# Question about Air Conditioning

**URL:** <https://boards.straightdope.com/t/question-about-air-conditioning/452874>\
**Category:** Factual Questions\
**Created:** [June 14, 2008, 12:03am UTC](https://boards.straightdope.com/t/question-about-air-conditioning/452874 "2008-06-14T00:03:01Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![twickster](https://avatars.discourse-cdn.com/v4/letter/t/ec9cab/32.png) [@twickster](https://boards.straightdope.com/u/twickster)\
**Post date:** [June 14, 2008, 12:03am UTC](https://boards.straightdope.com/t/question-about-air-conditioning/452874/1 "2008-06-14T00:03:01Z")

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Why is one supposed to wait 5 minutes after turning off an air conditioning compressor before turning it back on?

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [June 14, 2008, 12:06am UTC](https://boards.straightdope.com/t/question-about-air-conditioning/452874/2 "2008-06-14T00:06:50Z")

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To equalize the pressure in the high and low sides. Once the compressor stops running, the gas starts to flow back through it, and this backflow makes it much harder for the compressor to start turning again. Once the pressure is equalized, the backflow stops and the compressor can start up more easily. Most modern AC units have built-in 3 minute start delay to save wear on the compressor due to trying to restart it too soon.

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**Author:** ![twickster](https://avatars.discourse-cdn.com/v4/letter/t/ec9cab/32.png) [@twickster](https://boards.straightdope.com/u/twickster)\
**Post date:** [June 14, 2008, 12:12am UTC](https://boards.straightdope.com/t/question-about-air-conditioning/452874/3 "2008-06-14T00:12:50Z")

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Very clear – and very quick. Thanks!

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**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\
**Post date:** [June 14, 2008, 1:35pm UTC](https://boards.straightdope.com/t/question-about-air-conditioning/452874/4 "2008-06-14T13:35:01Z")

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One other reason (and I agree with Q.E.D.) is that A/C compressors are _ **vapor** _ pumps. A time delay prevents liquid being in the compressor due to rapid starting/stopping (it’s called “slugging” the compressor).

Compressing a liquid is essentially hydraulics, and an A/C system is designed to compress vapor _ **only.** _

Without the time delay you may "slug’ the compressor and “scramble” it, a nice way of saying “destroy it.”