As the dryer overheats, mechanical failures can cause sparks, which can lead to lint trapped in the dryer vent catching fire. This is often due to the exhaust pipe being too close to the floor surface. Any defect that restricts the flow of air through the exhaust ventilation system of the clothes dryer will increase the time it takes for the clothes or other contents of the dryer to dry properly. To ensure your clothes dryer continues to operate safely and efficiently, it is important to perform an annual cleaning of the dryer vent.
Installing a ventilation control right at the outlet of the dryer may help avoid backward air outlet, but it won't solve the problem of overloading the shared ventilation system and the backpressure that the dryer will face. The owners had been running the dryer only at a low temperature for several years after observing that the clothes were always very hot at the end of the cycle. It is unlikely that installation instructions for your clothes dryer provide for sharing a duct system, and if you contact the dryer manufacturer for advice, they may be hesitant to give you advice due to lack of data. When installing a new dryer vent system, it is essential to consider duct lengths and calculate the total effective length of the dryer's exhaust vent to ensure that size and flow of exhaust ventilation are adequate and safe.
Proper installation of clothes dryer exhaust grilles, layout, materials, and lint protection are critical in preventing building fires, clothing overdrying time, and dryer operating costs, humidity, damage to buildings, decay, putrefaction, or insect attack on the structure, saturation of building insulation, and mold contamination in buildings. These devices use a movable plastic cylinder that lifts to allow exhaust gases from the dryer to be vented when it is operating. After having installed, cleaned, repaired and replaced exhaust systems, ducts and terminations of dryers since the 1970s, it is my opinion based on experience that there is no difference between the propensity of dryer lint to adhere to ventilation grilles or metal fumes; what does make a difference are variations in humidity, air velocity and actual geometry of ventilation openings. It is important to note that plastic grilles and metal mesh on ventilation openings are prohibited.
For more information on dryer vents that resist lint clogging and about using (or recommendations not to use) supplemental lint filters in dryer ventilation systems and ducts, see Frequently Asked Questions on Clothes Dryer Ventilation.