How to Install a Bathroom Fan

Replace your old, noisy bath fan with a quiet, energy-efficient model

Step 1: Plan the bath fan replacement

A new generation of efficient and quiet exhaust fans is now available at home centers and from heating, ventilation and air conditioning suppliers (under “Heating Equipment” in the Yellow Pages). The new fans are virtually silent. They're also much more powerful and use less energy than the older models. They can even be left on full time if you need continuous ventilation. (Most ordinary fan motors would burn out.) See “Shopping for a Quiet Fan,” below, for more details.

In this article, we'll show you how to remove an old bath fan and install a new quiet one. In most cases you can do this in less than a day with little or no ceiling repair. While we're at it, we'll show you how to replace typical 3-in. uninsulated duct with much superior 4-in. insulated ductwork.

This project involves electrical wiring, so call your local electrical inspector to find out if you need a permit.

Installing a fan requires only elementary carpentry and electrical skills. You'll need basic hand tools, a power drill and a jigsaw. The basic operations include hand sawing a small drywall opening, driving sheet metal screws into aluminum ductwork, cutting aluminum duct, climbing on the roof and sawing a larger roof vent opening, and disconnecting and reconnecting electrical wires. If you run into problems you can't handle, such as complicated electrical wiring, tight duct clearances, water damage at the roof vent cap or a steep roof pitch, don't hesitate to call a licensed electrician.

You'll have to go into your attic and walk on your roof, so play it safe. Wear a long-sleeve shirt, long pants, boots, a hat and a dust mask to protect yourself from dust and insulation in the attic. While on the roof, use roof brackets, roof cleats or a safety harness for secure footing and fall protection. And if your roof is too steep or you don't feel confident up there, hire a pro for this part. The bath fan we're replacing is fairly typical. It's located in a ceiling with an accessible unfinished attic above.

The fan is properly wired and ducted with 3-in. uninsulated ductwork to the roof. If your old fan unit has additional features like a light or heater that operates off a second switch, your electrical wiring will be more complex. If the rewiring confuses you, consult a licensed electrician to work out the details. If you have a second floor above the fan, measure the height of the space available. The new, quieter fans are taller than the old ones, at least 7-1/2 in. tall. While it should fit into typical floor joist space, check the fan dimensions to make sure.

If you don't have an attic above, as we show, you'll have to do the entire installation from below. This means you'll have to cut open the ceiling a bit (and patch it later!) to get the fan in and run ductwork to a wall vent cap. The connections will be the same as we show in the photo series. However, changing to a larger wall vent cap can be more complex if you have brick, stucco or vinyl siding rather than wood. If you're unsure how to proceed, check with a siding specialist for advice.

Step 2: Remove the old fan

Before you start, you'll have to turn off the electricity to the fan at the main panel. With the fan running, flip circuit breakers or loosen fuses until it stops.

Put on your safety goggles. As you pull down and snap off the old grille, watch out for falling debris! You'd be amazed at how much crud can spill out. Then unplug the motor and remove it from the housing (Photo 1). It will be held with clips or screws. Wear gloves. The edges may be sharp.

The electrical connections are usually in a small splice box (Photo 2). Check the wires for power with a voltage tester to make sure it's off. Then disconnect them.

Next you have to go up to the attic. You need a power drill, screwdriver, utility knife, metal snips, wire cutter, wire stripper and a work light. To avoid extra trips, carry all the tools with you in a bucket. To make it easier to work in the attic, find a small piece of plywood to kneel or lie on—approximately 2 x 3 ft. In the summer, work in the morning. Attics get hot on warm days. Push the insulation back from the old fan housing and remove the housing (Photo 3). Most older fans will be mounted directly to a joist. The screws might be hard to remove. A power drill simplifies this task.

Caution: Aluminum wiring

Aluminum wiring requires special handling. If you have aluminum wiring, call in a licensed pro who's certified to work with it. This wiring is dull gray, not the dull orange that's characteristic of copper.

Step 3: Install the new fan and duct

Now go back down to the bathroom. Hold the base of the new fan housing to the ceiling and draw the new opening size (Photo 4). Most new, quiet fans are larger than the old fans. If not, you'll have to patch the ceiling with drywall. To simplify cutting, line up two sides with the existing hole. Wear goggles while cutting.

Before going back into the attic, cut a piece of insulated flexible duct approximately 18 to 24 in. longer than the old duct and cut one piece of 4-in. dia. aluminum ductwork 6 in. long. Take them into the attic along with the fan, fan mounting brackets, aluminum elbow, sheet metal screws, drywall screws, electrical cable staples, electrical clamp, caulk and duct clamps. You may also need blocking to attach to the joists to support your new fan housing.

You'll need a helper to mount the new fan. Slide the mounting brackets on and extend them to the joists. Align the fan with the new drywall hole. Then ease it down to your helper, who will hold the housing flush to the ceiling while you screw the brackets to the joists (Photo 5).

After installing the fan, run the ducts. Attach the aluminum elbow to the exhaust port (Photo 6) and the duct liner to the elbow. Then measure and cut the flexible insulated duct. The ideal duct run should be as direct as possible, with the fewest turns. Extend the duct from the exhaust port to the roof opening, but don't stretch it so tight that it kinks. Then finish connecting the insulated duct to both aluminum ends (Photo 7).

With our fan, the wiring is done from the attic. Some models wire from below. In either case, clamp the electrical cable to the housing with a standard clamp with 6 in. of wire extending into the box. Strip 5/8 in. of insulation off the end of each wire and connect the wires (Photo 8). Staple the cable to a framing member within 12 in. of the cable clamp. You may find that the existing electrical cable is too short to reach the new fan. If the cable runs to a nearby junction box, you can replace it with a longer section. Otherwise, you'll have to install a new junction box. If you're not familiar with the rules of wiring, call in a licensed electrician to do this part. In any case, have your work checked by the local electrical inspector.

Seal the perimeter of the fan housing with flexible caulk (Photo 9). This is critical for reducing sound transmission as well as air leakage. Replace the insulation, making sure to cover the fan housing. Look around for all your tools before you go back down!

Step 4: Install the cap

Complete the roof work in one trip. While working on the roof, you'll need the new roof vent cap, a scrap of the straight aluminum ductwork (cut one end to match the roof angle), a jigsaw, a tape measure, roof cement, caulk, a utility knife, roof nails, 1/2-in. Phillips self-tapping sheet metal screws, metal snips, a pry bar, a carpenter's pencil and a hammer. Wear a tool belt or apron or you'll be going up and down the ladder retrieving stuff that slides off!

If you're not fully confident working on your roof, install a pair of roof brackets along with a 2x10, or nail a 2x4 cleat to the roof a few feet below the vent opening. It'll give you a more secure area to place your feet as you work on the roof.

Replace the old vent cap with a new one. Remove the old one care fully so you don't damage shingles. They can be brittle, especially on a cold day. Then enlarge the old 3-in. hole to fit the 4-in. duct, using the 4-in. duct as a template. The opening will be oval-shaped in the direction of the roof slope (Photo 10). Reach through the roof and pull the straight aluminum duct up through the hole, with the angled end extending 3/4 in. above the roof surface. Screw the duct to the sheathing (Photo 11). A magnetic drill bit is helpful here. Drive a roofing nail through the side of the duct to temporarily anchor it. Seal around the edge with caulk to keep condensation from running back down the duct or under the shingles.

Check the screen and the damper in the new vent cap before installing it to make sure both are in place. When you're done, fill any nail holes you put in the roof with roofing cement.

To complete the job, install the decorative grille. Now is the moment of truth. Turn the power back on and listen to the fan—if you can hear it!

Shopping for a Quiet Fan

A quiet fan has a “sone” rating between .5 and 1.5. A sone is a measure of loudness. The lower the number, the quieter the fan. By contrast, typical low-cost bath fans range from 2 to more than 5 sones (which you can easily hear in the next room). Select the ventilation capacity of the fan that fits the size of your bathroom. To quickly estimate the cfm needed (cubic feet per minute, or how much air a fan moves), a “rule of thumb” is to multiply the length of your bathroom by the width and round up. For example, an 8 x 10-ft. (80 sq. ft.) room would need a vent rate of 80 cfm and would require an 80-cfm fan. Quiet fans have better motors (rated for continuous operation), heavier-duty housings and larger ducts, usually 4 in. They cost considerably more than standard fans.

It's quiet!

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