When I use A/C, I typically set the temperature to the coldest (blue). Sometimes, that's too cold, so I dial it back toward the middle.

That got me wondering. Is the A/C actually working less hard and robbing the engine of less power? Or does the A/C simply work at one level of intensity, with the reduction in cooling achieved by enabling the heater?

 

The heater is not disabled. As I understand it, there's probably always coolant flowing through the heater core, and the temperature knob simply shifts a baffle in the ductwork to make more or less air flow through the heater core. (It may be that there is also a valve to shut off the flow of coolant to the heater core when the temperature knob is all the way on the cold side—I have seen that on at least one model of car, and I haven't studied the Fit's setup in any detail.)

I also don't think the temperature knob affects how the A/C cycles on or off. One big reason for this is that the A/C isn't just used for cooling, it's also used to dehumidify the air, such as when using the defroster to defog the windshield.

 

Interesting. I previously owned a car without A/C, and the defrost only turned on the heating lines on the back window to get rid of ice on the exterior. But it makes sense that if you have A/C, cooling the air also condenses the moisture out of it, so why not use it to help get rid of fog on the inside.

 

Fits and Civics do not have anything that controls the hot coolant flow through the heater core. Honda wanted an all-electronic system and a motorized flow control valve was too expensive and complex for these economy cars. Heat is controlled by an air door. You essentially have 200 degree coolant flowing under the dash all the time. I believe this is why it takes a long time to cool your car down with the AC in hot weather. Park your car, and eventually that heat gets released into your interior.

 

Well that would certainly make A/C a necessity. In my old car, I could tolerate not having A/C by just using the vent. Seems like this is not sufficient in the Fit, and I simply attributed it to me getting spoiled.

 

interesting question, but the a/c is either on or off. even in the winter when i have the defrost running at full heat mode, the a/c cycling rate is the same (the faint click you hear when the compressor starts)

 

OK. I now understand that the A/C is either on or off. Thanks for that.

Right now, I'm interpretting that to mean that it is either fully on or fully off at any single instant in time. But you also mention that the cycle rate is the same. Could you please explain what you mean? Same as what other thing?

Also, the fact that it cycles at a certain rate opens up the possibility that the proportion of the time for which the A/C is on can be varied. Would the temperature of the heating dial control this, or is the percentage "on" time essentially fixed, with the temperature controlled by directing some of the air to the heater core?

 

The A/C compressor is either on or off at any time, based on the (electric) clutch built into its pulley. In operation, it cycles on and off as the car deems necessary, based I think on the temperature of the evaporator in the ductwork and possibly some other sensors.

How much of the time its on is presumably indirectly affected by the amount of cooling done by the evaporator, which is in part a factor of the fan speed and temperature knob setting. I don't believe there's any direct connection between the compressor control and the temperature knob—I think the temperature knob is purely a mechanical connection to one or two dampers in the ductwork.

(Oh, and by the way, the rear window defogger grid is entirely separate and unrelated to the A/C—the defroster that interacts with the A/C is the settings on the ventilation knob that send air to the windshield vents.)

 

Thanks, DrewE.

 

yah, wat DrewE said. been offline from this site a few days.

 

So you guys are saying that the AC is either on or off, meaning turning the AC dial lower or turning down the fan, while it makes the car less cool is actually not actually having any positive effect on the AC at all?

So cranking it full blast if we want to be really cold, is no different in fuel consumption and operation of the AC than if was simply set to a tolerable level?

 

yes.

Might be a tad difference using a lower fan speed and/or recirculate as this will not warm the evaporator as fast causing the compressor to cycle more frequently and saving power. But in actual use I don't know if the difference is measurable in mpg. Some here claim it does have this effect but there are too many other variables to isolate the impact of how you set the fan/recirculate.

Temp knob won't have any impact so if you want to be most frugal, reduce airconditioning by turning down the fan speed until you're comfortable. If at 1 and too cold then turn up the temperature. But as stated, I don't know if you will see a difference.

 

Hmm....Yes and maybe no. Fan speed does have a small impact as there is a thermo resistor i think that senses air temps. What i noticed with my car in particular is the AC clutch cycles off alot less when fan speed is set to 2 or higher. Also the engine fan stays on constant when fan speed is at 2 or higher but when i click down to 1 the engine fan cycles on and off with the compressor frequently.

 

There is a thermistor ( temperature sensor) in the evaporator(which is what makes the air cold), which tells the computer how cold to make the evaporator. Once it exceeds this coldness, it shuts off the compressor and let's the evaporator warm back up until it's above the target temperature. At that point the compressor will turn back on, hence the cycling.

So what some people are saying is that if you have the fan speed higher, it will pull in more warm air from outside or in the cabin per second, thus making the compressor run longer in order to maintain the target temperature.

The temperature knob is only connected to a door that controls how much air flows over the heater core. Air is always flowing over the evaporator no matter what however.

So as far as efficiency, it will make zero difference whether you're all the way blue or halfway dialed down. The computer will maintain the same evaporator temperature regardless, and all you're doing is just mixing in a little warm air to make the air less cold.

 

Wouldn't mixing warm air with cold air make it harder for the A/C to achieve the threshold temperature at which the compressor turns off?

 

No, because the air passes over the evaporator first

 

Yes, but if air is also being heated by the heater, the overall cabin air is warmer. Shouldn't it take more work to cool it down to the threshold at which the compressor turns off?

 

If you've dialed it slightly lower than max cooling my assumption is it's already cold enough.

I suppose the air coming back in would be fractionally warmer(assuming you're on recirculate), but the difference would be negligible. It's more about what you're feeling on your face at that point. A person breathing out warm air in the cabin and sunlight through the windshield would make more difference.

 

Thanks for your detailed description of the entire air heating/cooling system.

 

This is a great but dangerous discussion.
Dangerous? Because my wife and I have disagreed for years about air conditioning in cars, she reliving ardently that the A/C "works" harder at colder settings and therefor taxes the engine or uses more fuel, and I maintaining that the A/C is simply on or off, producing a certain amount of 'coolness' of this amount you use more, or less as needed, with the cabin settings.
I see now that it is a bit more complex than either of us thought, though I think I was more on the correct side.
The danger comes in how to use this knowledge. Probably better to let 'sleeping dogs lay' as they say.