Central AC Problem

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Old 05-27-07, 10:02 AM
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Central AC Problem

I installed my Central AC system 10 years ago and it has worked great until yesterday when it stopped blowing cold air. My first thought was I had a leak and my freon was low. However, an inspection of the system showed the High Pressure line input to A coil was covered in frost, Air volume from vents was very low, Compressor fan was blowing, and it appeared that the thermostat was working fine.. I turned system off and replaced a completely blocked air filter and let the high pressure line thaw out. When I restarted the system about 2 hours later the vent was pushing full air flow at 56 deg F. I also verified the proper operation of the dehumidifier pump. However, the system experienced the same freezing problem a few hours later. Any suggestions?
 
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Old 05-27-07, 10:50 AM
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Freezing up is usually caused by low air flow or low freon. If air flow is good, filter is clean, and coil is clean, I would suspect low freon caused by a leak.

When you say high pressure input line, I'm not sure what you mean. The high pressure line or liquid line, is the smaller diameter of the two. I'm assuming you meant to say the suction line, the larger of the two.
 
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Old 05-27-07, 11:33 AM
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Freezing

With the line being frozen, no air thru the vents, & the air filter being completely clogged, I suspect you did not give the indoor coil time to completely thaw. This will cause low air flow & subsequent re-freezing. I tell customers to give it at least 8 hours to thaw. If you can remove the panel in front of the coil, this will help. Another possibility is the coil is dirty. Look at the inlet side of it after it thaws & dries.
 
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Old 05-27-07, 05:42 PM
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Grady, thanks for the response. After my investigation and reading the responses to my original post I think my problem started with the clogged air filter. That puppy was covered with a very thick layer of dirt and dust that could be peeled off with your fingers. I'm surprised any air was getting through at all. This explains why my system froze and why my air flow dropped to virtually zero. I left the system off for about 5 hours today and when I turned it back on I had good air flow at 54 deg F. Is that to cold? My concern is that the A coil is only partially thawed out and I am forcing air through coils that have ice on them. I guess the easy way to confirm is to remove the access panel and look. The only problem is there is no access panel. This also presents a problem If I wanted to clean the coils like you suggested. It appears that my only options would be to remove the Humidifier unit which would provide a relatively small access hole, or cut a new access panel in the side of the air plenum. If I gain access how do I clean the coils? Wire brushes, vacuum, etc.. If I have good air flow do I still need to open up the system? It looks like a big job at this point. I installed the unit so that there is no vertical clearance. It touches the bottom of my floor joists and now has gas lines, AC lines, air ducts and other obstructions that would prevent the removal of the upper portion of the air plenum.

I also wanted to confirm what Jim-Connor said. The line that froze was the larger low pressure line. Thanks for the correction Jim. I was wondering if I still need to be concerned about a leak and low freon level after measuring the cooling air at 54 deg F?

Thanks to all for the help.
 
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Old 05-27-07, 08:04 PM
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Turn off the ac, flip the fan switch on thermostat from auto to on and let it run for another 4-5 hrs. to be sure it is thawed.

You want to measure return air temp. and supply air temp. rule of thumb is 15-20 temp. difference. 54 degrees is fine if your return is ranging from 69-74 degrees. I'm not sure what filter you use, but if you use the cheap one's your coil is more than likely dirty. Try it and see what happens.

good luck
 
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Old 05-28-07, 07:34 AM
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Thanks sgthvac, I have the fan running now and will try that for the next 4 hours. This is in addition to the 10 hours the system was off last night. If that doesn't do the trick, I guess I will need to rip into the air plenum to gain access to the A Coil.

As a side note, last night I noticed gurgling sounds coming from the AC coolant lines. However, this was when the air flow volume at the vent had noticeably dropped. Could this be a sign that my system is low on freon or is this a result of the frozen A Coil? And yes, I did have a 20 deg delta between ambient air and vent output.
 
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Old 05-28-07, 08:15 AM
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A-Coil

With the filter being that dirty, I think it is worth a look at the inlet side of the coil.
 
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Old 05-30-07, 09:05 PM
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I wanted to thank everyone that helped me with my AC problem. I thawed out my A coil with the help of the circulating fan and since that the system has been working great for the last several days. I was surprised when I recently learned that other people I know have experienced the exact same problem. I guess preventive maintenance is very important. Change Those Filters Before They Bite You Where The Sun Don't Shine!
 
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