A dryer vent exhausts hot, moist, linty air (and resultant gasses of combustion if it is a natural gas dryer) from the dryer to the outside of the house. Although I've seen dryers vent into crawl spaces, attics, joist cavities, or just straight into their own laundry room, none of these scenarios are permissible. The dryer must vent to the exterior.
To create the dryer vent, a route must be established from the dryer to either the roof or exterior wall that is within the overall length guidelines and also is able to be efficiently routed. This dryer vent system is comprised of several items:
I hope these basic help you understand your house's dryer vent better.
Here's a quick post regarding the three most common items needing replacement in a dryer vent system.
1) Transition Duct -- this is the duct that joins your dryer duct to the main duct. This should be made of heavy metal, but often it is not. There is a sticker on the back of most dryers that clearly specifies this.
2) Termination Hood -- this is where the exhaust system meets the exterior. The termination hood can be through the wall or roof (either is ok!) and there needs to be a damper integrated into it (whether a single damper or louvers). Plastic termination hoods are not recommended because most crack and break apart after a few years.
3) Main Duct -- sometimes a builder has installed an improper dryer vent and it needs to be replaced. "Slinky foil", vinyl, and semi-rigid are all insufficient materials to act as the main duct. In this case, the main duct will need to be replaced with heavy-gauge rigid material.
Each of these can vary much in replacement difficulty and complexity. A handy homeowner might tackle easier scenarios, but for more challenging situations a professional installation may be required.
Today, as I serviced one customer's house, the homeowner and I were discussing the importance of dryer vent cleaning. In our conversation I think she summarized it well: the main impetus for dryer vent cleaning is to mitigate the fire hazard, and shorter drying times and less energy usage are merely extra benefits.
You want to be cleaning the vent before excessive accumulation occurs, otherwise you begin flirting with the fire hazard. This means that if you follow the industry recommendations for intervals between cleaning (typically 1 year) you probably won't experience a dramatic change in drying times or energy usage (unless birds get in your vent!), but you will be following 'best practice' in order to mitigate the fire hazard.
It is always satisfying when I leave a home knowing that the owner(s) will experience substantially shorter drying times. It is impressive removing large amounts of lint or multiple feet of a bird nest because I know that there will be a drastic difference in airflow before and after cleanings. However, truth be told, I am much more satisfied leaving a home that has been maintained with regular cleanings and has removed marginal accumulation because I know that the family, house, and appliance are being safeguarded.
There is a reason that cleanings are recommended annually, and not every 5 or 10 years.