How to prevent dripping vent fans in cold weather

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Dripping exhaust fans (also known as ventilation fans or vent fans) in wintertime is a common problem in colder climates. A simple internet search of the topic reveals a variety of explanations on the issue, almost all with differing, and sometimes conflicting, viewpoints. In this article, we’ll take a look at the causes of dripping vent fans and a few remedies that may solve the problem.

Why and how it happens

A pervasive cold weather problem, there are several reasons why vent fans can produce an annoying water drip, including:

• Intermittent fan operation -  The fan is turned off and not run continuously

• 4” & 6” ducting - Duct size can limit airflow; bigger diameter ducting allows for higher volumes of air to flow easier.

• Vertical or horizontal ducting -  Does the house have ducts running to a roof cap, wall or soffit?

• Cold outdoor temperatures -  Essentially any temperature that can affect the potential for condensation

When a fan is ducted out the side of the house (or through a soffit), cold air from outside can migrate back through the ducting and chill the fan can. Then internal humidity aerosolized by warm inside air can condense on the inside of the chilled fan can. This built-up condensation then drips back through the grill and onto the floor, annoying homeowners in cold weather states all winter long.

The dripping occurs mostly in the morning hours when moisture loads in the home are increased due to morning activity and when the vent fan isn’t running. If the ducting is in the vertical, stack effect can carry more air up the stack, helping to create natural continuous ventilation. In turn, this draws warm, moist internal air up and out of the house. If the fan is ducted horizontally, stack effect won’t help.

In fact, it is possible for cold air to be drawn in through the horizontal duct and chill the fan can, making condensation inside the can more likely. 

In the ducting world, bigger is not better

One problem that causes cold air to cool the fan housing is often related to 6” ducting.

For example, a 4” duct has slightly more than 12.5” of area (area = (Pi*(r2))) with which we drive air out of. In comparison, a 6” duct has in excess of 28” of area. A 28” hole in the wall provides a big pathway where cold air can be drawn into your home. Many vent fan manufacturers think bigger ducting will lower static pressure and make their mediocre fan perform better.

The thinking that bigger ducting is always better is one of the most misunderstood concepts in the residential exhaust fan industry, specifically with intermittently (non-continuous) operating fans. So - short of waiting for summer - what should we do?  Basic physics tells us we have to do one of two things: 1. Warm the condensing surface or 2. reduce the humidity in the air.

BUILDER TIP: The air-moving power of Panasonic ventilation fans, with high-efficiency performance at .375” of static pressure, is key to solving “the dripping fan problem”. Conversely, other fan manufacturers routinely advocate larger ducting to artificially enhance fan performance.

What we recommend to builders, architects & engineers

  • Always recommend at least one continuously-running fan in the home with 4” ducting. Continuous run provides both an IAQ and a moisture reduction strategy. 4” ducting done correctly satisfies 99% of all residential air flow and ducting requirements.

  • Always recommend moisture sensor controls on all fans. Automated triggers are essential for preventing moisture problems before they begin.

  • Always recommend quality backdraft dampers in conjunction with external wall assemblies. For soffit applications EZ Soffit Vent™ is the best on the market.

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An inexpensive fix that works

Panasonic EZ Soffit Vent™ features a high quality seal which prevents air from being drawn back through the ducting. A proven and popular solution among HVAC contractors for preventing condensation and dripping from inside the ventilation fan.


Ken Nelson- BPI Certified Building Analyst & Channel Sales Manager, Indoor Air Quality, Panasonic Life Solutions

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