A/C Compressor Not Coming On, on hottest days


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Old 06-16-09, 11:31 AM
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A/C Compressor Not Coming On, on hottest days

Okay guys. This started last summer. We have two units. One for downstairs and one upstairs. Both furnaces are in the attic.

Last summer the upstairs unit compressor started not coming on, on the 90+ degree days. We have a lot of those in Oklahoma. At first I thought capacitor because I didn't realize it was only on the hottest days, so I replaced it. After the problem continued and there would be a day here and there when it worked just fine, I realized it was only on hot days this was happening. So now I'm thinking a thermal switch of some kind? I didn't think the A/C unit itself had one but I know the furnace has flame roll out switches. Our furnace has 4. But could one of these be bad and tripping in the heat and cause the compressor not to come on? On the 90+ degree days it's probably around 120+ in the attic (just guessing).

What do you all think? If it could be a flame roll out switch how do I test the switches or figure out which one is the problem? These things can be expensive and I don't want to replace one if it doesn't need it just to do some trial and error to find my problem.

Thanks in advance for any help.

Frank
 
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Old 06-16-09, 12:28 PM
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The compressor does have an internal thermal over load, but there could be multiple reasons the unit shuts down on hot days. Everything from refrigerant charge to dirty coils to age could be an issue.

Have you ever had the unit serviced?

How old is it?

Does the unit provide good air flow?

Could be a brown out situation
 

Last edited by dun11; 06-16-09 at 12:29 PM. Reason: forgot something
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Old 06-16-09, 12:33 PM
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As Dun stated there are several reasons this could be happening. I would 1st start by giving the outdoor unit a good cleaning. Pull the disconnect killing the electric and also kill the electric to the furnace. Then remove the top of the outdoor unit being carefull not to damage any of the fan wires. Hose the coils from the inside of the unit outward. Put it all back together and see how it runs then. If you continue to have the problem a service call is in order.
 
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Old 06-16-09, 12:36 PM
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Originally Posted by dun11
The compressor does have an internal thermal over load, but there could be multiple reasons the unit shuts down on hot days. Everything from refrigerant charge to dirty coils to age could be an issue.

Have you ever had the unit serviced?

How old is it?

Does the unit provide good air flow?

Could be a brown out situation
The compressor itself is about 2 yrs old. The rest of the unit is older. When the compressor was put in was the last time it was serviced. A coils are clean (checked them during the spring), and the a/c unit has been cleaned out and washed down in the spring. It's got great air flow as there is nothing around it at all.

Everything else in the system comes on. Blower motor, A/C motor etc. I just don't hear the compressor kick on and of course it doesn't pull any heat out of the house. Once eve. starts to arrive and the temp outside drops everything is back to normal. The A/C unit itself is shadded most of the day as we have a 2 story house and there is another 2 story next to us. The unit never even gets hot to the touch. That is what made me think it must be something going on at the furnace.
 
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Old 06-16-09, 12:59 PM
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It shouldn't have anything to do with the furnace, when you cleaned the coils did you do it in the manner "mattison" described? Did you use a coil cleaning product or just water?

You say the compressor is two years old and this started last summer, hum

Are you sure the correct cap was used, is the charge correct?

I also think you should have the unit serviced, short of cleaning the coils there are to many variables.

My best guess would be in the refrigerant side of things, tough to tell from here though.
 
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Old 06-16-09, 02:04 PM
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No mention was made (unless I missed it) if the compressor tries to start. Anyway, I really think you need a tech to look it over. Leaving it this way may damage the compressor.
 
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Old 06-16-09, 03:18 PM
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I suspect the system standing pressure is quite high on hot days. I have seen some compressors that don't like to start against the higher pressures and they kick out on the thermal overload which is inside of the compressor. The overload will reset as the compressor cools. Usually adding a hard start capacitor, such as a Supco SPP7, will cure the problem.
 
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Old 06-16-09, 03:48 PM
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I decided to test and see if the signal is getting to the compressor to come on. It is not. However the fan comes on. There is power there (at the block) but the actual turn on signal isn't getting to it when it's hot out. This again makes me think something is going on in attic. Both our upstairs and downstairs thermastats are the same digital honeywell model so I brought the one downstairs to the upstairs spot and plugged it in just to make sure it wasn't the thermastat. It wasn't. I didn't figure it would as it's the temp outside that seems to cause the problem. Any more ideas?
 
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Old 06-17-09, 01:05 AM
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What do you mean by the compressor does not come on? Do you mean the entire outdoor unit does not start or ONLY the compressor will not start? when you say the fan comes on what do you mean? indoor fan or outdoor fan? What do you mean by signal to the compressor? Do you maen the 240 volt power to the compressor terminals or do you meanthe 24 volts that goes to the magnetic coil of the compressor contactor? I have seen furnace limits in hot attics trip. When the compressor does not come on, does it seem like the indoor fan runs all the time?
 
 

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