Almost all homes in Mississippi are built on a concrete slab. That fact can present a problem if the plan designer has the laundry room drawn to sit on an interior wall.
It’s best to vent the clothes dryer directly to the outside of the building. In fact – the building code requires it to be terminated outside the building. For the few homes that are built on a crawl space, that doesn’t mean you can or should terminate the dryer under the house. The building code says that’s a no-no too.
In some cases the dryer vent may need to pass through an adjoining room on its way to the outside of the building.
When I inspect your home there are a few vent duct challenges I want to confirm. Let’s run through a few scenarios.
Your dryer vent runs through the slab.
Why should you care? It’s not a good design. Sometimes it may take multiple cycles to dry a load of laundry.
If your vent pipe is in the slab – I’m concerned.
Most in-slab dryer vent pipe is made of PVC plastic. The clothes won’t dry in a single cycle because the hot moist air blows through a cool plastic pipe buried under your slab. The result; condensation will form in the cool pipe. The pipe will collect and hold water. You will have to suck the water out of the pipe (regularly – depending on how often you dry your clothes) with a shop vac. That won’t be fun – but it will work in terms of keeping the water from collecting in the pipe.
NOTE: Not only is running the dryer vent in the ground a bad idea – PVC pipe isn’t a code approved dryer vent material. Download the 2018 Mechanical Code Sections here that addresses dryer vent installation. You’ll want to refer to it later in this article.
The designer has chosen the option to run the vent through the roof.
When you were a kid did you ever try and blow or talk through a long rubber garden hose? If so, you found that no matter how hard your blew on one end of the hose your lungs weren’t strong enough to move the air through the hose. The shorter the hose, the easier the air will move through the pipe.
Air has weight. It’s what keeps us alive on earth. The weight of the air close to the ground is heavier than the air above the clouds. The weight of the air exerts a pressure upon us. It’s called atmospheric pressure. The total weight of the atmosphere exerts about 14.7 pounds per square inch of pressure at sea level.
Air is hard to move through a pipe. Moving air through a crooked pipe is even more challenging because it’s affected by the pipe’s turns and twists.
The best advice: don’t install long runs of pipe and don’t try to push air through a pipe with with lots of turns and twists.
If the dryer vent pipe is run vertical into the attic and terminates through to the roof or through the siding, it may need an in-line fan to help move the air through the pipe. A booster fan, equipped with a pressure activated device (diaphragm), will help do the job. The diaphragm recognizes when your dryer is operating and will automatically start the fan operation.
The fan will need to be sized according to the pipe length and the number of elbows (turns) in the pipe. You may also need an inline lint trap (filter) to help protect the booster fan from getting clogged up with lint. Consider this option when the duct length between dryer and the booster fan is less than 15 feet. It’s best to install the lint trap where it can easily be accessed (and inspected).
Home inspections are, by default, visual inspections. If I can’t see it – I can’t inspect it.Gary Smith – SafeHome Inspections
Also – keep in mind – the 4 inch pipe will need to be protected from being damaged by drywall screws and nails when installed. Protective shield plates should be installed on the face of framing members. See the Mechanical Code (download above) for specifics.
Finally – a dampered roof mounted vent cap should be installed on your roof. The cap’s vent screen will need to be inspected and periodically cleaned. The screen will help prevent birds from nest building. The damper will help prevent wind blowing back into your home.
The vent terminates too close to the air conditioner.
I see this condition way way more than I’d like. Here’s a photo of what that looks like.
Because of the way the condensing unit operates, lint from the dryer can and often does affect the air conditioner’s operation. Actually, not keeping the condenser fins free of debris (including lint) can damage the system.
Inspect your outside condensing unit often during the summer. Keep grass cuttings, dust and airborne debris from clogging the fins.
For best operation keep the dryer vent termination about 8 feet or more from the air conditioning system.
The vent pipe is too long and has too many elbows.
The maximum length of exhaust duct shall be 35 feet. Where fittings are used, the maximum length of the duct shall be reduced in accordance with the following table. If you use a power ventilator, check with the dryer’s installation instructions to determine the maximum length of the duct.
Example: For each 90 degree fitting on the duct run, reduce the overall allowable length of the duct pipe by 5 feet. Download and read the Mechanical code reference above for other sizes and more information.
|Vent Fitting type||Equivalent Length|
|4 inch 45 degree fitting||2 foot – 6 inches|
|4 inch 90 degree fitting||5 feet|
The vent pipe terminates in the crawl space.
It won’t work. You’re asking for trouble. Stop thinking about it. Consider another alternative. DO NOT vent the dryer into the crawl space.
It will cause mold to grow on the framing lumber and it will coat the area with lint – which is considered a fire hazard.
When hanging the vent duct in the attic or in the crawl space:
- securely support the pipe every 4 feet
- the insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow
- the duct shall not be joined with screws that protrude more than 1/8 inch into the side of the duct.
- if and when the pipe is install in a wall cavity the pipe shall not be deformed
The dryer terminates in the garage.
You’ll find numerous homes in Metro Jackson, Mississippi where the municipality has allowed the builder to vent the dryer to the garage or carport. I don’t like this one any more than terminating it in the crawl space. The main concerns are lint and moisture.
Venting into the garage WILL CAUSE MOLD to grow on the drywall.
In our climate zone, in the garage, you probably already have some form of mildew growing. Now add moist dryer air and lint to the mix and you have the complete package.
One way to help control lint in the garage is to install an inline lint trap. It will need to be cleaned and maintained. The trap won’t help with moisture. Take care! During the cooler months of the year you could slip and fall in the dampness.
Join my 8,000+ member Facebook group for homeowners and inspectors.