Clothes Dryer Info

The length of domestic CLOTHES DRYER VENT is limited by the Uniform Mechanical Code (UMC - 1994, 1997, 2000 editions) to 14ft including two 90degree elbows. The International Mechanical Code (IMC - 2000 and 2003 editions) limits dryer vent length to 25ft no elbows, subtracting 5ft per elbow; or a maximum of 15ft and 2elbows.

For longer length dryer ducts, both UMC (Section 504.3.2) and IMC (Section 504.6) Codes offer a single "Exception": install a high-pressure dryer rated for at least the additional extended-length of 4" duct including elbows according to it's “Installation Instructions”. This method is acceptable when the high-pressure dryer is in place, but what happens when the owner moves and takes the high-pressure capacity dryer? A new "normal" replacement dryer as selected by the new owner may not have the pressure capacity to exhaust properly when connected to the existing 4" extended-length dryer duct system. We believe an occurrence such as this is very likely (probably happens even daily), and unfortunately contributes to an unsafe condition resulting in reduced airflow, dryer (gas or electric) elements possibly overheating – increasing potential of a fire hazard, and extending dryer run time unnecessarily.

According to National Fire Protection Association (NFPA), there are over 14,000 dryer related fires each year (see "Links" for NFPA Internet site). The majority of fires reported were from lack of maintenance. We understand most of these dryer fires are on Code complying 4"round 14ft 2elbow dryer systems.

Both UMC and IMC codes state a "4 inch minimum" dryer duct is to be used, implying a larger diameter dryer duct is therefore acceptable if properly designed and installed.

Using the ENGINEERED DRYER VENT SYSTEM that incorporates the Equal-Friction Calculation Method, any domestic dryer can be used! (NOTE: All domestic dryers on the market are listed with a pressure capacity of at least 14ft 2elbows). Higher-pressure capacity dryers will experience less pressure loss and therefore exhaust air more efficiently when connected to an Engineered Dryer Vent system.

According to 2000 UMC 502 Definitions, a domestic dryer duct is classified as an ENVIRONMENTAL AIR DUCT (moisture removal), not intended to be a product-conveying duct. A moisture removal duct can be designed using either of the "Equal-Friction" or "Equal-Velocity" methods. It is interesting to note that Product Conveying Ducts require sizing only by the "Equal-Velocity" method (typical for design of ducts carrying wood chips, dust, etc). 2000 UMC Code states that "Product Conveying Lint Ducts" require velocities over 2,000 ft/minute, whereas the velocity in a 4"round dryer duct is under 1,500 ft/minute (125 CFM). It is our opinion that dryer ducts are not intended to be listed as product-conveying.

A standard engineering Air Pressure Drop Calculation is used to analyze the total pressure drop of the proposed larger diameter Dryer Duct system. The calculated air-friction (or also called "pressure-drop" and sometimes as "backpressure") of the proposed Dryer Duct system is compared to the Code complying 14ft 2elbow dryer duct system. Using a target airflow of 125 CFM, the calculated air friction of the Code 14ft 2elbow dryer duct is based on approximately 0.300"WC total pressure-drop (ANSI Z21.S.1 gas dryer info). This is commonly called the "Target Pressure Drop" that shall not be exceeded when analyzing optional duct sizes using the Equal-Friction calculation method. Note that dryers with greater CFM air flow capacity (more than 125 CFM) will calculate a greater "Target Pressure-Drop" but will also have the same ratio of acceptable dryer length when larger diameter duct systems are analyzed. In other words, the actual CFM capacity of a domestic dryer 4inch dryer duct system is taken into account with the ENGINEERED DRYER VENT SYSTEM.

A CODE CHANGE is proposed for both UMC/IMC regarding domestic dryer ducts that exceed 14ft and two elbows to remove the reference of the present “Exception” of installing a high dryer according to Installation Instructions. We recommend the Exception to include the ENGINEERED DRYER VENT METHOD evaluating the entire dryer vent system using the "Equal-Friction" method verifying the total pressure drop is equal to or less than a Code 14ft 2elbow dryer vent system. Wording should also include the requirement of oval cleanout openings with edges ground smooth, covered with a gasketed metal curved sheet, held in place with exterior metal removable duct straps. Access (fire rated as required) shall be provided in all larger diameter straight sections of dryer ducts, maximum 20ft on-center, to allow maintenance. When dryer ducts are located in an unheated attic, we recommended insulation be wrapped around the exterior to reduce condensation buildup.