DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > Lennox 50/7.5 capacitor question

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Old 07-14-2010, 08:40 AM  
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Default Lennox 50/7.5 capacitor question

Repairman replace the outside Lennox 50/7.5 capacitor with a 50/5.0 one as well as the fan motor. The old capacitor registered 4.8, is this 5.0 really enough for this unit? Repairman says it is.

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Old 07-14-2010, 11:23 AM  
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Though I'd prefer to have the proper mf, It will probably work just fine. You didn't say what size unit or hp of fan you have. Sounds a little larger...4-5 ton?


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Old 07-14-2010, 12:22 PM  
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Originally Posted by suzy View Post
50/7.5 capacitor with a 50/5.0 one as well as the fan motor.
The old capacitor registered 4.8,
is this 5.0 really enough for this unit?
It becomes a question of likelihoods. The guy might have gotten lucky.
If the 7.5 had a tolerance of +/- 50%, the value can be between 3.8 uF and 11 uF 95% or 99% of the time, depending on the quality control of the capacitor manuf.

Probably 4.8 uF does not work if his diagnosis was correct. Without looking up the Normal Distribution curve and all that
about 1/6th of the 5 uF caps that he picks at random will work in this application.

When you add in that the 5 uF also has a tolerance, this 1/6th becomes less.

He might have gotten lucky. And if that cap drifts off of 5 uF in the wrong direction it will fail sooner than one whose value is centered on 7.5 uF.

It was probably late in the day or he was tired.

On second thought -
In a way, there is a contradiction here. If the design center value is 7.5 uF and he declares 4.8 uF to be unacceptably out of tolerance then he should not turn around and replace the 4.8 with a 5.0. He might as well have left the 4.8 in place.

So my first objection was on the grounds of bad practice, and my second is that he is contradicting himself. I hope you at least got a good price.

Last edited by Wuzzat?; 07-14-2010 at 12:41 PM.
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Old 07-14-2010, 03:37 PM  
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It depends on what the cap requirements for the new motor are and it could be that the old motor was still good and just had a bad cap.
However, on most residential systems, the fan motor will come on just as soon as the compressor does so the cap requirements for the fan/compressor combination must be accounted for otherwise there's a good chance you'll burn up one or the other (fan or compressor) in the long run.
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