Dryer vent booster fans: what the code allows and what the makers require
The 2021 International Residential Code says domestic booster fans shall not be installed in dryer exhaust systems (M1502.4.5), and that domestic dryer exhaust duct power ventilators shall conform to UL 705 and be installed to the maker's instructions (M1502.4.4). With a power ventilator, the ventilator maker's instructions set the maximum duct length (M1502.4.6.3). Tjernlund rates its LB2 for 30 to 150 equivalent feet of 4-inch duct, and Fantech rates its DEDPV-705 for up to 125 feet at sea level. Both say to mount the fan at least 5 linear feet from the dryer outlet. HomeGuide puts booster fans at $200 to $400 for the unit alone.
| Requirement | Tjernlund LB2 | Fantech DEDPV-705 |
|---|---|---|
| Listing | DEDPV supplement to UL705 and CSA C22.2 #113-12 | UL to the DEDPV supplement of UL705 |
| Duct length it is made for | 4-inch ducts from 30 to 150 equivalent feet; do not install where the equivalent length is 25 feet or less | Up to 125 equivalent feet at sea level, 80 feet at 12,000 feet of elevation; do not install where the equivalent length is less than 35 feet |
| Distance from the dryer outlet | Not less than 5 linear feet; the manual's chart sets the minimum by total duct length | Not within 5 linear feet or further than 40 linear feet; a secondary lint trap between them if under 15 feet |
| Distance from the outside hood | For best performance, no closer than 2 feet | Not stated in the manual |
| Dryers it may serve | Residential capacity dryers, gas or electric; not dryers designated as high output | A single residential dryer, gas or electric; not high output dryers |
| How it turns on | Senses the change in duct pressure when the dryer turns on or off | Positive pressure sensing switch |
| Warning panel | Notification panel, permanently installed in the dryer's space | Indicator panel near the dryer, with a 50-foot cable |
| Hottest air allowed | 167°F | 167°F |
| Upkeep | Clean the dryer vent at least every 2 years; check the hood flapper yearly | Inspect the fan at least every six months |
Dryer vent booster fan code: booster fans vs. power ventilators
The 2021 International Residential Code (IRC) uses two terms. Section M1502.4.5, titled "Booster fans prohibited," says domestic booster fans shall not be installed in dryer exhaust systems. Section M1502.4.4 says domestic dryer exhaust duct power ventilators shall conform to UL 705 for use in dryer exhaust duct systems, and shall be installed in accordance with the manufacturer's instructions. The 2024 IRC prints the same two sections with the same words. These rules bind where your state or city has adopted the code.
The explanation of the difference comes from the makers. Fantech's manual says dryer exhaust duct power ventilators (DEDPVs) are different from the term dryer booster because the product has to comply with strict safety requirements. Tjernlund's LB2 instructions say the 2015 IRC restricts the use of dryer booster fans to those tested to the DEDPV supplement. So when a contractor proposes a booster, ask for the UL 705 DEDPV listing on that model.
When a long dryer vent can use a power ventilator
Section M1502.4.6 gives three ways to set the maximum length of a dryer exhaust duct. The first is 35 feet from the transition duct connection to the outlet terminal, reduced for each fitting (M1502.4.6.1). The second is the dryer manufacturer's installation instructions (M1502.4.6.2). The third is the manufacturer's instructions for a dryer exhaust duct power ventilator (M1502.4.6.3). Where the equivalent length exceeds 35 feet, the code wants it stated on a permanent label or tag within 6 feet of the exhaust duct connection (M1502.4.7). Our page on how long a dryer vent can be has the elbow chart.
A fan is not the only way past 35 feet. Whirlpool says each of its dryer models has a maximum rated vent length, shown in the product literature supplied with the model, and its venting guide refers to long vent dryer models. Tjernlund's product page says that if a dryer's vent run is over 25 equivalent feet, or the dryer is taking a long time to dry, it may need a booster fan, and adds that some dryers can exhaust beyond 25 equivalent feet without one. Fantech's FAQ says that, according to some dryer installation instructions and local building codes, fans listed to the DEDPV standard are a requirement when the duct run exceeds 35 linear feet.
Do dryer vent booster fans work?
The claims come from the makers. Fantech says installing its DEDPV can assist the dryer in overcoming the added restrictions of a long or restrictive run, and its manual says that on 4-inch galvanized steel duct the unit can overcome equivalent lengths up to 125 feet and hold an air velocity of 1200 fpm. Another line of the same manual gives 130 feet of that duct as the maximum for 1200 fpm. Whirlpool's target: duct air velocity of at least 1,200 feet per minute to keep lint moving, which in a 4-inch pipe takes at least 105 CFM.
A fan can also be too much for a short run. Fantech's manual lists a minimum equivalent duct length of 40 feet, "as this will remove air from the dryer at an air velocity over 2200 fpm," which it says will decrease the performance of the dryer. Fantech's documents differ on that floor: the manual's caution says not to install the fan where the equivalent length is less than 35 feet, and the product page says to add it only when the length is between 25 feet and 125 feet. Tjernlund's instructions say not to install the LB2 where the equivalent duct length is 25 feet or less. The code points to the manufacturer's instructions (M1502.4.4), so go by the manual that comes with the unit.
What the dryer makers say about booster fans
Whirlpool's venting guide says "Do not install a booster fan in the exhaust duct." The warning does not apply to engineered multiple-dryer exhaust systems. The guide also says there is currently no UL classification for booster fans and their direct connection to dryers (to ensure that the dryer won't run if the booster fan fails), so Whirlpool does not provide recommendations for booster fan use. It does not approve third-party vent accessories that are not licensed by Whirlpool.
Tjernlund's instructions say not to install the LB2 in the exhaust duct of a dryer whose instructions prohibit a booster fan. Fantech's manual says not to install the DEDPV-705 in a dryer's exhaust system if it is not permitted to do so. Read your dryer's installation instructions before you buy a fan.
How to install a dryer vent booster fan
These steps are the makers' own. Fantech's manual says the procedures must be done by approved personnel, and Tjernlund's says the installation must be inspected and accepted by the local authorities having jurisdiction.
- Clean the duct first. Make sure an existing duct is clean and free of lint, which may take a professional duct cleaning company, or the fan may pull lint into the impeller on start-up and eventually stop (Fantech FAQ).
- Add up the equivalent length. Count 5 feet for every 90-degree elbow and 2-1/2 feet for every 45-degree elbow, and check the total against the fan's range (Fantech manual; Tjernlund instructions).
- Pick the spot. At least 5 linear feet from the dryer outlet (both makers). Fantech sets 40 linear feet as the farthest and wants a secondary lint trap between dryer and fan if the fan is under 15 feet away. Tjernlund's chart sets how close the LB2 may sit for each total length, with one minimum distance without a secondary lint trap and another with one; its LT4 secondary lint trap, if needed, goes about 4 linear feet from the dryer, with the LB2 after it. For best performance, Tjernlund says to install the LB2 no closer than 2 feet from the outside termination and to avoid installing elbows within 2 feet of it. Tjernlund says the fan must be accessible after installation, and Fantech says to leave space to service it.
- Mount it the right way up. Tjernlund: never with the discharge facing down, and discharge up if mounted vertically. Fantech: the pressure switch must be vertical, and a vertical mount is recommended to reduce condensation buildup.
- Connect metal duct. Fantech says to use only metal ductwork and not stretch flex duct. Tjernlund says not to use screws in the duct installation and to secure the duct with reinforced foil duct tape. Tjernlund also requires an outside hood with a flapper or a backdraft damper, which the IRC requires at every dryer termination (M1502.3).
- Mount the warning panel. Both makers say it shall be permanently installed in the space where the dryer is, readily visible without opening doors other than those needed to reach the dryer.
- Power up and test. Tjernlund: with the duct complete and clear of lint, the dryer off, cool and empty, lint traps clean and the door closed, plug in the LB2 and let it calibrate for up to 45 seconds. Fantech: start the dryer, and the fan starts five seconds later, then checks duct pressure every five minutes.
Are dryer vent boosters safe?
Both makers say the dryer depends on the ventilator, and that running the dryer without a working one will cause inefficient operation, excess energy use and a possible fire hazard. Tjernlund's LB2 shuts off and shows a steady red light when it senses high temperatures, and flashes red when it senses a blockage; its thermal fuse is set at 186° to 196°F. Fantech's thermal fuse opens if exhaust air is hotter than 196°F, after which the manual says to replace the product and speak to the dryer maker. Tjernlund says not to operate the dryer if the panel shows no green light, and to stop the dryer immediately and have the vent system inspected by someone qualified when it shows a red light. Fantech says not to use the dryer if the panel light starts to flash, and its panel label says to see the manual or call for assistance if the light is flashing or off while the dryer runs.
Dryer vent booster fan cleaning and maintenance
Fantech's manual says the impeller may collect lint or debris and to inspect the fan a minimum of every six months. Its FAQ says an annual inspection, so the manual is the stricter of the two. Tjernlund recommends cleaning the dryer exhaust vent at least every 2 years, depending on use, checking yearly that the hood flapper or backdraft damper is free of lint and moves freely, and recalibrating the LB2 after any impeller cleaning. Fantech recommends its DBLT 4W secondary lint trap to cut lint in the duct, and its product page says that trap is not suitable for gas dryers. A dryer vent cleaning company can clear the run before a fan goes in and on the schedule above.
Dryer vent booster fan installation cost
HomeGuide says booster fans cost $200 to $400 on average for the unit alone, for blowing air out of an extra-long dryer vent or one with multiple bends. HVAC labor runs $75 to $150 per hour, it says. Both fans plug in on a cord (6 feet on the LB2, 5 feet on the DEDPV-705, which Fantech says not to field wire), and if there is no outlet at the fan location, HomeGuide puts a new electrical outlet at $150 to $350. Rerouting the duct instead is covered on dryer vent installation, and a crushed or disconnected run on dryer vent repair. For a quote on a fan, a cleaning or a new run, use the form below and we will send your request to a company that serves your ZIP code.
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Sources
- International Code Council, 2021 International Residential Code, Chapter 15, section M1502 Clothes dryer exhaust: codes.iccsafe.org
- International Code Council, 2024 International Residential Code, Chapter 15, section M1502 Clothes dryer exhaust: codes.iccsafe.org
- Tjernlund Products, "Dryer Duct Booster Fan Model LB2" installation instructions, P/N 8504216: tjernlund.com
- Tjernlund Products, "Model LB2/LB2XL No Clog Dryer Duct Booster" product page: tjernlund.com
- Fantech, "DEDPV-705 Dryer Exhaust Duct Power Ventilator" installation, operation and maintenance instructions, 483443, dated 2025-12-03: fantech.net document library
- Fantech, "DEDPV Dryer exhaust fans" product page: fantech.net
- Fantech, "Dryer Exhaust" frequently asked questions: fantech.net
- Whirlpool, "Dryer venting specifications" (W11688180A): whirlpool.com
- HomeGuide, dryer vent installation cost, page dated January 5, 2026: homeguide.com