Installing a bathroom fan that works

Cold weather is on its way, and this means bathroom exhaust fans will become more important.

Cold weather is on its way, and this means bathroom exhaust fans will become more important.

Without the ability to open a window all the time to let out steamy shower air, winter is the season when the integrity of your home depends on effective mechanical ventilation.

So how’s yours doing? Does it create more noise than good? There’s still time to make things right before temperatures drop and windows steam up, and work begins with a simple calculation.

To be effective, a bathroom fan needs to change all the air in your bathroom at least eight or nine times an hour. Ten is even better.

Any less than this and excess moisture will have a chance to damage walls, ceilings and windows before it’s drawn outdoors. If the moisture ever gets out at all, that is.

Begin by determining the size of fan you need by multiplying the bathroom’s length, width and ceiling height in feet. Set this number aside for a minute then look at the fan you’ve got (or the one you’re planning to install).

All exhaust fans are rated in cubic feet of air moved per minute (CFM), so you’ll have to multiply this number by 60 to get an hourly figure. Can the fan you’re looking move at least eight times the total volume of air in your bathroom in 60 minutes?

If not, you need a bigger fan, but watch out. Standard bathroom exhaust fans only work properly in the tiniest of bathrooms. If you’ve got anything larger than an ordinary bathroom, you definitely need a larger than ordinary fan.

Noise output is another issue. Larger fans are usually slightly louder, though you should never go for a quieter fan that’s not big enough. Even today’s largest bathroom fans generate a background sound somewhere around one “sone”. This is a unit of sound output used in the exhaust fan industry, and a one sone rating is quiet indeed.

The last time I installed a bathroom fan in an existing bathroom, the unique shape of the fan unit I chose made the job much easier.

It was a Panasonic WhisperFit model designed especially for retrofit situations. The unit is thinner than comparable fans made for new construction, with a housing depth of 5 5/8 inches versus eight inches for standard designs.

One of the biggest challenges when installing an exhaust fan where none existed before is electrical supply.

It’s one thing to cut a hole in the ceiling and route new ducts to the outdoors, but it can be very disruptive indeed to cut into your walls to re-route wires and install a switch.

An easy way around this is to tap into an overhead light fixture to power your new exhaust fan.

It’s usually a matter of running a few feet of new cable to the fan itself, directly from the box powering the light fixture nearby.

This approach eliminates the need to install a new wall switch. Your exhaust fan simply comes on and off with the light.

While the fan you choose is certainly important, the way it performs also depends on the kind of duct you select and the care with which it’s installed. If your duct is travelling through a heated area on its way outdoors, you’ll get best results using four-inch diameter metal pipe and elbows.

To maximize the output of the fan, and to keep moisture from escaping into attics and ceiling cavities, make sure all seams are covered with a layer of aluminum sealing tape.

This works much better than duct tape because it never lifts or peels off in time.

For installations where duct will travel through an unheated attic space, you’ll need to use flexible, insulated ducting to prevent warm, moist air from condensing on its way out of your house. Never let an exhaust fan vent into the attic space.

Exhaust fans are one of those home features that are usually less than they should be.

Invest the time and effort to address the important details and you’ll enjoy an effective installation that works like it should.

Steve Maxwell is Canada’s award-winning home improvement expert, and technical editor of Canadian Home Workshop magazine. Sign up for his free homeowner newsletter at www.stevemaxwell.ca

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