A/C not coming on with thermostat


  #1  
Old 06-19-12, 04:00 PM
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A/C not coming on with thermostat

I have an occasional problem with my central A/C -- which are the worst kind! It is a Heil compressor with a Lux programmable thermostat. During normal operation the compressor and fan sometimes do not come on when the thermostat calls, even though the thermostat is blinking the cooling icon, saying it has activated. Changing the set temperature has no effect. What does work is to turn the system off and back on at the thermostat (and wait the normal delay time). Everything then works fine, with the system cycling on and off as the thermostat demands, until days later when the same thing happens.
Am I looking at a thermostat problem or something more serious?
Thanks for any guidance
 

Last edited by hvactechfw; 06-20-12 at 04:09 PM.
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Old 06-19-12, 06:33 PM
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If you are a DYI person, you can check the following yourself. WHen T-stat call for cool, but outside unit is not working, you should check to see if you have 24V to the outside condenser and contactor (the samll wire from your air handler to the condenser, you have to open the condenser panel to do that). If no 24V, push the contactor button by using a wood stick, to see if that start the outside unit. If the unit starts, then you have to find out why the 24V did not get there. (Be careful, high voltage in there). If you do have 24V at the contactor, then you may have contactor or capacitor, or other problems.. post back here to continue....
 
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Old 06-21-12, 01:42 PM
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Thanks. Sorry for delay in responding. It only happened again just now. I thought I would try removing the thermostat front and replacing its batteries first (althought I suspect hey have nothing to do with anything except the program and light). The compressor started as soon as I put the thermostat front back on its mount.

Assuming there will be a next time, I will follow your suggestion and let you know. I might take a look in the condenser panel before that to get familiar with what you are describing ahead of time.

But before I do that, two questions, please: First, is the intermittent nature of this a clue to you? Does it mean something physically sticking sometimes? Or an electronic component unreliable with heat (I THINK this problem may be related to significantly hotter weather)? Second, the furnace fan doesn't come on either. Is the fan usually in series with the compressor? Normally I believe the compressor starts and then the fan shortly afterward. However, I thought in the past they started together.

Thanks.
 
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Old 06-21-12, 02:26 PM
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Wait, you just said the furnace fan does not come on either, that will be a whole different story. If this happen everytime, the ouitside condenser may not be the problem, all your problem may be Thermostat related and I believe you just fixed it already. If by replacing the battery, clean the T-stat cover, and reattached the cover does not fix your problem(another word neither the furnace fan nor outside condenser comes on), then you may have a bad transfomer or control module which is in the air handler right next to the fan. Intermittent problem may be caused by old rusted T-stat contacts, or problem with control module. Not sure if this is related to hot weather, but if you ask me, my answer is no. Your furnace fan should come on same time as the outside condenser.
 
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Old 06-21-12, 04:07 PM
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We'll see if what I have done with the thermostat does fix it. The batteries were down somewhat (1.32v), but there was no low-battery icon. So the batteries do have something to do with the thermostat calling? I thought the system control voltage did everything except the display light and programming.

Remember that I said the fix previously has been to turn off the system at the thermostat then turn it on and wait for through the delay. The fact that that worked does seem to indicate a thermostat problem.

We'll see. I'll let you know. I have a message in to Lux. Perhaps they will confirm batteries/terminals, but my experience is that they usually blame someone else's compunents!

Thanks for the help.
 
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Old 06-22-12, 10:07 AM
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Lux replied with a voice message, suggesting I slightly close (tighten, I suppose) the RC power for cooling and Y compressor tangs on the thermostat panel. So I guess they agree with you!
I have probably effectively improved the contacts just by removing and replacing the panel, so I'm going to leave well enough alone. But if the problem returns I will clean/adjust those contacts.
Again, thanks for he help.
 
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Old 07-26-12, 04:18 PM
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Just to close this out, for the benefit of anyone else trying to follow a thread about thermostat anomalies...
I did the closing-the-wall-plate-prongs thing several times, first with needle nose pliers (little to grab onto) and then by twisting with a screwdriver, as suggested by the Lux guy. Each attempt worked for two or three days, finally down to 12 hours between problems, when once again the compressor and furnace fan would not come on when the thermostat showed demand. But it was sinking in that contact problem was not the issue if I could fix the problem, albeit temporarily, by switching the system off and on at the thermostst without touching anything else. So I asked about resetting the thermostat. (I had to ask, this was not suggested to me.) It turns out there are two small white buttons on the rear circuit board side of the panel that detaches from the wall plate, one for hardware reset and one for software, press each for five seconds and you're done. Nine days now without problems. I would have thought we should all have thought about that first, but so it goes.
By the way all the settings are lost, so best to write them down first.
 
 

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