Dryer manufacturers have gone through great strides in R & D in providing a safe and energy efficient dryer. Building codes on the other hand have made no effort in prohibiting the use of exhaust terminations that are flat out inefficient.
Even some of the new dryer vent hood designs do not take into consideration the resistance values of the airflow path. The objective of a vent hood for the dryer is basically two fold; prevent back drafts and reduce or eliminate any surfaces that can promote lint build-up. In addition to that, preventing bird or rodent entry as well as nest building capability is very important. Personally, for me, these bullet points sum up what I find critical in designing a vent hood specific to venting the dryer:
- be aluminum or galvanized metal to minimize UV degradation
- feature a small or low profile design to minimize unsightliness
- incorporate a clever light weight damper impervious to rodent penetration
- allow access and entry by a rotating brush and bendable rod for cleaning
- not provide a “bird house” like environment on top of or within the damper
- the least obstructive airflow path
- absolutely not incorporate a screen or mesh that would catch lint
- provide a minimum area of 12 inches squared through the entire termination and 15 inches square through any alteration in airflow direction greater than 30 degrees
- incorporate a integral 4” collar into the base of the vent for positively securing the rigid dryer conduit pipe to
- offer clever design to minimize any water infiltration
- provide zero back pressure
- Complies with IMC 504.4 & IRC 1502.3 building codes
- (location) not terminate less than 10 feet from any exterior mounted air conditioning equipment
- (location) not provide direct flow onto a patio, porch, window or door
In new construction, a roof vent hood (generally referred to as a roof jack) designed for the dryer looks almost identical to the one used to vent a bath fan. With that, the roofer commonly installs the dryer vent at the wrong penetration. Vents provided for the bath fan are smaller and feature a screen, a major no-no for dryer venting.
Vent hoods in new construction are generally provided by the HVAC contractor which they attain from the HVAC supply house. Since most of the duct materials are manufactured from galvanized sheet metal, the vent hood has ended up in most cases to also be constructed in galvanized sheet metal. Many of the manufacturers continue to provide legacy designs and contractors continue to install them because no one has questioned their efficiency.
To give you some idea, there are popular wall AND roof vent hoods that provide over .41 inches of water column pressure, approximately 3 times higher than a duct cleaning industry self-imposed rule of thumb.
The makers of the Dryerbox and the Dryer-Ell have recently introduced a super efficient series of roof vents and a wall vent specifically for venting the dryer. The DryerJack is a major improvement to what is available for venting the dryer to the roof. http://www.dryerjack.comhttp://www.dryerwallvent.com
Store links: http://store.dryerbox.com/DryerJack--Low-Profile--Scoop-Design_p_109.html http://store.dryerbox.com/Premium-Flush-mount-Metal-Dryer-Wall-Vent_p_93.html