YSK because the first hot spell is when most people discover their AC unit is broken.
Several years ago I waited to turn on my AC. This is how I found out about one of the busiest times for repairmen and had to sweat for two weeks. Now I do a test run a couple of weeks before it heats up. Same goes for the furnace at the end of Autumn.
Edit to add: YSK line (Rule 2)
I think it’s generally good policy to run ACs every month or so.
Why? What purpose does this serve? This is a real question, not a snarky comment. What do I not know?
Refrigeration tech here. Don’t run your home AC in the winter unless it was specifically designed for that. I think the person you’re responding too was assuming that because that’s what you’re supposed to do for cars that you should do it for your home too.
In cars the AC compressor runs off the belts so there is a shaft seal that can dry out and crack if the system is not run every so often. On residential AC systems the compressor is run by an electric motor that is sealed inside the compressor housing. There is no shaft seal that can fail. Running your home AC in the winter can actually damage it if your system wasn’t designed for cold weather running. When it’s cold outside liquid refrigerant will tend to condense inside the compressor. When the compressor starts in those situations it will immediately suck in and try to compress that liquid refrigerant. Unless you’re a physicist, liquids are incompressable. So you now have your compressor trying to compress something that can’t be compressed. This is not good for the compressor to put it lightly. The best case scenario here is the compressor manages to force the liquid refrigerant out adding a whole lot of premature wear in the process. The worst case scenario is that something inside the compressor goes pop and it dies.
On systems designed to run in cold weather there will be a little band heater on the compressor to keep it warm and prevent refrigerant from condensing inside it. However that is not the norm for residential AC just because most residential AC systems don’t need it and it would just be a waste of electricity.
Most machinery lasts longer when you run it every month or so at least. For an AC unit there is lubricant in the refrigerant, so circulating the refrigerant once in a while keeps the seals in good condition. It would also keep the moving parts of things like fans from corroding and seizing up.
For vehicles, running the them occasionally circulates the fluids like oil and antifreeze. The oil keeps the moving parts of the engine coated and doesn’t allow things like condensation to rust the internal parts.
Same reason why if you have part time 4WD in a vehicle you should put upwards of 10 miles per month with 4WD engaged (just try to keep it to straight roads). Check your owners manual first, but every 4WD vehicle I’ve owned has recommended it.
This is definitely good advice for vehicle AC because like you said, it keeps the compressor seals in good shape. However it isn’t as important for residential AC systems because they use fully hermitic compressors. There is no shaft seal that can fail. Depending on your outdoor temps, running your home AC in the winter can actually be very bad for it.
When it’s cold outside liquid refrigerant will tend to settle in the compressor crank case that means when that compressor starts up it will immediately suck in and try to compress that incompressable liquid refrigerant. Depending on the compressor type this will drastically reduce it’s lifespan or just straight up kill it. On systems designed to be used in cold weather there will be a crank case heater on the compressor to boil off any liquid refrigerant in the crank case. But if your system wasn’t built with cold weather running in mind then it won’t have one of those.
Today I Learned, don’t run a house AC when it’s cold.