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Recently installed a range hood and it's not sucking up smoke very well. It's been uninstalled and checked for obstructions and none have been found. The dampers are opening and closing properly. One at the unit the other on the roof top vent. The unit is a four speed fan (450 to 920 cfm). Using a facial tissue I have to move said tissue to within 1" of the range hood to get it to suck up. I'm no electrician but can follow simple instructions after 40 years of handyman work. The only thing left to my mind is that the fan is running in reverse. So once again my question is: If the receptacle I have plugged this unit into is reversed polarity would that cause the fan motor to run in reverse?

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    Most a/c motors aren't polarity-sensitive in that way, but clearly that's not the problem here anyway. Your tissue test would've gone very differently if it was. You have either a poorly-designed fan or a flow obstruction. – isherwood Jun 16 '16 at 19:07
  • How long is the duct? What size duct is it? How many 90° bends are in the duct run? What material is the duct? – Tester101 Jun 16 '16 at 20:56
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    No AC motors are polarity sensitive; they all are designed to function from alternating current (which isn't polarized). – Daniel Griscom Jun 16 '16 at 21:02
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    Most low-end range hood fans have notoriously low suction - my range hood barely moves air even at the highest setting (which is so noisy that I rarely use it. At low setting, any smoke or steam rises up to the hood, then filters out around the hood into the kitchen rather than being sucked away. I've confirmed that the filter is clean, the outside damper is open, and the duct is clean and is straight 8 foot shot out the wall with a single 90 degree bend. – Johnny Jun 16 '16 at 21:07
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Just crappy fan design and misleading ratings (450-920 cfm with no duct at all, and terrible fall off with any normal resistance - or else it's an outright lie.) What is your actual duct arrangement (size, length, number of turns?)

If the fan was running in reverse, the tissue would be blown away, not sucked up from any distance. A/C motors run on current that reverses polarity many times per second, and their direction of operation is not affected by that.

Either return it, or scrap the fan and replace with a quality duct fan designed for range hood service (grease, moisture, heat) - might as well go for quiet as well while you are investing in quality.

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If the fan were running in reverse the test tissue would not get sucked up at all even if it brought right up to the fan. It would blow it away.

Single phase AC motors cannot be reversed with the supply wires. Reversible ones have terminals in the connection box that get swapped. Fan motors like this are usually hard-wired and cannot be reversed by the customer.

Twice in >30 years I have seen motors that were not properly attached to the shaft from the factory. The motor runs at normal speed but the shaft only spins slowly. If you can access the fan itself you can see if light pressure against the fan will stop it from rotating. If the shaft is still spinning then it is bad and will have to be replaced.

If you can't access the fan then I would take the hood back anyway for a replacement. Wire it up before installation to verify it moves enough air for you.

Good luck!

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No, because AC motors don't have polarity. At least not single-phase ones. (Because single-phase power doesn't have polarity, it switches polarity 100 or 120 times a second.). These motors choose their direction by a "starter" circuit built into them.

Just for amusement, I'll mention that three-phase motors are a different deal. Three-phase power is sequenced, first phase 1 peaks, then phase 2, then phase 3, which defines motor rotation. They are reversed by switching any two wires (1-3-2).

Also, many DC motors also don't reverse if you reverse polarity. The armature reverses, but so does the field, so they still go in the same direction. Reversing the field wiring does the trick.

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