# Effect of “fan speed” setting on air conditioner efficiency

My air conditioner has several switches, two of which are: (a)mode="fan_only/cool" and (b)fan_speed="low/medium/high". I never use mode="fan_only" since I have a separate, more efficient window fan when the outside temperature's cool enough for that. Otherwise, I always run the AC in mode="cool" with fan_speed="high", thinking that as long as the compressor has already done all the work to liquefy the working fluid, higher fan speeds transfer as much heat as possible during each pvt cycle, with the compressor doing more-or-less the same mechanical work regardless of fan_speed. (Note: I'm talking only about work done by the compressor, not by the fan.)

You can google lots of hits on "heat pump" or "air conditioner thermodynamic cycle", etc, but I wasn't able (in 5-10minutes) to find anything that explains whether the above conjecture is right or wrong.

Aside: while googling, I came across Brayton, Rankine, etc cycles. But, as you can imagine, I'm only familiar with Carnot, Otto, Diesel. So which cycle describes your standard window air conditioner?

Edit: clarification-- From @CountTo10 's comment, I just want to clarify that my real question is whether or not the conjecture about fan_speed is right or wrong. That is, more explicitly, are there any situations where running the fan at a lower speed might get greater efficiency, i.e., more btu's/watt of cooling?

It occurred to me that if the conjecture's right, then why does that fan_speed switch exist at all??? It would just be asking the question, "Would you like me to run more efficiently or less efficiently?" Yet its very presence probably encourages people to choose a lower fan speed when it's not very hot outside, thinking they're saving energy. So either the conjecture's wrong, or the manufacturer's done a silly thing.

• It's probably a Rankine cycle, but in reverse, the working fluid changes back and forth between gas and fluid, but the fluid needs to have a much lower boiling temperature, HFC-134a is one replacement for freon – user108787 Sep 9 '16 at 6:53
• Thanks @CountTo10 . Please see edit/clarification above. Was the original question worded too vaguely, or did you get it and were simply commenting on just that last part? (P.S. And just to clarify, should I be counting to myself or out loud?:) – John Forkosh Sep 9 '16 at 7:10
• Count is a reference to my aristocratic family pedigree......., nah not really, it's to remind me to not make quick assumptions. I was only commenting on the last part because by coincidence I had read yesterday about the Rankine cycle used in power stations. The first question is clear to me, but I can't answer, sorry, i haven't read the chaper yet, tbh. It's a pity there isn't any indication, apart from compressor hum/noise, so you could test does it run longer at high fan speed. – user108787 Sep 9 '16 at 7:29
• This is not a thermodynamics problem. Fan is fan. With higher speed, more air circulate. You can feel it. You may argue that with high flow, it may take away more heat, may need to consume more energy and may be less efficiency. But it depends on the AC design. – user115350 Sep 9 '16 at 16:21