Is using a higher RPM fan a problem?

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Old 08-02-19, 05:29 PM
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Is using a higher RPM fan a problem?

I'm branching this off from a different thread, as I'm not sure anyone who might be able to help with this question would have read the other thread.

The 1/8 HP, 825 RPM fan in my condenser died. I ordered the exact model as a replacement but they sent me a 1/3 HP, 1075 RPM motor instead. Same capacitor value as the old. As my system had been down for longer than I wanted I just figured it's not under-spec'd in any way, so it should be alright. Apart from using more electricity, is there going to be a problem with what I did?
 
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Old 08-02-19, 06:20 PM
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Condensers get rid of heat.Yours might do it a touch faster. This might lower suction pressure a bit but not enough to make a lot of difference imo. At most might affect it a little more on cooler outdoor temperatures.
 
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Old 08-02-19, 07:40 PM
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I thought we had this discussion. It may not have been with you. One big problem with increasing the motor speed is the fan blades may not be rated for higher speed.
 
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Old 08-03-19, 05:23 AM
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No PJmax , that must have been another person. That sounds a little scary. Would you be able to add a link here to that thread? Thanks.

guyold , could you elaborate on this behavior in cooler temps? Am I putting my system at risk in some way?
 
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Old 08-03-19, 01:32 PM
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The motor amperage draw goes up exponentially as you increase the rpm.

This is because velocity energy is the square of velocity (e = 1/2mv^2) and resistance to flow increases as you raise the speed. It can be to the ^3 or even ^4 when all is said and done.

A 25% increase in rpm can require a 50 to 75%++ increase in required hp. There are fan laws which can be used to calculate, couldn't be bothered to look them up.

Increasing airflow above manufacturer's rating reduces refrigerant pressures, potentially causing operational problems in cooler weather. (evenings, nights)

What happens is the high side/condenser pressure drops, potentially causing under-feeding of the indoor coil, leading to reduced capacity. In extreme cases the indoor coil can freeze - particularly if u already have low airflow across the coil, the return air is cold and it's 70f or lower outside.

You must use the correct RPM motor.
 
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Old 08-03-19, 03:28 PM
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[email protected] Thanks user 10 . I'll have to see if the place I bought it from will do a cross-ship with the correct motor.
 
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Old 08-03-19, 07:20 PM
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The only place I could find online that has the exact needed Genteq part number is an outfit that apparently has a horrible reputation. From what I can tell they have a website, and an eBay presence, both located in Sellersburg, IN but under different names.
 
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Old 08-04-19, 08:23 AM
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You may also want to make sure that they send you a motor with the correct voltage.
 
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Old 08-04-19, 09:31 AM
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I've decided to try a local HVAC place, if I can find one. Given my AC is one among thousands of the same in my area I have to believe someone has the motor stocked.
 
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Old 08-05-19, 01:39 PM
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That's a huge difference. Id try and get it within 50 cfm
 
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Old 08-05-19, 04:14 PM
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I was able to find another supplier with whom I was able to chat to confirm that what they were listing was the correct motor. It will be here on Wednesday. Fortunately the first motor was bought through Amazon so I've already confirmed I'll be able to send it back as a "wrong item sent" with no consequences.
 
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