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Dryer Booster Fans | Are They Legal?

Booster fans are supposed to be the silver lining when you are faced with dryer ductwork long enough to make the appliance inefficient. However, there is a lot of information floating around about them being against code. So, is there no hope of a boosting device?

Before you sign up for a massive alteration project or go out and purchase that ventless dryer, you should know that boosting devices are not all illegal. However, there are certain requirements for such an appliance to be code-compliant and safe.

Dryer booster fans are against IRC regulations for safety reasons and are, therefore, illegal in most states. But it is prudent to check local codes for amendments to the international code. However, the IRC does permit UL705 listed and labeled dryer exhaust duct power ventilators.

Dryer Booster Fans Are Against IRC

The International Residential Code (IRC) guides the regulations for safety in homes. In Section M1502.4.5 it states that:

“Domestic booster fans shall not be installed in dryer exhaust systems.”

So, this means that no, generally, booster fans are not legal. However, this may be different according to local codes, and it depends on what you are looking at as a booster.

Domestic booster fan installation is not allowed in dryer exhaust

Some Local Codes Allow Booster Fans

It is always important to look at your local codes. The IRC gives the basic universal safety standards for America, but the local codes are the modified version specifically dedicated to the circumstances of your state or district.

It is also crucial to understand what your local codes are saying. For example, this source for California may appear to be allowing booster fans because it uses the two terms interchangeably. However, if you look a little closer, the source indicates that it uses the term “booster fan” to refer to an exhaust duct power ventilator.

It also specifies that these “booster fans” are only allowed if they meet the exception criteria. The exception requires the booster to be listed and labeled according to the UL 705 and that you must adhere to manufacturer installation instructions.

Power Ventilators vs Booster Fans

The source above uses the terms booster fan and power ventilator interchangeably. Yet, in Section M1502.4.4 of the IRC, dryer exhaust duct power ventilators (DEDPVs) are allowed.

Spruce Automatic Clothes Dryer Exhaust Duct Power Ventilator Booster Fan

The prohibition of booster fans is listed in the IRC in blue, meaning it is a recent addition to the code. It’s possible the terms are synonymous, and the IRC compliers made an error in not removing the section on DEDPVs. However, I suspect this isn’t so and is instead the result of slightly confusing terminology in the source.

As I mentioned, the source specifies what is meant by “booster fan” and looks at what is allowed in terms of such a device. An exhaust duct ventilator does, in fact, boost the exhaust power, but calling it a booster fan is a little misguided since that can refer to unregulated devices as well.

So, what is the difference between a DEDPV and other booster fans? The difference is the UL 705 listing of the DEDPVs, which has approval for the safety standards of the IRC.

Difference of DEDPV and other booster fans is the UL 705 listing

The Underwriters Laboratories (UL) listing on DEDPVs means they have been tested for functionality, safety features and requirements, correct airflow for lint, and the ability to maintain them properly. DEDPVs are, therefore, boosters that are proven to be safe and appropriate for assisting in dryer exhaust systems.

Why Are Booster Fans Prohibited?

Booster fans are intended to boost the airflow through your dryer exhaust vent to combat the difficulties associated with longer vents. Those difficulties are decreased appliance efficiency, longer drying times, and lint buildup.

The prohibition of booster fans seems to be attributed to safety regarding the appliance. It is dangerous if you have an unnoticed failure, if lint can catch on the extra surfaces, and if the ducts are too long.

If the booster fails, your dryer is no longer being appropriately ventilated, which is critical for dryers to operate safely (unless they are ventless). The result of this with a gas-powered dryer would be particularly insidious, as the harmful by-product carbon monoxide will not be exhausted properly.

Heat and lint can build in the ducting if the booster fails, resulting in a major fire hazard since dryer heat is often the source of lint ignition. In addition, lint buildup can cause blockages in the vents, also causing heat to build in the ducts.

Cleaning the dryer vent full of lint

Lint can also gather because the fan is another surface to catch on. It is dangerous whenever there is a buildup of lint, especially if you are not regularly cleaning the fan as well as the vent and because there is heat from the dryer.

Another reason that could be attributed to the trouble with booster fans is that they can be used to compensate for ducting that isn’t code compliant since they assist when you have long ductwork. However, there are limits to the allowable length of dryer ducting.

According to Section M1502.4.6.2 (IRC), you must adhere to the maximum duct length in the manufacturer’s instructions. However, Section M1502.4.6.1 indicates that the overall maximum duct length for any dryer is 35’ between the transition duct on the dryer to the output. Bends equate to a certain measure of straight length, so adding bends reduces the allowable length.

 Are Codes Equivalent to Laws?

The IRC is intended as a model code based on trade professional expertise and practices amongst national and international trade committees and councils.

Essentially, this is the compilation of best conduct and safety practices from experts in the relevant fields, which is designed to be adopted by governments with appropriate amendments where necessary.

According to the International Code Council (ICC), their code is accepted in 49 states (not Wisconsin currently), the Virgin Islands, Guam, Puerto Rico, and the District of Columbia.

This means the organizations that approve of and conduct constructions, alterations, and installations in your home are guided by this code. These include contractors, plumbers, electricians, home inspectors, and municipalities where you get permits and approvals. Even insurance companies will use this as a standard for evaluating the validity of claims.

So, yes, the codes are equivalent to laws, provided you follow any local amendments, because the legal bodies acting in your state have standardized these building codes into their legislation.


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