Suction line freezes up at sundown, regardless of outdoor temperature. We are stumped.

• Approx. 1000 sq. ft. house
• R22 units
• 2-ton 13 SEER Weather King condenser unit, installed in 2012
• Evaporator coil & furnace are approx. 36 years old. Belt and blower motor have been replaced over the years. Filter is changed frequently and is kept clean.
• Furnace is 49,200 BTU.
• Evaporator coil is clean as far as we can tell/see. It is not easy to access, but everything we can see is clean.
• Unsure of evaporator coil size/rating, but we think it is probably 1.5 ton and the tech guessed it was around 10 SEER, I think.
• No visible sign of leakage indoors or out
• Fixed orifice/piston metering device - original to the unit as far as I know, so 36 years old
• If there's a filter drier, it's factory installed and inside the condenser unit, but I've not seen one
• Condenser coils have NOT been kept clean. A couple of weeks ago was the first time they were cleaned, and there was a thick blanket of dust and grass around the whole thing. It's spotless now, but this has not seemed to make a difference in any symptoms except perhaps noise.

First 5 years the new condenser unit worked great. Last year, the condenser fan was turning slow and we determined it was a bad capacitor. Replaced the capacitor.

Last year, the AC started to freeze up at sundown every day, regardless of outdoor temps. It might be 88 degrees outside and it would still freeze up as soon as the sun began to set. It was almost the end of the season and we limped through it. Two people in the house also felt the condenser was much louder than it had previously been, while one person felt it wasn't.

This year, same thing. Put the gauges on it and suction pressure was around 20. Obviously there is a leak somewhere, but given that it's taken this long to show up (and that we can't see any sign of leaking at either end) we assume it's in the pipes and probably small. We had some R22 on hand, so we added some. I don't have all the numbers you guys are going to want, but I do know that when we got the suction pressure up to 47, the head pressure was running around 225 and we had a superheat of 11.4 (which the book tells me is a little bit low, as normal range is 12-15).

Also, electrician checked amps and connections, etc. and everything seemed to be as it should be. Compressor was reading fine.

We stopped charging when suction pressure reached 61 PSIG. Problems persisted. Suspected air in the lines, so we called a tech, who did his thing and agreed. He recovered the refrigerant, pulled a vacuum, charged with new R22. Said we had overcharged it because the air was giving us inaccurate pressure readings.

Tech was about to leave right at sundown, and unit started to frost at the suction line. The only reason he could think of was that the indoor and outdoor units are different SEER, but he kept repeating that that doesn't seem to fit since they were mismatched on installation, and worked fine for 5 years before problems started. Head pressures were still running higher than expected (again, I don't have the specific numbers).

Fast forward to now. It's 90 degrees outside, sunny, and humid, and the frosting/freezing is now happening during the day too. I can run it maybe 30 minutes before it starts to frost up outside. (It takes a lot longer before it will start to frost or freeze inside.)

Obviously, I'm aware that the indoor unit is well past its expected lifespan and the first thing anyone is going to do is tell me to replace it. I am not interested in replacing it at this time, for many good reasons that I won't go into here.

What I want to know is what could be causing this. I have scoured the internet and found a few people with similar issues, but no resolution. The troubleshooting chart in the installation manual for the condenser unit doesn't even have an entry that matches low suction pressure and high head pressure, much less a slightly low superheat. I thought it might be a restricted metering device, but from what I can tell that should cause high superheat, which we don't have. I thought it might be a failing compressor, but the electrical readings were normal.

EVERYTHING says if the suction line is freezing, it's either low airflow or low refrigerant. The charge has been corrected, so it shouldn't be low refrigerant. The fans & motors seem to be operating properly, the belt is brand new and adjusted correctly, the evaporator coil is as clean as it's ever been, the filters and condenser coils are all clean. I can't seem to find where there could be improper airflow.

Is there ANYTHING else that could cause these symptoms??

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