Sporadic AC problem (sometimes blows warm, turning off for an hour resets)


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Old 08-20-17, 05:09 AM
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Sporadic AC problem (sometimes blows warm, turning off for an hour resets)

The history is, an oldish (15 year) AC unit, that's worked fine for a long time. (Lenox elite CB29N 21/26, and I need to go outside when I can for the outside unit, but Lenox also)

Starting perhaps six months ago, it'll blow uncooled air now and then. Fidgeting would fix it, and over time, I learned it wasn't the fidgeting, it was the resting, so now, mostly, turn it off for an hour or two, and it'll work fine again, until the next time. It varies from every few days to a few weeks of working right.

What we've tried:
  • I did have an AC repairman come out. He confirmed the refrigerant wasn't low. It was all working reasonably well the day he came too. He inspected the coils (looked fine, not frozen), the filters, etc. He didn't see anything obviously wrong.
  • The capacitor was working, but old/dodgy. I agreed to replacing it, but in the end, that didn't resolve the issue.
  • AC repairman said something about a valve, I forget the name, that might be the issue.
  • I'm in Florida: The AC industry has everyone convinced that everyone needs a new AC every 8 years. This person was better than most in that respect, he didn't turn on his 'replace' sales pitch too much.
  • When working fine, if I touch the tube from the hot side leading into the house, it's cold. And of course, when not working, it's not.
  • My other unit turns 25 years old this year. That one maybe, I should do something about... :-) (In fact, this one started acting up just this morning, but it's less of a mystery there, what's wrong. but it reminded me to ask about the other unit)
  • Turning off for about an hour: that seems to reset it until next time.

Any ideas or suggestions? Thanks in advance!
 
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Old 08-20-17, 02:22 PM
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Turning off power to a condensor for an hour to reset it sounds like the unit is shutting down on overheat. The one particular problem that comes to mind is failure of the fan motor.
 
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Old 08-20-17, 04:39 PM
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@PjMax... Hmmm, extremely interesting theory... The condenser fan outside seems to blow strong and fine.

But, a big but... Is there anything on the air handler inside susceptible to an overheat shutdown, because by coincidence, the air handler is in an extremely hot attic (130 degrees), and at pretty much the same exact time that the problems started, the big oak tree that kept the same attic in shade fell, so the attic is even hotter. Is there a way to tell?

Thanks
 
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Old 08-20-17, 06:21 PM
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If it still blows air then its not in the furnace. Did the tech test the cap before he replaced it ? Hot side ? The large or small copper line ?
 
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Old 08-21-17, 04:38 AM
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@PJ and @Skaggs, no, definitely the cause is at the condensor, since I can confirm when it's not working, the main line inside will be warm, not cold. PJ's comment was intriguing, because the problems definitely started in the same time frame that the attic got even hotter than before.

And @Skaggs, keeping in mid you're dealing with a moron... So you're saying there's a small copper line? ;-)

The tech was reasonably straight with me, he did say the cap was looking old, but it wasn't clearly failing either. Knowing (I've been in this house all 15 years of this AC) the cap was 15 years old, I figured I'd gotten my money out of the old one and replaced it.

Thanks again!
 
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Old 08-21-17, 05:50 AM
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There are two copper lines connecting the condenser (outside unit) to the evaporator (inside) coil. There is a larger diameter line and a smaller diameter line. When the A/C is running, the larger line should feel cold and the smaller line should feel warm/hot.
 
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Old 08-21-17, 08:33 AM
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Gotchas to you both...

So definitely, when it's working the larger line is nice and cold, and when it's acting up, the large line is not chilled at all. I haven't specifically checked the small line, I will later, and also know (well, I know NOW...) to check it in the future.
 
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Old 08-21-17, 10:40 AM
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One other possibility that could cause an intermittent problem is a defective Time Delay Relay (TDR). Not all outside units have them. My 25 year old Bryant (Carrier) unit does have a TDR and a few years after the system was installed, the A/C would sometimes not turn on when the thermostat called for cooling. Sometimes it would work, other times it wouldn't. After checking the wiring diagram on the inside of the cover on the condenser unit, I saw that it had a TDR. When it wouldn't turn on, I got my multimeter out and checked some voltages. I had 24VAC (control voltage for the contactor) going into the TDR but nothing coming out. I bought a digital thermostat and just jumpered across the TDR.

The purpose of the TDR is to prevent the compressor from turning on too soon after it shut off. Typically, it won't allow the compressor to come back on for 5 minutes after it was last running (turned off). All digital thermostats have that 5 minute delay programmed into them, so the TDR isn't really needed if you have a digital thermostat (as opposed to an old style mercury thermostat). The only advantage that the TDR gives you is that it would protect the system if you had a momentary power outage, as the thermostat wouldn't know about that.
 
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Old 08-22-17, 04:49 AM
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The person who came out also said something about the possibility of contamination in the refrigerant, which at times causes a blockage, that was where the valve whose name I don't remember came into the picture as one of his theories.

Regarding the TDR, I'll look at the schematic, but it is a digital (circa 2001, is it possible just a digital display but an analog thermostat?) thermostat.

Thanks again for the ideas.
 
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Old 08-22-17, 06:54 AM
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If your thermostat has a digital display it is a digital thermostat. The "non-digital" ones use a mercury filled bulb and a temperature sensitive spring. The old round Honeywell thermostats are the most common type of non-digital thermostats.
 
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Old 08-28-17, 08:56 AM
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Update: I'm considering an attic fan, to alleviate the attic heat somewhat. Keeping in mind it's the Condensor unit outside, air-handler in attic, would lower temperatures in the attic affect the operation? (Obviously, a less hot attic would help energy bills and stuff too, but specifically, the sporadic shutdown problem?)

Thanks again!
 
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Old 08-28-17, 09:01 AM
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Do you hear the compressor and the outdoor fan motor running outside when this issue occurs?
 
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Old 08-28-17, 11:01 AM
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The outdoor fan definitely does operate. The compressor part in particular, I need to spend more alone time outside to make sure I can isolate the fan from the compressor sounds. The larger pipe definitely isn't chilled when the problem is occurring.

That's because circumstances are, I only go outside when things are acting up, so my frame of reference on what sounds from where is is challenged. I'll give a better answer when I can. Thanks!

P.S. I was looking for an excuse to buy a mechanic's stethoscope from Amazon... ;-)
 
 

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