I have a very small bathroom with an electrically powered air vent (extractor fan). When I turned on the light, it seemed to break down, and has not been working since then.

I want to try and fix the issue, but I have no experience in the matter.


  • Can I just measure the current extractor fan, and replace it by one with the same size that I would have bought on line? I assume I have to be careful to choose one for a bathroom. Any other element I should pay attention to?

  • In terms of installing the new extractor fan, is it simply a question of connecting the wires? Is it easy?

  • The fan in one of our bathrooms quit working. I redid the connections in the fan housing connecting the wires (hots or neutrals) and that solved the problem. I think the bad connection was a wire nut connecting 4 or 5 wires. Our house is wired in 12 AWG and 10 AWG aluminum and the original install had not used a large enough wire nut. In your case it might be the fan unit needs replacing, but since it could be a bad connection why not check that first? If you turn off the wall switch, that shuts off the hot at wall so it should be safe to work on (if the wiring is correctly done) Jan 9, 2017 at 17:46
  • Most of the bathroom fans I have repaired or replaced are shaded pole motors so if they stop spinning the motor is still usually good unlike other motor types that burn up. What I have found the bearings have dried out and need a little light weight oil, although many on this site don't like wd40 I have used it with great success to loosen the old dried out grease / oil then a few drops of motor oil may bring the fan back to life for a few years before a few more drops of oil will be needed again. I have saved myself, family members and customers $ with this method.
    – Ed Beal
    Mar 9, 2018 at 15:32

3 Answers 3


If you're looking to replace your fan here's what you need to consider

  1. Sound - Measured in sones, a lot of older square fans are fairly loud. Those are 4-5 sones. A lot of newer fans are a lot quieter. I was able to install a 1 sone and it's so quiet you can't hear it outside the bathroom. The boxes typically display this prominently
  2. CFMs - Cubic Feet per Minute. This measures how much air the fan can move. The small, noisy fan probably moved 50 CFMs, if that. My new fan is larger, but moves 90 CFM. With the new fan, I can take a full-on hot shower and have the small bathroom (maybe 50 square feet) de-humidified in about 5 minutes.
  3. Ducting - If you can go into your attic, you should. Try to figure out where fan sits. Sadly, it's very possible that the fan does NOT duct outside. This was what I found in my bathroom (the insulation was filthy from years of dust dumping). This is a problem if it doesn't (can cause mold to grow), and there's a good chance that, if they did duct it, it's not insulated. Especially if you live further north, you want this line insulated (warm, moist air into a cold zone will cause condensation). Most ducts and vents are 4", but if you go above 100 CFM, the duct size tends to go up to 6" line. You can buy insulated flex ducting (check the ducting area of your hardware store, as fans are sometimes sold near light fixtures).
  4. Venting - Sadly, the only roof vent I could find was in a kit that came with uninsulated flex line. It's a compact 4" duct, and can be cut with either a large hole saw, reciprocating saw, or jigsaw. Be sure to buy some appropriate roofing or outdoor caulk. DO NOT USE BASIC SILICONE ON YOUR ROOF. If you're scared of cutting a roof hole, you can also buy a soffit vent, but be aware that the warm, moist air will go out the vent and some might come back up through the vents of the soffit back into your attic, where it can cause mold. If possible, vent out of the roof.
  5. Wiring - Honestly, if you've already got a fan wired in, you can keep the same wires. Just be aware that it's important to ground these fans. Your previous fan may not have one (or they may not have attached it). Don't skip that step. Make sure it's grounded. Also, consider putting in a timer switch. That way your family and guests can set the fan to run after they're done with the bathroom

Nutone actually makes some fans that are designed for easier DIY replacement. Some of their models can be replaced entirely from the bottom, instead of trying to get to it from the attic. This may involve some drywall cutting to accommodate a larger form factor, but they keep the height of the smaller, traditional 6" x 6" (typically 3"). You keep the same wiring and venting.


Replacing a bath fan is a simple project on the surface, but it can get complicated for various reasons. It involves mounting, wiring, and ducting.

When purchasing a replacement unit, physical size is one consideration, but more important are sound level (sones) and air movement capacity (CFM) of the new fan. Most will tell you on the package what room size they're suited for. These fans are designed for use in bathrooms, so there's no special rating for that purpose.

If you're replacing an entire unit, you may need to slightly enlarge the opening in your ceiling or wall, but that's usually not an overwhelming challenge. This applies to box units, not in-duct units such as are common in the UK and elsewhere.

Where it can get tricky is in mounting the new fan. Ideally, you have attic access, from which you can disconnect and remove the existing fan, mount and connect the new one, and repair any disturbed insulation and vapor barrier.

Another possible issue is ducting. If the new fan has its outlet in a different position, or is a different size, you may need to move or alter the duct. Many are flexible, so then it's a matter of adequate length.

I suggest that you purchase a fan kit, read the directions completely, then make an assessment as to your ability to complete the project. You can always return the fan and hire someone to do the job.

  • 1
    FYI: The "If you're replacing an entire unit, you may need to slightly enlarge the opening in your ceiling or wall" would be unlikely to apply in the UK, where the vast majority of fans fit everything within a 100mm (4") duct diameter, hence the opening will always be a 100mm circle. See image search for "bath fan UK" for clarity.
    – AndyT
    Mar 9, 2018 at 9:38

I learned the hard way and had to pay $200 for the Mechanical permit that is required. It should have been $80 but because I had done the work already I had to pay double $160. The problem is that if you change out the fan and it burns your house down you could be held responsible for not properly permitting.

  • 3
    Hello, and welcome to Stack Exchange. Building codes are different everywhere, so where are you located? Mar 8, 2018 at 3:20
  • A permit to replace like with like? That's a bit over the top! Certainly wouldn't be a problem in the UK. I feel your pain, and thank you for sharing, though as Daniel has said this would be much more useful to future visitors if you give your location.
    – AndyT
    Mar 8, 2018 at 17:03

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