Thoughts on running AC when it's 55 outside vs sweating in an 80-degree room?

I dealt with this exact setup in my old place with a GE Zoneline unit. Facing west with massive sealed windows, my place would hit 82 while the street was in the mid-50s. I figured that heavy indoor heat load would keep everythign warm enough to protect the system, so I ran it anyway. It seemed fine for thirty or forty minutes, but then the indoor coil turned into a solid block of ice.

The issue isn't the heat going into the return air; it's that the cold outside air drops your head pressure thru the floor. Without enough pressure across the capillary tube, the refrigerant can't feed properly, the coil temprature plummets below 32 degrees, and the compressor risks drinking liquid. After paying for a burned-out compressor out of pocket on that unit, I learned my lesson.

If you absolutely must kick it on for 15 or 20 minutes just to knock the edge off, keep the fan on high—never auto—so you maximize airflow across that coil. But doing it regularly at 57 degrees is asking for trouble unless your unit has a low-ambient kit installed, wich most stock condo PTACs don't.

The coil freezing up happens becuase the condenser outside gets too cold, wich collapses your head pressure and drops your suction temprature below 32. But you don't necessarily have to sit there sweating in an 80-degree living room just because the manual lists a 65-degree cutoff.

In my last place with an Amana PTAC, our building maintanence guy showed me an old commercial trick for shoulder seasons: artificially raising the head pressure. If you can safely reach the exterior grille, blocking roughly a third of the outdoor coil with a piece of cardboard or plastic restricts that chilly 57-degree airflow. That keeps the refrigerant condensing at a normal temperature so the indoor side won't freeze over.

Also, make sure you force the indoor fan to constant "High" rather than Auto. Max airflow keeps the warmest posible air washing over that evaporator coil continuously. I ran my 12,000 BTU unit thru three springs like that on sunny 55-degree afternoons without ice buildup or compressor rattle.

Pop the front plastic shroud off and check for the manual outdoor damper lever. On most chassis like the Friedrich WallMaster or an Amana J-Series, there's a mechanical slide tucked right by the indoor coil that physically opens a vent door to the outside. Crack that slider fully open and leave the blower locked on High. You'll pull actual 57-degree air directly into the room without ever kicking on the compressor. I used to drop my corner unit down 7 degrees in 45 minutes doing that, completely bypassing the low-ambient risk.

Folks forget about what happens to the oil. Back when I worked institutional maintanence in the 80s, running in cool weather meant refrigerant migrating right to the compressor shell. You kick it on, that oil foams up like a shaken soda, washes out of the crankcase, and chews the bearings to scrap in no time.

Your real enemy here is radiant solar gain. In my old high-rise place, I slapped reflective film and heavy lined drapes over the west-facing glass. Knocked the room temp down eight degrees by four o'clock without ever risking the compressor.

Quick update — I pulled the front shroud off to check for that fresh air damper lever. Found the slider and pushed it wide open, but running just the fan barely brings in any cool air at all. For anyone who’s used that trick, is the airflow trough that vent door ussually this weak, or coud there be a clogged exterior screen behind the chassis?