unusual AC failure mode (home, not car)

[friendly_jacek]

I'm a bit stumped. You turn central AC on and it works OK the whole cycle, but when it cycles off and back on again, only warm humid air comes from registers forever. Condenser fan works but compressor is not running. Then I turn the system off again, restart and it works great but only for one cycle. It has been doing it for a second day and it's too dark to troubleshoot after work. Could low refrigerant do it?. I had a slow leak many years ago and fixed it, but don't remember doing the same thing, just weak cooling then. This is old unit with no fancy controls, but there is a control board that shows OK light on. Contactor and caps look OK and were replaced in the last 1-2 years. Starting relay replaced this spring. I guess it could be that. Thanks! PS: there is no need for AC this time of year, but this year is clearly not normal.

 

[JohnnyJohnson]

Is it a heat pump or just an AC?

 

[Eddie]

Low on Freon would be my guess. When the system turns off after the first cycle the pressure side is to0 low and the sensor stops the compressor. When you restart it later the pressures have equalized somewhat and this allows another cycle. Ed

 

[friendly_jacek]

It's heat pump alright. I'll check state of charge tomorrow.

 

[StevieC]

Could it be a faulty reverse valve or the thermostat not holding it in the right mode?

 

[Kuato]

Could be lots of things...this happened to me and it was a bad compressor - it would work fine, but got hot and for the second compressor cycle it would not turn. Time for an HVAC guy to take a look; I hope you don't need a whole new heat pump!

 

[Highboy]

Could be a bad start/run capacitor. Easy to change. You can get a new one on Amazon or eBay for 10 or 15 bucks. Lots of YouTube videos on how to do it.

 

[friendly_jacek]

Originally Posted By: Highboy

Could be a bad start/run capacitor. Easy to change. You can get a new one on Amazon or eBay for 10 or 15 bucks. Lots of YouTube videos on how to do it.

Thanks, but I replaced caps on this unit a few times and it's not that. Looks like it's low on refrigerant, but I'm not 100% sure as the temps are jumping around and I misplaced good thermometer. I went ahead and ordered R290 and will see what happens. Thanks all for replying.

 

[friendly_jacek]

Originally Posted By: Highboy

Could be a bad start/run capacitor. Easy to change. You can get a new one on Amazon or eBay for 10 or 15 bucks. Lots of YouTube videos on how to do it.

Actually, you were righ and I was wrong. It was a start cap that was partially failing (weak). This is what happened. I found my good thermometer and realized it was only slightly low on freon. I guess the stop leak I used 7 years ago worked. Then I swapped start relay and no change. Then I swapped start cap and it fixed it. It looked fine and it was only 6 months in service. Cheap Chinese junk I guess. My unit keeps pressure up when thermostat calls for end of cooling. Then compressor could not start against that pressure with weak cap. Turning the system off equalize pressure and helps start compressor. I learned something new. I like fixing AC and this is why I keep those 21 year old units (2 of them almost identical).