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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7542
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My HVAC stopped working. Control unit works. Capacitor stores

Resolved Question:

My HVAC stopped working. Control unit works. Capacitor stores enough charge to make huge spark. Heat pump outside won't turn on. Telling the control unit to run only the blower motor doesn't work. Never smelled a burned motor. It's 26 years old though... I went ahead and bought a new motor online but didn't match the capacitor uf. Mine came with a 10 and the new motor is a 5. Can I just get a 5 capacitor for my system or will that screw things up? and is this problem even my motor?
Submitted: 6 years ago.
Category: HVAC
Expert:  Phil replied 6 years ago.

Hello, yes you can get a 5 uf run capacity for the fan motor the voltage rating is important. thats the volts out of the capacitor... it can be very high 370 to 440. try to match what the motor mfgr specifies. you might get by with 370v on a fan motor though (its low torque)


If the compressor doesn't run try using a well insulted screw driver to push the contractor closed, see if it runs.


... in that case...if it runs then the problem is in your control circuit. If it doesn't run, check power to it. If thats fine, kill all the power, triple check that its OFF, then triple check it again.


Then pull the wires off of the compressor and check using an OHM meter to ground...any read at all = bad compressor. then check OHMS C to R, C to S, and R to S. R to S should add up to total reads you get on C to R and C to S


any infinite resistance reads indicate an open internal over load.. if the compressor is cool and the internal overload is open... its toast.



these will be low resistance reads, in the 1 to 7 ohm range each. If thats good and the compressor just hums when you shove in the contactor, check its run capacitor again... if it sparked it was probably OK... (watch out however those can explode)


Do all that and get back to me.







Customer: replied 6 years ago.
ok well first thing's first, and I wanted to see if it was the blower motor. So all of my replacement blower fan spec's match up to the old fan except for the new one is a 5uf capacitor and my old one is a 10uf... So I'm not sure if you were saying it's ok for me to get a new 5uf capacitor to match my new blower motor, or if it's ok to try my old 10uf capacitor on my new 5uf motor... I also have a new 7.5uf capacitor (ordered the wrong one) All of the other numbers match (voltage, etc)
Expert:  Phil replied 6 years ago.

Don't use the 10uf capacitor on the new motor if it calls for a 5uf capacitor. Doing that will cause a greater phase displacement between its windings that it was designed will run, but it will run hot (too warm)..and that can damage its motor windings.


Capacitors are cheap. I would get the 5uf it calls for.


Keep me posted, if we get this 26 year old heat pump running it might be a new world record. 10 years is ancient for a heat pump.. 26 years is unheard of. Its worth going after and if you have managed it...the feat speaks very highly of you.



Customer: replied 6 years ago.
I got my matching capacitor to the new blower motor for my inside unit. I was able to manually start my outside heat pump by pushing in the contractor like you said, but I don't know if that's the problem. My thermostat gives me an option to turn on the blower motor only, and that wont work. Nothing works. I took my thermostat off (it's battery powered) and tested it with a multimeter and saw no voltage changes when turning the fan, heat, or A/C on.
Expert:  Phil replied 6 years ago.

Hello is good news that you can run the compressor manually.. that means its a controls problem... if you changed thermostats that is most likely where the problem is.. these don't always replace wire for wire... One stat fits everyone these days, but you have to add jumpers etc to make them work and they cover that on their wiring diagram...however... each original system my have anomalies that make life difficult even for a pro, on scene that can see what you have.


we should be able to sort it from here though.



Starting with the blower not coming on...that is actually good news. It points to no power at the thermostat. Check voltage at the 24v transformer, red to should read 24v... if not thats the problem.


If you get voltage at the transformer, use a jumper wire and jump red to green at the thermostat. that should bring the fan relay on... if not its a bad stat, bad relay or wire not connected. Let me know how those checks work out and we will go from there.




Customer: replied 6 years ago.
ok well i haven't changed the thermostat... so i can't say with definity that it's not the problem, but it is new compared to the HVAC... probably only 5 years old...

I'm assuming the transformer is the all silver-metal thing about the size of a fist. It's got an orange and a white wire in one end and a black and red wire on another end. how do i test voltage?
Expert:  Phil replied 6 years ago.

Hello, first I notice that time on your question is about to may want to extend that.


To test the transformer put the meter on V for voltage, not mV, or Vdc, but *Vac' and set it in the mid range, then test it in a wall socket and see if you can see it reading 110 volts or close to it.


If so, you are ready to rock with the meter.


Check across white and Orange and let me know what you get. (should be 24v)

Then check across black and red and let me know (should be 110v)




Phil and other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
sorry but i don't know how to extend the time... but i'll only accept from you since you're doing a good job walking me through this... my multimeter is this and i don't see that option, but i know how to probe a wire... guess i'll just have to take off the caps that are holding the two wires together to check....
Customer: replied 6 years ago.
ok i had 122 going in one (the white and orange) and like 2.5 for the black and red... so this would be the problem then? I have 122V going to my capacitor when the main power is on, regardless of whether or not i'm trying to run the system
Expert:  Phil replied 6 years ago.


Hello again, thats not the color code standard but the transformer appears white and orange are the 120v power supply,... if that read was 122v that was close enough.


However in that case the other side, black and red, should have read 24v... instead it read 2.5 which indicates you misread the meter, or the voltage on that side is bad...and indeed,. 2.5 volts where you need 24v would cause the problems you have.


But its unusual to see a partial failure like that... so... lets be sure you are reading the meter right on the 24 volts.. go test it on some other 120 volt sources around the house and see if the transformer reads 1/4 of that scale, or 24 volts...not just 2.5 volts... thats a suspect reading.


Another issue is the color is the reverse of the color code standards for such we need to double checks all of those issues.



Customer: replied 6 years ago.
ok well the orange and white wires that were holding the 120V charge,
the orange was spliced with a wirenut to a red wire, and
the white was wirenutted to another white wire.

the wires with the low voltage (probed each wire again, fluctuated around 2.4 - 2.5)
the black one is wirenutted to a black/blue wire and
the red is wirenutted to a black/red wire.
Expert:  Phil replied 6 years ago.

Hello again, this is what we call in the controls business a 'hair ball' that is all sorts of wiring cobbled together not following any of the industry standards.


When I run into these in the field I remove all of the affected wire and much of what it connects to, even if that part is standard....out. Then rewire from scratch. That works.


I will 'opt out; of this question, leaving it open for another expert who might be able to assist. I will contact a few directly, If you don't here from someone this morning (Saturday AM) I recommend you call a local technician who can sort this our for you hands on, Furnaces can be significant hazard if not attended to property and this one is need of such help, beyond the wiring issues there are others as well that a technician on site can address,,, make sure you get a good one,, check references beforehand.


Good luck, Phil

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