I am told that my whole house fan motor is no longer repairable. It is a 1/2 HP Robbins & Myers dating from 1966 and weighs about 51#. The Hunter fan documentation says the motor is the right size for the fan, which is 42" in diameter. The motor repair place claims that to get the necessary power in a new motor, I should upgrade to 1 HP. Why would that be?

  • 1
    Are you asking us why someone else said something? Seems like a question for them.
    – isherwood
    Aug 4 '21 at 23:49
  • Your wording suggests that you've had to repair the motor previously, implying that it's failed a few times already. You might be able to answer your own question based on the failure mode(s) of the motor (windings, bearings, capacitor if equipped, starter+start windings if equipped).
    – HikeOnPast
    Aug 5 '21 at 17:55

To my knowledge motor HP has not changed. I would go with the manufacturer's recommendations, the original motor lasted over 50 years. If you go to a 1HP motor the shaft size will probably change. You also need to consider the electrical side of this. Is the fan electrical feed large enough to operate a 1 HP motor? Are the thermostat and switches rated for that HP they have to be rated for the 1 HP rating you will need. A 15A breaker is OK for 1/2 HP however a 20A breaker is borderline for a 1HP motor. Breaker is sized at least 1.25 times the full motor load. Is it direct drive or pulley driven. With the above I would ask them to explain why the larger motor and what would the advantage be. You already know you get a lot of life from a 1/2 HP motor.


When I had to replace the fan motor on my whole house fan I increased the HP to the next size which in my case was a 3/4HP motor. My professional opinion is that most older motors would actually produce more HP than advertised. Also, to make my fan more user friendly, I installed a 2 speed motor that had to have a 2 speed switch and wiring change that would yield a lower air flow at a much reduced noise level for say when running the fan at night when you are trying to sleep. My wife was pleased and thanked me for the exhaust fan upgrade.

  • You did this but provide no evidence that your wiring was up to code for the rated power Aug 5 '21 at 16:54
  • Pardon me, @Carl Witthoft, I didn't know that I had to write a complete essay on the change I made. I was just telling the OP what I did and that it performed well. And why yes, I had a registered electrician do the work so I knew it was up to "code" in my area.
    – d.george
    Aug 5 '21 at 18:34
  • @dg when you throw in "professional opinion" then one expects some professional substantiation. All you had to say was that a licensed pro did the install, and state whether rewiring or a change in breaker was required Aug 5 '21 at 18:38

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