Does the AC use more power when you turn up the fan?

[Kelemvor]

This is something I've been wondering for a long time.

Most cars these days have a button you push that turns on or off the Air Conditioning. Then there's a separate knob you turn that controls the blower fan.

Does turning up the fan make the AC work harder? Or is the AC just on or off and the fan just controls how much and how fast the air goes through it? I'm thinking it's the second one but just thought I'd ask here.

 

[steppinthrax]

Depends....

On some cars the compressor stays on all the time or never ?de-clutches? from the accessory belt so in that case it would make no difference. However other cars the compressor cycles on/off. In these cars if the blower motor is kept low it will probably stay off longer. There is a pressure switch and or sometimes an evaporator coil thermostat.

 

[cprince]

The second one is correct. The compressor is driven by the engine, so it depends on engine RPM. The fan control just controls how fast air is moving through the evaporator coil.

 

[Kelemvor]

Yeah, I'm talking about one that does NOT have climate control. Just a slider that goes from hot to cold.

Thanks.

 

[Raduque]

I've never noticed my compressor going on/off when changing the fan speed. I don't have auto climate controls or anything, or a slider. I have a knob that goes from hot to cold =P

 

[OrganizedChaos]

higher fan speed will cause the evaporator to absorb more heat. this will
1. increase low side pressure causing the compressor to cycle more often in orifice tube style systems.
2. cause an increase in high side pressure (all that thermal energy has to go somewhere).

the increase in high side pressure means more drag on the engine and the increase in condenser temperature will cause your vicious clutch to provide more lockup(if you have one) and thus more engine drag as well.

 

[Shawn]

it's either on or off.

 

[Pacfanweb]

Originally posted by: OrganizedChaos
higher fan speed will cause the evaporator to absorb more heat. this will
1. increase low side pressure causing the compressor to cycle more often in orifice tube style systems.
2. cause an increase in high side pressure (all that thermal energy has to go somewhere).

the increase in high side pressure means more drag on the engine and the increase in condenser temperature will cause your vicious clutch to provide more lockup(if you have one) and thus more engine drag as well.

Hmm....you're on the right track, but not exactly right.

Running the fan will not cause the compressor to cycle on and off more, it'll cause the compressor to just stay running....but even this depends on the weather. If it's hot, then the compressor will basically never cycle off. So theoretically that will use more power, not by causing a heavier load on the engine, but simply by staying on all the time.
If it's not so hot out, but you need a bit of A/C, the compressor will cycle on and off as needed, and will therefore over time use less power, because you have periods of time where the compressor is off and not running.

Oh, and an increase in low side pressure causes the compressor to come ON. Low pressure turns it off. That's in any system, not just orifice systems. So if you run the fan on high in hot weather (which is the only time I could see running it on high for any length of time), then that's basically never going to let the low side pressure get low enough to cycle the compressor off, which will cost more power over time.

 

[Pacfanweb]

Originally posted by: steppinthrax
Depends....

On some cars the compressor stays on all the time or never ?de-clutches? from the accessory belt so in that case it would make no difference.

Name one, that's less then 20 years old.

 

[steppinthrax]

OK,

The 91 to 99 Saturns when you push the A/C button the clutch is always engaged until you turn it off.

 

[Pacfanweb]

Originally posted by: steppinthrax
OK,

The 91 to 99 Saturns when you push the A/C button the clutch is always engaged until you turn it off.

Try having someone start it up with the A/C on and see if the compressor is turning, too.

And try running the A/C in cold weather and see if it doesn't cycle on and off. I'm willing to bet that it does.

 

[RU482]

It will make a difference. Will you notice it at the pump (having the fan on 1 vs 5)...probably not

 

[Zenmervolt]

Originally posted by: Pacfanweb

Originally posted by: steppinthrax
Depends....

On some cars the compressor stays on all the time or never ?de-clutches? from the accessory belt so in that case it would make no difference.

Name one, that's less then 20 years old.

I was so close to naming the system in my 951, then I realised that the car was built in 1986 and is now 21 years old. I know that the Volvo I have uses a TXV to regulate refrigerant pressures though and, in theory, should not require that the compressor de-clutch ever (though I've never checked either, I only know that on previous cars I could hear and feel the compressor cycle and on these two I cannot). Granted, on both there is a temp sensor on the condenser coil to shut the system down if the condenser gets too cold, but that doesn't normally come into play.

To the OP: In the academic world, it will have a small effect. Will you notice it in real-world applications? No.

ZV

 

[Pacfanweb]

ZV: I suppose my real point is/was, in reality, how many cars still are like that?

And on your Volvo: Doesn't the engine cut the compressor off when you accelerate?

 

[Zenmervolt]

Originally posted by: Pacfanweb
ZV: I suppose my real point is/was, in reality, how many cars still are like that?

And on your Volvo: Doesn't the engine cut the compressor off when you accelerate?

Oh, I'm just having fun with theory now.

Stopping to think about it, you're right, there are a myriad of reasons to cut out the compressor. I guess that I'm thinking of the older orifice tube systems that would cycle the compressor a couple times a minute to maintain temperature rather than the newer systems that cycle the compressor for efficiency reasons.

ZV

 

[OrganizedChaos]

Originally posted by: Pacfanweb

Originally posted by: steppinthrax
Depends....

On some cars the compressor stays on all the time or never ?de-clutches? from the accessory belt so in that case it would make no difference.

Name one, that's less then 20 years old.

i was under the impression that all the manufacturers were switching to variable dispacement scroll compressors. there supposedly more compact and have better volumetric efficiancy. instead of using a clutch the swashplate adjusts to set the dispacement to near 0. in motor magazines last A/C service issue there was a big article on it.