible transition vent and insert any of several types of tools to scrub and clean the inside of the dryer vent.  This should be fairly easy, because the vent is smooth metal inside because you installed it according to the rules.  Once again we see that following the rules makes everything safer and it makes most things easier.

Here’s a quick summary of these rules for venting a dryer:

• venting must be done according to the dryer manufacturer’s instructions
• use a single piece of flexible vent, no more than 8 feet long, conforming to UL 2158A
• the flexible vent must not be concealed
• rigid metal vent pipe after that
• no screws at the joints
• no screen on the terminal outside

There are other rules for dryer venting that I haven’t mentioned here, but these are the rules that I see broken most often.  Please be safe.