This is topic HVAC capacitors - a cautionary tale in forum Film-Yak at Film-Tech Forum ARCHIVE.


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Posted by Leo Enticknap (Member # 534) on 07-23-2016, 12:54 AM:
 
About two years ago, our outside air cooler unit stopped working just after I left for work on one of the hottest Sundays of the year. My wife simply couldn't stand the heat, and so we had to pay an emergency callout contractor $300 to replace a $20 motor run capacitor.

After that incident, I educated myself on domestic forced air HVAC systems in American houses (they don't really exist in England, where I spent the first half of my life), quickly discovered that the motor run capacitors are the thing that fails in them most often, and bought and filed spares both for the exterior and interior units.

Last year we moved house, and since then it has been in the back of my mind that I needed to look in the units in our new house to see what the capacitor ratings were, and, if different from the system in our old house, buy spares.

I never got around to it; inaction that has just bit me (plus my wife, son and cats) in the ass big time.

About two hours ago, the exterior unit went out. It's still 96 degrees outside, despite the sun having gone down over an hour ago. Needless to say, the dual capacitor for the exterior unit at our old house is a different rating (compressor = 40uF / motor = 5uF) to the one here (compressor = 70 / motor = 7.5). Checked with a capacitance meter - the motor side is toast. So, a very hot night (not the nice sort) now awaits us. Thankfully, there's an HVAC spares place three miles away that opens at 8am tomorrow, and I don't have to leave for work until 9.30, so after getting through tonight we should be OK.

According to the news sites, pretty much the entire United States is about to start a week of abnormally hot weather. So, here's a suggestion for everyone: open up your exterior cooler unit and check the rating of the dual run capacitor in there, then look in the inside furnace/blower unit and check the rating of the single motor run capacitor in there, and order spares on Amazon, plus a cheap capacitance meter (if you don't already have a high end multimeter with a capacitance measuring function), so that if this happens to you, you can have cool air back up again in 20 minutes and for $20, not in several hours and $300.
 

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Posted by Buck Wilson (Member # 5885) on 07-23-2016, 01:27 AM:
 
Great post. My unit has a brand new capacitor, and I've got the specs on my desk for a spare I'll buy in the next few weeks
 

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Posted by Richard Fowler (Member # 893) on 07-24-2016, 08:20 AM:
 
Due to South Florida heat we have our capacitors mounted outside the compressor / air handlers to allow a quick change......
 

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Posted by Leo Enticknap (Member # 534) on 08-26-2016, 06:25 PM:
 
F-T's collective wisdom appreciated on the following issue: can I cause damage to an interior furnace unit blower motor by putting a capacitor of too high a rating on it?

Got a call for help from a family friend this morning: interior unit blower motor stopped working. Quickly diagnosed a bad cap (should be 5uF - measured 0.0something). Problem: the only new spare I have in stock is 7.5 (which is what our system uses). She was getting really uncomfortable, and so after quickly Googling and finding this page, came to the conclusion that as long as it wasn't in there for a long time, it was probably worth the risk of putting in the 7.5 I had. This I did, and the motor appears to be working normally.

I've ordered a new 5 from Amazon Prime that should be here by the end of the day, but would appreciate any thoughts on how quickly I should get over there and put it in, and/or if we should really look at replacing the motor as well. New motors of that model number go for $150ish online, so even if we do end up needing to replace it, we're probably still ahead relative to the cost of getting a local contractor to replace the cap, with the added advantage that she had cool air back within one hour than the minimum of 5-6 it would probably have taken anyone local to show.

The impression I get is that using a higher than necessary rating will damage the motor if left in long-term and in the short term will result in increased power drain, but should be OK to get you out of trouble for a day or two.
 

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Posted by Louis Bornwasser (Member # 3063) on 08-26-2016, 08:48 PM:
 
My experience is what you expected. Some variation is OK. I think you are well within the range of acceptance. Remember that even the 5mfd is plus minus some percent; perhaps 10-20%. 7.5 should be permanently OK. I wouldn't change the motor unless it shows signs of badness.
 

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Posted by Leo Enticknap (Member # 534) on 08-26-2016, 11:17 PM:
 
Many thanks, Louis. The 5 cap from Amazon arrived just now, but a very busy work weekend is about to start and it won't be easy to get over there before Monday. Good to know that I'm not rapidly frying her motor by having the overrated one on there, which was the one little niggle in the back of my mind.
 

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Posted by Jim Cassedy (Member # 4115) on 08-26-2016, 11:59 PM:
 
I've run into this problem (having to make a temporary cap
replacement until I could get an exact value) and at that
time I pulled some info off the internet that I still have
in my computer. I don't have the link, but here's what I've got:

First of all- - I know you're smart enough to know that the
replacement cap needs to have a voltage rating equal to or
greater than the one that failed.

Generally speaking, if you need to make a temporary substitution,
as a general 'rule of thumb' it's better to use a cap that's slightly
higher (in MFD's) than to use one that's lower than burnt out one.

A 'larger' START capacitor will usually have no effect on the motor start,
but one that is smaller will cause the motor to take longer to come up
to speed, or in worst case, it won't start at all.

A slightly 'larger' RUN capacitor will usually not have any major effect
on motor operation, although the website didn't give any specific figure
as to what a "slightly larger" value was.

Depending on the combination of motor & cap, it may, or may not,
have some effect on overall motor efficiency

The website recommended not using a run capacitor with a lower MFD
value than the 'bad' one your are substituting for, for any length of
time, since lower values are more likely to lead to unstable operation
and possible overheating.

Apparently there is some 'wiggle room' in the cap values and some
variables depending on the exact motor design and load conditions,
so all this should be taken as "general info" for substitutions until
an exact replacement part is found.

Remember; I'm a projection-guy, not an HVAC tech!
 

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Posted by Leo Enticknap (Member # 534) on 08-27-2016, 09:47 AM:
 
Thanks Jim - this squares with everything Prof. Google and others have told me.

quote: Jim Cassedy

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First of all- - I know you're smart enough to know that the replacement cap needs to have a voltage rating equal to or greater than the one that failed.

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