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I recently replaced the capacitor on my external A/C unit. After replacement it started back up and was working fine for day or two but for the last 4 days it has been freezing up every single night after sundown. It's about 85 deg F. during the day and it runs fine and keeps the house at 70 degrees all day long. Air flow is normal and unrestricted all day. But almost immediately after sundown it starts to slowly freeze to the point where I have to turn it off and run just the fan overnight. I turn it back on every morning and it does the same thing. I have absolutely no clue what's causing it since the unit works perfectly during the day when the temps and humidity are at their highest, but after sundown it freezes.

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A low charge can cause evaporator freezing. If your cap was bad that usually indicates an older system. Micro leaks have caught up, probably R22 and it is getting expensive. A 5 ton system will take 4-6 lbs normally to be fully charged but may give you another 5 trouble free years. In most cases the icing will get worse. It could also be a throttle valve that is sticking but it seems more likely low Refrigerant.

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  • I don't know much about refrigeration, but all my reading on SE pushed "low refrigerant" right to the top of my mind when I saw the post. – fred_dot_u Jul 3 at 17:28
  • System is from 2002, so that probably falls into the "old" category. Is there any reason that this freezing would be so isolated to the evening hours? It's running right now in the 88 degree heat with no issues whatsoever. (Turned it on at 5am this morning, it's 3pm now) – The Shoe Shiner Jul 3 at 18:45
  • You should not have to guess what the refrigerant is. It should be listed on a sticker somewhere on the condensing unit. As the outside temperature drops the compressor may be lowering the pressure on the suction side to a value that gives a temperature below freezing. – Jim Stewart Jul 3 at 21:53
  • @JimStewart Is there anyway to control that pressure? – The Shoe Shiner Jul 3 at 23:03
  • As was said above this could be due to low refrigerant level. And if it is, then you have a leak. You need a professional servicing. Have you determined what your refrigerant is? Do you know if you have a fixed orifice or a variable expansion valve? – Jim Stewart Jul 4 at 0:16

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