Central AC Unit Acting Weird
 

I’m laying here on the couch listening to my central AC unit cycle on and off, the thermostat is calling for cooling, the compressor kicks on but shuts off after several seconds, over and over.

Thoughts? Anything easy to check? Or shrug, flip breaker off, call HVAC company?

It is a Lennox, about 25 years old, has been a champ. I imagine it’s a power hog though and maybe I wouldn’t be worse off replacing it.

Is the outdoor unit fan coming on? If the fan is coming on and it's cycling on/off quickly, it's probably low on refrigerant. If the outdoor fan isn't coming on it's probably kicking out on a pressure switch. If the outdoor fan isn't coming on, it's probably a bad capacitor.

Yes, outdoor (compressor) fan is kicking on and quickly shutting off.

I’m with Nick
Probably a bad capacitor, they’re the weak link. I’ve replaced tons of them

You probably already checked, but are the coils free of debris? Been a heavy cottonwood seed season in MN and those seeds will blanket an ac unit. Not sure if that would make it behave like you described or not though.

If bad capacitor, motor wouldn’t start up at all, correct?

If the pressures equalize, a bad start capacitor will not always keep the compressor from starting.
There is typically a start switch. Potential or Current Switch. It has been a long time since I have done this work but it is a good idea to change the start switch with the caps.

Don't let the compressor cycle itself to death. Get some gauges on it and see if there has been a loss of charge.

Not sure if your condensing unit would have pressure switches on it but it could be cycling on a low pressure switch or inherent protection if the compressor motor is too hot. (could be other stuff too...)

Refrigerant cools the motor. Lack of refrigerant = hot motor.

It's a split capacitor which has a side for the compressor and for the fan that moves air across the coil. The fan side can go bad while the compressor side still works. I've been on a lot of service calls due to a bad capacitor where the fan side failed. If you're sure the outdoor fan is working, the system is most likely very low on refrigerant. When it's low on refrigerant, the compressor will come on, run until the low pressure switch kicks it out, turns off until the system balances out and then restarts. But it has to be REALLY low on refrigerant for this to happen. Typically you'll notice the suction line at the evaporator, and sometimes at the compressor, freezing up and once it's finally so low it just cycles off of the LPS.

I don't know exactly what your issue may be, but I think this is called "short cycling". My GF's unit was doing this (in a brand new house) and the HVAC guys came out and fixed it easily.

Again, don't know the cause of yours, but I think the behavior is called short cycling.

System is short cycling, probably
low on charge. Check around the flare fittings or shredder fittings for refrigerant oil, usually an indication of refrigerant leak. Best to put gauges



on the system to check pressures. Without seeing system pressures, you’re just guessing.

My own system was recently low on Freon, but it was not short cycling. Even though I am a mechanical engineer and 'know how these things work', they can be hard to diagnose on your own without tools. Call the man.

But. if it is truly 25 years old you're looking at a new one most likely. That is old age for an HVAC unit.

Below is a link to a helpful tool for deciding whether to upgrade or not. Keep in mind too that 410a has been phased out and coming across a new 410a unit is a bit difficult. It's more likely you'll end up with R32 or 454b unit which, I think, requires a sensor in the furnace or air handler. At a minimum your system is rated at 10 SEER if it's 25 years old.

https://www.seerenergysavings.com/

You should proactively look @ your 25 yr old compressor contactor, the short cycling occurring can cause a failure where the contacts weld themselves together.

Get a high SEER unit and save energy.

You’re correct. There’s a sensor attached to the a-coil for units with the new refrigerant. I think it has butane or propane? A former co-worker keeps me updated with work related stuff.

The suspense is killing me! Was a tech brought in?

Mine is a similarly aged Lennox, lost a circuit board earlier this year and the board in unobtainium. Helpful AC guy bypassed it.

Ha ha suspense. More like sweating. At least we still have the minisplits on second floor.

I'm getting an HVAC service to come out.

Take a long handle screwdriver and spin the outside fan clockwise, if it starts it’s a bad capacitor. Set temp low to keep it running.

Good luck with the repair and please keep us posted.

I plan on doing a Mini-split for my place and keeping the central system as well. Sounds crazy but I wouldn't run them both at the same time. Just keep them both for contingency.

On the subject of contingency.....I have a window unit in my LR that stays there year round. I run it during the summer to supplement my central. Not that my central can't do the job. But so the central doesn't have to run as much - saving wear and tear. Much easier to replace a window unit that an outside condenser unit.

Also my window unit serves as a backup just as your mini-split is doing just now.

Contingency!

I also have a larger window unit still in it's box for more contingency. :)

It's times like this that we have to ask ourselves, WWJD (what would Jebediah do?) Think like the Amish! :D