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I noticed this article about [Good to Run A/C in winter]. Well, it used to be true that it was a good idea to run your AC every few weeks in the winter, just to keep the seals in good condition. I suspect this may no longer be needed due to improvement in materials, but I'd like to hear from someone who knows.

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Yes, it is still a good idea. Luckily, whenever you run the HVAC system on the "Defrost" setting, the A/C compressor is activated. As a result, you have probably been periodically running the compressor even if you were not aware of it.

  • Not on all cars. The Vauxhall Agila A doesn't do that. On most buses, you almost always have to turn the passenger AC on before you can turn the driver's AC on to defrost the windscreen. Can't use an huge 560-650 cc displacement compressor to only cool such a small area. – Al_ Apr 16 '18 at 10:39
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Nowadays the compressor cycles on and off even when the A/C is not in use. This occurs all year long to maintain system pressure and maintain the integrity of the seals. In the old days and I am old enough those days. Because of the fact that the A/C wasn't periodically run the refrigerant and the oil would pool in bottom of the condenser core. The seals would dry out and let moisture in. The moisture would react with the refrigerant and the oil which is hydroscopic absorbs water and turns to acid and rots out the core.

  • 2
    Do you have any reference on this? Do all recent vehicles do this and if so, starting from what year? – Ives Sep 29 '17 at 7:46
  • I can only say that the vehicles I have owned have do so since 2001 ran the compressor year round. The car with the rotting was a '80s vintage Plymouth – Old_Fossil Sep 30 '17 at 5:18
  • Until you have positive pressure in your system, nothing can get inside it from the environment. That's the same reason as for why a system has to be regassed either through the low side with gaseous refrigerant and with the compressor operating, or (the best option) through the high side with the compressor off, liquid refrigerant, and after pulling a vacuum. – Al_ Apr 16 '18 at 10:34
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My car (Jaguar) specifically states to leave the a/c on all the time and just control the temperature as needed - helps control the humidity is the reason in the manual...

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All cars & trucks since at least the 70s turn on the a/c compressor when ever defrost mode is selected to remove moisture in the passenger compartment. a/c oil is not hydroscopic, the only fluid I'm aware of is brake fluid which will absorb water & thats why manufactures recommend changing it or flushing it.

P.S. The only thing that would prevent this is a low freon level. When this happens the low pressure cut off switch prevents the compressor from running to prevent system damage Jim

  • Note- not all vehicles with A/C have a cutoff switch to prevent the compressor from running with low refrigerant pressure. – Jimmy Fix-it Sep 29 '17 at 19:39
  • Not all cars turn the AC on with defrost on. My 2003 European Vauxhall MPV doesn't. It even applies for buses, especially when they only come with a single large displacement compressor designed for the cooling of the entire bus: you have to turn the whole bus AC system on to enable the driver loop, and it means that AC will turn on for the passengers too, otherwise defrosting will only be done with heated air. Newer cars with internally controlled variable displacement compressors rarely come with an LP sensor or switch, and that's because the compressor's stroke is controlled by LP pressure. – Al_ Apr 16 '18 at 10:20
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Definitely helps keeping all O-rings and seals (especially the compressor's shaft seal) ok and a good charge inside for a longer time. That's because the compressor lubricant oil, circulated with the refrigerant (and the best oil circulation rate is attained with a full system charge), keeps them from drying up and then letting the system's contents escape to the atmosphere. Let your compressor run a few minutes each week.

This, however, won't apply with a clutchless compressor: these compressors, with the engine on, are always turning and pumping something around the system. With these, you only have to care about not letting the car sit around too much without being turned on.

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