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September 2021
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Bathroom Fan Upgrade

Bathrooms are the wettest rooms in your house. You installed those new fixtures, cabinets, lights, floors and tiles. You probably didn’t think about the fan. Proper ventilation of your bathroom will go a long way toward keeping down mold, mildew and other moisture related problems from shorting the life of your bathroom and its fixtures.

Fans come in a wide range of sizes and performances. The two most important are the CFM, (Cubic Feet per Minute) and noise level rated in Sones. The Home Ventilation Institute[1] (who knew) recommends a minimum of 8 air changes per hour. A lot of older bathrooms are 5×8 to about 8×10. Calculation of size is width x depth x height divided by 7.5 to get you a minimum CFM rating. There is a quick table here[2].
A 5×8 bath would need a 40-45 CFM fan. A 8×10 bath would need a 85-90 CFM fan.

Sone[3] Ratings are a little different. 4.0 Sones is the sound of your TV. 1.0 Sone is the sound of your refrigerator. The smaller the Sone Value the Higher the cost.

A real good indicator of needing an upgrade is having to wipe down the mirror when you are done with your bath or shower.

Now that you are on the upgrade path, the other important part of the ventilation equation is a timer. Just because the water is off doesn’t mean the moisture is gone. Remember wiping the mirror?
Here are two products that I installed recently. A 140CFM Fan and a Timer.
This is a Broan Elite Series Fan. About 140 bucks. It has a 2.0 Sone rating making it one of the quietest fans available. Bought at Lowes. Quiet costs money.

This is a great fan but it is much larger than that small fan you will be replacing. More on that later. One of the other nice features beyound the three year warranty is the fact that the motor unit is available separately, so if you ever need to replace it, you will not have to crawl in the attic again. Also included in the kit are some black plastic spacer blocks if your joists happen to be “I” joists. The one thing that is not mentioned any where in the instructions is the size of the unit. You will have to measure it, befor cutting your rough opening.
This is a Pass and Seymour Decora style 7 button timer. Around 30 bucks. It allows times between 1 minute to 1 hour. Personally I recommend setting it at 1 hour, turning it on before you start your shower/bath and let it run after you are finished.

The cool thing about this switch is once you decide on a time, you only have to turn it on, until you change the time. Very cool. The only odd thing is the led’s are lit when it is off and go off when it is running.

Okay now that you have the fan and timer, it is remodeling time. Here is a short list.

Location of breaker to shut off power.
Always a good idea when working with electricity.
Access to the old fan and vent.
Most fans have tabs outside the box that mount them to the side of the ceiling joists so you will need to crawl around in the attic. You will also need to disconnect the romex from the fan as well as disconnect the vent pipe.
If you are lucky you can reuse the romex connector. If not, put one on your list. Fans don’t come with them.
Screws to mount the fan(nails will work themselves loose)
4” metal vent pipe and elbow(s)
Metal tape to seal the joints.
Caulk to seal the perimeter of the fan opening.
Keyhole saw to cut new opening.
Save the piece to cover the old hole.
Drywall mesh tape and mud of you are moving the fan.
Dab of paint to make your repair invisible.

Moving on to project planning.

While you are up there, check to see if you can just enlarge the existing opening, if not and you need a new hole, you need to see if you have enough romex to move the opening for the new fan, or a place to run new cable if you are moving it a long distance. In most cases the fan wire is a switch leg from the old switch. If you are moving it a long distance you will need to be able to run the new cable.
You will also need to get a rough measurement for the vent pipe to join the fan to the roof vent. 4” steel pipe is recommended. The fan has a 6” option but it is extremely rare to see a 6” vent in a house. Don’t forget a bit of metal joint tape to seal all your joints. We want to eliminate the moisture and or mold problem, and not move it into your attic.

Here is a recent installation.

I have removed the old fan and vent pipe which was the wrong size. I made a mark on the ceiling to see if I could the existing opening. You can see that it is a larger fan.

No such luck. The fan is too close to allow a vent pipe. Plus there are some other challenges above. I have enough romex for a new location, but because of the existing romex I can only go so far.

Measure and cut your new opening, saving the scrap to fill in the old hole.
Here is the new opening. because of the vent outlet of the new fan I am attaching it to the next joist over. This gives me a straight run to the roof vent with only one elbow.

Here is the new fan mounted. Notice the flanges on the bottom of the new fan. You will need to measure the overall width and length and add a little bit to allow you to get the fan in the hole and mount it properly. Having the original hole allowed me to measure and assemble the vent pipe, attach and seal the joints (remember the metal tape on the list above?) from the bathroom rather than crawling in the attic which in this area is only about 14” high.

Fan in place and hole patched with scrap from new opening. See that there is a gap on the side from making the opening large enough to install the fan. This will be filled with a piece of mesh tape and drywall mud. At this point we caulk the perimeter of the opening to eliminate air leaks around the fan body.

Here is ceiling mudded with fan grille in place checking that we have covered the gap.

Here is our finished project.

[1]Home Ventilating Institute
[2]Sizing a Bath Fan
[3]Sones: the Psychoacoustic explanation