Occasionally, water can accumulate within your dryer vent system so that there is wet, sopping lint. Normally, the moist air from the dryer dampens the dryer vent ductwork, the moisture in the exhaust is largely brought to the exterior, and the dampness remaining in the duct evaporates away. When there is a significant blockage or restriction such that the dryer is unable to bring the moisture outside, the water accumulates within the duct and creates problems. First, it can be very difficult to clean the dryer vent (higher cost) and can also require an additional cleaning a week or two later (extra cost). Second, large amounts of condensed water can leak out of the duct and damage your house. Third, this same water can corrode the galvanized metal ductwork, especially if it doesn't have a way to leak out of the duct. This can require anything from a more isolated repair to a complete dryer vent replacement. This can all be avoided by simply staying up on regular dryer vent cleanings. All these problems become quite costly and can be mitigated by proper maintenance.
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.