Question about replacing a capacitor

Hi all…

New to the forum…hope its OK to ask a question…

I am having a problem with my Heil air conditioning condensor unit…unit will click on and hum, but neither the fan nor compressor runs…every minute or so I also hear a louder hum…

Have done some testing…240 volts at the disconnect…checked the contactor and it looks OK…seems to be working…clicks on/off…

Looked on the web for clues, seems to indicate a bad capacitor even though it looks OK externally…mine has a dual capacitor, marked 40 + 4, 370 volt…

I figured I would replace the capacitor but all I can find anywhere is a 40 + 5, 370 volt…the 40 + 4 is apparantly no longer made…the fellow at the HVAC place seems to feel it will be fine…but a place or two on the web warns against any more than a 10% difference…

The part is ordered and wont be in until tomorrow…am I risking catastrophe buy trying the 40 + 5?

Thanks for any help you folks can provide…

10 over is generally the rule 40 is for the compressor the 4 is for the condenser fan motor and no you are not going to burn up the motor by using the 5 but I would just use it temporary

There is also a safety embedded in the windings of the compressor that will open thermally if the compressor does not start when the contactor pulls in.

This can only be checked with an ohm meter and requires the removal of three wires which I don’t recommend you fool around with if you do not know what you are doing

my best advice would be to pay the service call and get a qualified service company to repair your unit

Thanks for the advice…

I checked the contactor again last night and got 240 at both sides, so I know the unit is getting and sending power…since niether the compressor nor fan run, I figure it has to be a common part…and the capacitor seems to fit that bill.

I have the capacitor ordered and will try that…if that doesnt work I guess then I will call in someone…

I tried calling 3 different places when it happened on Saturday…one guy sounded intoxicated and did not want to seem to disturb his Saturday, though he was nice enough I guess…one flat out said he wouldnt come out on a weekened…and the other place wanted an $89 fee just to come out (reasonable) and then “$0 to $400 to diagnose the problem”. That means I could be looking at $500 for him to tell me what is wrong and that doesnt get it fixed…He said “Well, it could be $0 too…”…In my expereience, when someone gives you such a huge range, chances are good that it will be at the higher end of the scale…when I questioned the huge range for cost he got rude and said “Ya know what, I better let you go dude…its gonna be a hot day so you may want to get ready for that.”

After those experiences, I have a bitter taste about calling anyone in to work on it unless I absolutely have to…

I am by no means an expert or even a practiced hand at AC stuff…but they say “necessity is the mother of invention”…in this case, figuring it out myself if I can. FOlks on the web are very helpful and there is a lot of info to be had out there usually…

I figure that if it isnt the capacitor, I have only risked $30. Lucky for me that my unit is older and seems pretty straightforward…I dont know how folks figure out issues with the new high tec computer-driven units.

Thanks again…

The humming thing sounds like the compressor starting.

Often a capacitor will only loose one side (that rated side can be replaced with an individual capacitor).

As for rating, you can go up, but don’t go down.

If you spun the fan blade as soon as you turned it on (before the motor tripped on internal overload) it would have run without the capacitor.

Anyway, sounds like your on the right track. Caps are soft and cheap.
If the compressor will not start after you replace the cap (but still hums) you will need a hard start kit which comprises a start capacitor and potential relay. They have them at any HVAC outlet. Just put the two wires across the compressor cap (per instructions).

A locked compressor may cause the cap to fail.

Thanks! Yet more great info,…

My biggest issue is getting parts…seems like no one around here will sell to the average guy…have to have a commercial account…I lucked out and found this one place…

I may try the hand-start fan thing just to see what happens…

So, if the fan side had died, the system will shut down anyways then because of no fan? The hum doesnt seem to be the louder hum that I normally associate with a compressor but I guess it could be???

I started a cash account at Johnstone Supply years ago just on the word that I was an HVAC student (I was at the time). They never questioned me beyond that. Later I changed the account name to my business. They are a very good, large wholesale supply house. I have saved hundreds of dollars over the years buying items for repair of my own home. Get on their mailing list for their catalog and sales and ours has an open house twice a year. Free food, give aways (really nice prizes) and a good way to kill a morning or afternoon.

Yes the system will shut down on internal over load if the fan is not operating and it will be in a matter of just minutes. One quick way to determine if the compressor is running when you first turn it on provided the compressor is cool and the internal over load has been allowed to reset itself is turn the 220-volt disconnect off at the condenser turn the stat to cool and lower it below set point. Go back to the outside unit and energize the disconnect. If the compressor starts and the fan does not the small copper line which is the hot gas line leaving the unit will get very very hot quickly to hot to hold your hand on it that will tell you immediately if the compressor starts or not.

How long? If it is constant, it’s the magnetic contactor (normal).

If it comes on and off it’s the compressor trying to start.
You will barely hear it hum. It’s trying to start, not starting. The hum comes from the contactor in many cases due to the high amperage draw.

Depending on how hot the compressor gets the hum will occur every 30-60 secs.

It is unlikely that the fan and compressor will fail at the same time unless there is an outside force like lightning, big voltage surge ect.

Low refrigerant pressure will not let the contactor pull in.
High pressure safety can be auto or manual reset. Any red buttons around on the refrigerant lines in the area of the compressor?
Low voltage circuit and t-stat is ok if the contactor pulls in.

Which one of you offering advice is going to take responsibility when the lawyer calls you to court for him being electrocuted because of “SAGE” advice. I am a licensed HVAC contractor for 30 years and have never reccomended a homeowner without specific training, certification and experience attempt to repair dangerous electrical components. Capacitors may still have a high voltage charge even after the line voltage is disconnected.

Better safe than sorry

Ralph Ramsey
TACLA 023462E

Might try a start assist booster from Johnstone Supply and see if the compressor starts.
Just BE SURE the power is OFF before doing anything.

Ralph, Seems you spend more time looking over your shoulder than ahead!

If you practice this in your business, I hate to see what you spend your time doing for your clients. 77 pages of "cover my a$$!?

This place and the Internet is about education. It is quite obvious that this guy is/has been working on his unit already. The only thing in an HVAC capacitor that is likely to cause serious damage is the PCB’s inside. More education may keep him safe.


We are talking about some wires - relays - motors - pumps - valves - pipe and some coolant here

This is not brain surgery

With your background I bet you could train a person to do your job in a week - No beer breaks

BTW – a Cap will not charge unless it is charged from direct voltage and a little bit from just sitting in a box. (Yes, some large caps will even have shorting bars installed for shipping but not in this range)

You have been around how long???

Check your electrical technical facts before posting


Send all of the responsibility COD to me I’ll take UR all

Good David LUV it