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I'm finishing a new basement bathroom and just tried the exhaust fan, and it appears to be blowing the wrong way. I can feel air blowing downwards, and the horizontal backdraft fins on the outside exhaust vent (it's just above ground level, on the house wall) are fully closed.

This is a brand new NuTone fan and it came with the metal box. I'm pretty sure it's all installed correctly and I don't think it's physically possible to put it in the wrong way.

The plug seems to have no polarity (2 identical flat prongs), but I tried it the other way just for shiggles and it made no difference.

Looking at the fan itself and just applying basic physics seems to indicate that it should spin the other way. As it currently stands, my understanding is that it's scooping air from the outside and pushing it into the middle, while it should be doing the opposite. Am I going crazy here? Pictures show the fan rotation. As I understand, this is a typical centrifugal fan and the wiki article seems to support my understanding that the blades should be curved AWAY from rotation. https://en.wikipedia.org/wiki/Centrifugal_fan

enter image description here

enter image description here

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    Is there a chance that the backdraft flap on the unit itself is taped shut? Could a screw in the ducting be preventing it from opening? Feb 10 at 23:36
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    @AloysiusDefenestrate I can easily open the flap with my finger when the fan is out.
    – Egor
    Feb 10 at 23:45
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    A reversal of thsi fan would be a very unlikely assembly error. Do not modify or tamper with this unit. Send pics to the mfgr and get them in on this case. The mfgr should wnt to know if this occurred in assembly. Feb 11 at 0:57
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    @mikes I don't know, but it wouldn't work anyway since the open center of the fan would then face upwards and air flow would be blocked by the cage.
    – Egor
    Feb 11 at 5:13
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    @JimStewart I already got the new one and it's working fine, air is blowing out of the external vent. I'm not sure what the deal was. The vent is really short, maybe 3 feet with a single bend in it, I can see most of it from the outside, there are no obstructions or anything like that.
    – Egor
    Feb 21 at 3:19

4 Answers 4

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This is a centrifugal fan. The fan blades can be straight, curved towards spinning direction or against spinning direction. This will affect the noise level and efficiency of the fan, but the direction will always be that air flows from the center outwards. The unit you have has forward curved blades, which gives larger pressure and less noise, but is less energy efficient.

Reversing the motor will not reverse the flow of air.

The fan must be installed so that the motor is on the intake side. If that is not suitable for your installation location, you need a different model of fan.

If the outlet of the fan is obscured or has too much backpressure, you may feel a small draft near the motor. This is due to a spinning vortex forming, but the flow will be much less than normal. To check, you can unmount the fan and run it on its own to feel which direction it blows.

A forward-curved centrifugal fan should also have a scroll which is not visible in your photos. This is the spiral-shaped enclosure with an outlet at the widest point. Without the scroll, the fan will not work correctly.

If the fan does not have a scroll (and assuming it's not just a missing part), then it is indeed supposed to be a backward-curved fan and Transistor's instructions on reversing the motor should help. Backward curved fans can operate without a scroll, but forward curved fans cannot.

You can also compare the orientation of the exhaust port to the images on Wikipedia - the exhaust port should be oriented so that the rotation "throws" the air towards the outlet.

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  • When you say "unmount the fan" I assume you mean remove the entire unit from the framing and test the direction of airflow, right? You do not mean to remove the fan from the assembly as it was from the mfgr, right? Feb 11 at 10:58
  • Would there be any point in the OP having someone hold open the outside louvers and using a shop vac or regular vac to establish airflow out, testing for this flow inside, then turning on the fan? Feb 11 at 11:05
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    @JimStewart Yeah, entire assembly. Or just unmount the hose, if that is easier to do - point being to eliminate the possibility that the exhaust pipe is just blocked.
    – jpa
    Feb 11 at 11:49
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    Thanks. I read up some more on these fans and found out that you can have forward-curved blades as you said. I'm not sure why it wasn't exhausting properly, the duct is only 3-4 feet long and I couldn't find any reason why air shouldn't easily flow through. I ended up going for a more expensive model and it seems to be working just fine.
    – Egor
    Feb 20 at 18:10
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That's a shaded-pole motor. It generates a rotating magnetic field by "shading" part of the rotor poles with copper rings (ends visible beside the rotor and there should be a matching pair 180° away). The coil generates an alternating magnetic field on the main poles at mains frequency. The shaded poles' magnetic field will peak a fraction of a cycle later and so introduce a rotational torque.

enter image description here

To reverse the motor:

  • Remove screws (1) and (2) and pull the bearing holder off.
  • Pull the armature assembly (laminated core and coil) off, noting the direction of the label (3) and put it back on in orientation (4).
  • Replace the bearing holder and screws (1) and (2).
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    Reversing the spinning direction of a centrifugal fan does not reverse the flow of air.
    – jpa
    Feb 11 at 9:12
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    @jpa, Ah, but I never claimed it would! I just showed how to reverse the direction of rotation.
    – Transistor
    Feb 11 at 9:34
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    Heh, that's indeed true and I have to agree that the reversing instructions for the motor are perfectly correct.
    – jpa
    Feb 11 at 9:40
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    Just holding a sheet of paper either side should give some indication of what's going on. A (disgusting) vape pen would be a handy way of generating some smoke to give a visual.
    – Transistor
    Feb 11 at 14:14
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    @Transistor Or make some smoke the old-fashioned way. Feb 13 at 19:18
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If the red arrow in your second photo is correct, it is spinning the wrong way and if it's brand new, send it back! The information about reversing the motor is interesting but just get one that works.

While you're waiting for the replacement you should verify the duct isn't blocked, because that would cause turbulence and backdraft. But if the fan is turning the wrong way that should be the first thing you attend to.

If the first picture (installed) is of the bathroom ceiling, it is installed the right way. As you wrote, you can't install it the wrong way (the duct would be in the bathroom). There is no A/C polarity, as you know although the wire-reversing sanity check is something we all might have done because nothing in the real world behaves like in a high school physics class. Taking it apart and observing the blade direction was the golden test.

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I recently had to reinstall one of these. The unit I had was a similarly cheap model and there were multiple place on the housing that were not air tight which let air escape and felt as if the fan were blowing backwards. I used duct tape(the actual metal stuff, not the fabric kind) to seal all of the holes I could see, turned it back on to feel for other drafts, rinse and repeat. This is a picture of the bottom, but I sealed that and all the exposed sides as well. Basically anywhere I put my hand and could feel air moving when the fan was on. bottom of cheap bathroom fan with metal duct tape sealing holes in fan housing

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