HVAC: Installing An Exhaust Duct For A Bathroom Fan, Clothes Dryer, Or Ventilating Fan

By Bruce W. Maki, Editor

Introduction:

The installation of an exhaust duct for a bathroom fan is about the same as the process for installing a dryer vent duct. This involves: 

• Locating a suitable area to cut through the outside wall. The best location is one that has as few bends as possible. Every turn in the duct increases the resistance to air flow, and sharp turns are much worse than gentle curves.
• Determining the exact center point of the hole.
• Drilling a small pilot hole through the wall.
• Cutting the large hole from the outside (typically) based on the exit point of the pilot hole.
• Installing the duct outlet hood and sealing it from the weather.
• Attaching the ductwork upstream from there, and connecting to the appliance.

Location, Location, and Location

By far the most important aspect of vent ducting is figuring out where to put it. Four-inch vent ducts can easily fit in the typical "joist bay" (space between adjacent joists), but under no circumstances are such ducts allowed to cut through a joist that spans any significant distance. The required hole would so badly alter the structure of the joist that it would be at risk of breaking. If the duct has to cross a joist it must pass below the joist.

In this project I installed a used bathroom fan in my basement so I could remove fumes from paint, stain, and urethane. It's cold in the garage and too expensive to heat the place just for drying paint and urethane finishes. It's quite possible that the air flow rate (measured in cubic feet per minute, or CFM) is not adequate to create a completely safe atmosphere. Fumes may still get into the house, but this is better than what I had before.

In my case I chose a location for the fan based on an easy and straight run of duct out the wall. This isn't always possible for other fans or dryer ducts.

The joist bay where I decided to install the fan and duct (arrow). I had to be certain that I would be able to cut through the outside wall in this bay. If I cut through the next bay to the right, the hole would go under the laundry room, which juts out from the house behind that wall.

The fan housing and a piece of duct.  I measured the distance from the left side of this box to the center of the duct outlet. I needed to cut the duct hole in the right location within the joist bay, or else the duct pipe would not be aligned with the fan outlet.

Once I determined the best location for the duct to break through the outside wall, I drilled a small hole through the rim joist. I used a spade bit with a couple of 12-inch extensions, which made a drill bit about 2½ feet long. I did this because there was a drain pipe in my way and I was too lazy to get a taller ladder so I could get really close to the action. Sometimes it's easier to drill holes from far back.

Besides, the foundation on this house is almost 2 feet thick, which means you just can't get close to the edge of the house to drill holes like this.

Note: Before drilling through a wall, it's very important to look around on the other side and make sure there is nothing in the way. There are few things that can ruin a pleasant Saturday afternoon quicker than the words "um, honey... I just drilled through the gas pipe".

I went outside to see where the hole broke through. At this point I realized that the duct was going to cut right through the water table molding (that piece of wood below the yellow siding... with all the paint missing). That's not a problem, just something I'd prefer to avoid.

Using the 4 inch diameter duct as a guide, I drew a circle big enough for the duct.

Cutting The Hole

I drilled a series of 5/8" holes around the circle.

I would normally use a 4½" hole saw for this, but my hole saw requires a ½" drill, and my only ½" drill has a broken gear, so I had to do this the low-tech way.

I used my reciprocating saw (with a narrow wood cutting blade) to cut out the circle.

Years ago I cut out a few holes like this, but I didn't yet own a reciprocating saw. I marked the circle and drilled a ring of holes tightly packed together. Then I used a keyhole saw to cut between the holes. That takes more time and effort, but it's certainly worthwhile if a homeowner has no desire to buy a reciprocating saw.

A jig saw should also work here, providing the blade is long enough to reach all the way through the rim joist. That could be well over 2 inches on many houses.

The hole wasn't very circular, but that didn't really matter.

The duct discharge hood fit just fine.

I marked the wood where it needed to be cut away to let the hood sit flush.

My initial plan was to cut the siding away so the white plastic would sit flush (or below) the wood siding. But when I discovered that the duct would cut through the water table trim, I changed my mind. It turned out that the siding and the white-painted board below the water table (whatever that piece is called) formed an almost-flat surface. The red arrow shows the small gap at the top when the bottom was pressed flush.

Or, when the top was held flush with the wood, this small gap appeared at the bottom. I figured I could cover this with caulking.

This dryer duct just a few feet away is an example of how not to install a duct hood. Rain can get behind the white plastic piece and enter the building. This is the laundry room annex that needs to be completely rebuilt, inside and out. That's why there is no paint on the siding... there's no point in painting something that will soon be removed.

Unless, of course, you are like me and take FOREVER to get around to these projects. In hindsight, painting this annex would have been a good idea because it's been an eyesore for many years now.

Which illustrates a point: Many people, myself included, defer repairs and maintenance on things they intend to remodel soon. But nothing ever goes as planned, I always say. New projects pop up, or some factor complicates the intended remodeling project and it gets pushed farther back. Suddenly, it's three or four years later and nothing has happened, except damage from time and weather. For example, it would be tempting to delay re-shingling a roof because you planned on building an addition. But when the addition gets delayed repeatedly, the roof can leak and cause structural damage, creating more headaches and expense.  But, I digress...

I drove in some 1½" stainless steel flat-head sheet metal screws to hold the duct hood in place.

I caulked around the perimeter with siliconized acrylic latex caulk.

The outside work was complete.

On the inside, I sprayed expanding foam insulation around the irregular-shaped hole to fill the gaps. Of course, I sprayed to much foam and it oozed out all over the place.

This plastic ring is meant to cover that irregular hole on the inside. After I scraped off some excess foam, I stuck this ring in place and screwed it to the wood.

I cut a piece of 4" duct with tin snips.

Round ducts just snap together. It's harder than it appears... I always seem to struggle with these things.

I applied a few pieces of metal foil tape to the seams, just to prevent them from popping open later.

All of the round ducting I've bought at Home Depot in the last couple of years has had this problem... the crimped end wasn't crimped heavily enough and won't fit inside an uncrimped end. So every time, I've had to improve on their crimping by making a series of twists with a pair of needle-nose pliers.

For example, the duct on the left is a piece I crimped myself (because I cut off the crimped part and used it somewhere), and the other duct is factory crimped. I should buy a crimping tool, but I don't do enough ductwork to justify it. My home-made crimps work okay, but they probably increase the resistance to air flow slightly.

Before installing any duct pipe I placed some fiberglass insulation against the rim joist.

I installed a piece of duct and the fan housing. The fan was mounted to the joists with small sheet metal screws at the ends of four steel telescoping brackets (red arrow).

I also taped the duct joints with metal foil tape. If you've ever used duct tape on heating ducts, you know that it doesn't last long. It does last a bit longer on cool ducts like this, but I only use foil tape on ducting. In fact, there's some university research web site out there (yes, I should find it and link to it) that points out how poorly duct tape performs and says that duct tape should not be used for ducts.

So why do they call it DUCT TAPE then? They should call it UN-duct tape.

I connected the supply wires.

If you've never done any maintenance on a fan, this might be news to you. Most fans (especially Broan and NuTone) have a removable motor and squirrel-cage unit that simply plugs into an outlet that is built into the fan housing.

The light is usually made the same way, plugged into a separate outlet. So when a bathroom fan is wired, all you are really doing is wiring one or two outlets.

The fan unit simply slides into place. It's held on one end by two tabs in two notches, and at the other end a single screw secures it to the housing. Quick and simple.

The completed fan. Normally there would be drywall around the fan, but this is just an unfinished basement.

What About A Dryer?

Connecting to a dryer is similar, except that usually a piece of flexible ducting is used to connect the dryer to the fixed duct. This allows the dryer to be pulled away from the wall and pushed back while staying connected. The type of flexible duct to use is the all-metal kind. Every dryer I've seen recommends against using flexible plastic duct, because they can overheat and ignite. They also get little holes and leak.

Copyright © 2002  HammerZone.com

Written March 7, 2002