A/C: fan spinning, compressor not kicking on. Cause?

[Beartracks]

My house is warming up and I realize I haven't heard the compressor kick in all afternoon. The furnace blower is still blowing, and the A/C's condenser fan motor is still working, but the compressor isn't doing it's job. For DIY yourself purposes, what might be the culprit?

- Blown fuse in the disconnect block? (Or would a blown fuse result in NOTHING working?)
- Start/Run Capacitor? (Or would capacitor failure result in the fan motor also not running?
- AC Contactor?
- Something else?

If it's the compressor itself that's actually gone bad, I might just need to get a whole new A/C since this one is already nearly 25 years old. But I'm wanting to make sure there's not something else (something cheaper) that might account for the symptoms I'm seeing.

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[NoMoreRepugs]

1. Ck drain

[Beartracks]

3. In the furnace blower where the evaporator coil is?

Also, wouldn't a clogged drain result in the entire A/C unit not coming on (that is, the condenser fan also wouldn't be spinning)?

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[Beartracks]

6. Update: I guess the whole unit ISN'T coming on now.

Well, whatever problem occurred earlier must have been while the unit was running. The compressor wasn't on while the condenser fan still was -- but now that I've switched it off at the thermostat, and then tried to restart it, NOTHING happens.

Does this shed any additional light?



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[DUgosh]

2. Could be thermostat

[Beartracks]

4. How can that be determined?

My thermostat displays a little snowflake when the A/C compressor is supposed to be on -- and that little snowflake is indeed lit up, so my thermostat at least THINKS it has properly signaled the system to kick on.

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[Beartracks]

7. Update: Well, it seems the whole unit ISN'T coming on now.

Whatever problem occurred earlier must have been while the unit was running. The compressor wasn't on while the condenser fan still was -- but now that I've switched it off at the thermostat, and then tried to restart it, NOTHING happens.

Does this shed any additional light?

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[getagrip_already]

5. Could be the capacitor...

Lots of experts on youtube (ahem), so do a search there but take it all as entertainment first. But there is some good info if you look hard enuf.

Either way, unless your are good with electrical systems, you will need a pro to look at it.

[Beartracks]

8. Hm, you may be right. It seems now that the whole unit is down.

Whatever problem occurred earlier must have been while the unit was running. The compressor stopped while the condenser fan continued running -- but now that I've switched the A/C off at the thermostat, and then tried to restart it, NOTHING happens.

Does this shed any additional light?

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[Hassin Bin Sober]

13. You can check the capacitor visually to see if it's bulged out on the top.

Don’t touch it - even with the power off because a capacitor holds a charge.

You can remove and check the capacitor with a multimeter as it could be bad without a noticeable bulge (after shutting off power and discharging the capacitor)


This guy is pretty good:

[moniss]

9. It may not be an

electrical component failure per se. Your A/C system has pressure/temperature switches that protect the system by shutting it down/keeping it off in the event that your pressure in the system is beyond a predetermined range or if the internal system temperature is too high. These are not things for you to try and check. The service technician can check the system easily. Those switches can easily go bad over a 25 year period. Either way you should call the service tech. If you do need a new compressor or system you can likely count on it being way more energy efficient than what you have now.

[orthoclad]

10. If you're thinking of replacing it, go for a heat pump.

The new high-efficiency ones work well below freezing for heating, and they'll handle the cooling as well. And under the IRA, you get a large tax credit for it.

I replaced both an oil furnace and an old AC with a heat pump, and it's saving me money. Especially with solar panels in the mix.

Get an estimate.

[flying rabbit]

11. The capacitor is usually the issue on this

But your later post seem to indicate other issues.

[hippywife]

12. It could be any of those things.

Ours is about 20 years old. The best place to start is to have someone with knowledge of electronics and a meter test it out for you.

There have been times when ours has gone out that it has been just a fuse, but the last time about a month ago, the husband checked the capacitor and it tested as bad. He got a new capacitor and installed it. It ran fine, but the unit was getting hot and it wasn't blowing cool air inside while running. He ended up buying a new motor and installing it, along with the previously purchased capacitor (since it too was still bad) and that fixed the problem.

In the last 5 years, he's also replaced the fan motor and some sort of controller circuit in the furnace.

[KY_EnviroGuy]

14. Check your HVAC control circuit board.

Typical unit only shown here, usually located inside the furnace compartment:



These devices detect system faults and can prevent the compressor from running (therefore, contactor not pulled in). It should have either a flashing LED as shown here, or possibly a display showing a fault number. Refer to your user manual for a list of faults. Most faults are for furnace (heating) operation, but yours could have some AC (cooling) protectives as well.

Other than that, if the feeder fuses are good, there may be a resettable overload device on the outdoor unit that has tripped, sometimes with a red reset button. It would be very informative to know if the compressor contactor is energizing when the thermostat calls for cooling. It's possible the contactor or thermostat has a problem. You should see 24Vac on the contactor coil during cooling demand. Be very careful around that device because it will also have 240Vac on it as well.

If the compressor capacitor or windings are bad, the compressor will not start and usually will blow a fuse in the disconnect box, or trip an overload device as mentioned above.

Best of luck!

KY