Most efficient setting for car air conditioner?

Factual Questions
1

I have a long running debate with my spouse about using the car A/C and couldn’t find a previous post about car airconditioners to settle this. I say turning the temperature dial towards red (hot) uses just as much energy as running it at the maximum blue (cold) setting because it mixes hot air from the engine with the airconditioned air. My spouse says it’s more efficient because the A/C simply isn’t working as hard, and that there’s no hot air from the engine getting mixed in. Who’s right?

Second question: what’s more efficient, turning the car A/C on & off as needed to maintain a cool but not too cold temperature, or turning the dial slightly towards the red setting and leaving it alone?

I’ve read the thread about the house A/C’s but wanted to know about car A/Cs.

2

The most efficient setting would be towards the red when your A/C unit is off. If the A/C button is pressed, it is running (as far as I know) and putting a load on the engine no matter how hot the air is coming out. So that would make you right. I always wondered why they don’t make it so that the A/C turns off when the heat is used. I suppose it could be used to dehumidify the air, but I’ve never used it for that.

I’m interested in what people have to say about your second question. I’m always think about it even though I have always figured it’s more efficent to just turn the A/C off until you need it on again, and have it go in cycles. AFAIK, the compressor runs anytime the unit is on, even if you’re mixing in hot air.

3

I used to use it like that all the time to defog/defrost my windows. It’ll clear them up really quickly when set to AC and hot.

4

Well, they used to. The air conditioning temperature control used to be a thermostat that energized or released a clutch on the air conditioner pully so that when the temperature in the passenger compartment got too high the compressor ran and when it was cool enough the compressor stopped.

I need to check my car to see if that clutch is still there. If it is then the compressor is disconnected when cooling isn’t needed.

5

I think that the answer to the second question could depend on your individual car. I imagine that a good design would use a thermostat, and would just cycle the AC on and off if you move the dial a bit toward the red. Another efficient design would be to run the AC itself at lower power (which might even be a more efficient heat transfer cycle, but you’d need an engineer to know for sure).

If it’s mixing hot air in after cooling, then that’s obviously not efficient, and the only question is whether the wear and tear caused by turning the AC on and off is worth worrying over. I doubt that it is, considering that every other AC cycles on and off on a thermostat.

6

I’ve wondered about this too,(and argued with the wife) I’m only replying here so that I can keep track of the final answers. I’m leaving for S. Dakota next week …so I’ll give ya’ll till Friday night to come up with an answer.

7

You won’t have a definitive answer without knowing exactly how the system in your car operates. They aren’t all the same. Some regulate temperature by blending refrigerated air with heated air. Others do it with a thermostat that controls the “on time” of the compressor.

Even when armed with such knowledge, I question whether there’s much, if any, real-world advantage to trying to sharpshoot the A/C control operation. You’re probably wasting more energy worrying and arguing about it than by just using the controls as they’re intended. :slight_smile:

8

Every automotive A/C compressor has a clutch. Not every system has a thermostat.

9

My Toyota Matrix (just to get an idea of what kind of car we’re talking about) will turn on the AC when the fan location dial is switched to vents or feet and vents. Even with the AC button off, it still turns on to defrost / dehumidify the front windows (I have disabled this feature but it’s the default).

The engine load is the same with heat/cold so in my case, I figure the compressor is always on and is simply dehumidifying my heat (the heat feels the same with or without the AC on).

I took a general automotive course many moons ago in high school and we took up AC systems. I can’t quite remember, but AC relies on more than just a compressor. I figure when on cold, the entire AC system is working, while in heat only the compressor is working to pull moisture from the air. Since the compressor is the only part that needs power, it’s all the same regardless of heat/cold.

The Matrix does not have a thermostat, I’m guessing only cars with “climate control” would have something like that. I only have hot/cold.

10

Okay I am more confused now. I think my car does not have a thermostat ('95 Celica). There are no fancy buttons to set the A/C to cool to a precise temperature like you see in the Lexuses/BMWs/Benzs; it’s just got the toggle button for A/C, toggle button for recirculate, dial for fan strength (off to #3) and the blue-to-red dial.

So basically, to sum up acc to the general consensus so far, if there’s no thermostat, the AC is working just as hard even if the dial is turned towards the red, is that right?

Or did badmana mean to disagree, and say that some part of the AC (the part other than the compressor) does not have to do work when the dial is turned towards red?

Who said anything about real-world advantage?? :dubious: We’re talking “Told-ya!” advantage here. :smiley:

11

Agreed. I think the temperature control on my Dodge sets the duty cycle of the compressor and I act as the thermostat. It functions as a bang-bang system. If I am too cold I turn the temperature knob toward warmer and the A/C cuty cycle is reduced so that the compessor doesn’t run as long in any given period of time and so doesn’t take as many BTU’s out of the air. If I’m too hot I turn the knob down and the compressor runs more and cools more.

So in my case, turning the temperature knob to a warmer, or less cold, setting reduces the amount of time the engine is having to pull the compressor along.

12

I used to have a '93 Ford Taurus that acted that way as well. No fancy dial to set actual temp - just somewhere in the blue side of the knob that was comfy. At idle (stoplights or in line at toll booths) it was evident when the compressor clutch engaged and released as the system would kick up the idle speed a couple hundred RPM. So actually, it wasn’t a totally brain-free system - it gave an input to the engine computer to control RPM.

13

There is a lot of misinformation in this thread.

I can’t state definitively that NO cars use a thermostat to regulate the AC compressor, but definitely the vast majority of cars do not. Automotive AC systems are regulated via refrigerant pressure, not temperature. When you hit the AC switch, the compressor is engaged and continues to run until a pressure switch in the high-pressure side reaches it’s limit and cuts the compressor. The pressurized refrigerant leaks at a controlled rate through a valve into the “evaporator”, aka “the thing inside the dashboard that gets really cold”. It evaporates due to pressure differential as it leaks through the valve, and due to evaporation it gets really cold. The evaporated refrigerant is then passed through the “condenser” up front by the radiator, where it sheds the heat it absorbed in the evaporator and turns back into a liquid. The liquid refrigerant then goes back into the compressor for another cycle.

As far as the “temp” dial in your car goes (not the fancy-pants electronic systems where you dial in a temperature, but the standard “blue-through-red” dial), all that does is control the amount of HEAT being added to the cabin air, either by controlling the flow of hot engine coolant through the heater core, or by controlling the amount of hot air from the heater core that is allowed to blend into the cabin air. In other words, by moving the dial from blue toward red, you are adding heat from the engine to the cold air produced by your AC, and thereby “undoing” some of the work it has done to move heat outside.

So to answer the OP, your husband is wrong. In a properly working AC system, the temperature of the evaporator is always the same. You merely control the flow of air over it with the fan switch. Adjusting the blue-to-red knob has NO effect on the function of the AC system, it just turns on the heat also.

2nd question: not enough information. Manually cycling the AC system results in loss of efficiency because the entire system needs to be brought up/down to pressure/temperature every time you turn it on, and that wastes energy. It’s a question of whether the energy you save during the periods that the system is switched off exceeds the energy wasted by repeatedly cycling the system.

Just FYI, if the AC is too cold for you, it’s better from an efficiency perspective to turn the fan down rather than turn the heat up. Turning the fan down does reduce the cooling load on the AC, turning up the heat does not.

Further FYI- not all car AC compressors cycle on and off. Newer VWs (and some others) have variable-output compressors that dont’ need to cycle all the way off.

All this is further confused by the fact that some (usually domestic POS) cars are designed to run the AC whenever the “defog” setting is selected, presumably because the designers thought the average consumer isn’t smart enough to manually use the AC to dehumidify as well as cool the air.

14

Really? I have a 2003 Corolla (which is close enough to the Matrix that it comes with some of the same manuals) and it doesn’t do anything like that. The AC is only on when the button is pressed and the green light is on. However, the recirculate feature does turn off when I turn the location dial to defrost or feet/defrost.

15

Actually, I tested it tonight and you’re right. The AC comes on when the vents are on either defrost setting. Is that something you disabled yourself, or something the dealer had to change?

16

Actually, this has been the case on all my cars for the past 20 years, foreign and domestic. Have I just been lucky in picking cars?

17

Since when is convenience a synonym for stupid? Why punch two buttons when one works? My climate control system is exceptionally convenient – oops, I mean it works well because I’m too stupid to turn the temperature knob and cycle the compressor when I drive from, say, White Horse to Orlando.

FWIW, though, if I had a regular system I would prefer an A/C button – but for different reasons. Cheapo A/C systems always seem to insist that you get A/C air from certain mixes of vents, while you get vented or heated air through other combinations (including the defogger vent). What the hell’s the deal with that?

18

It’s the same thing. One of the really cool things about R-12, R-134 and other refrigerants is that if you know the pressure, you know the temp and vice versa. Compare these pressures and temps for R-12
50F = 46.9PSI, 60F = 57/9PSI, 65F = 64PSI, 70F=70.4PSI. So temp = pressure, since if I know one I know the other. As far as a thermostat goes, why would you want one? All that is needed is a switch, at this pressure switch in the compressor in, when the temp drops to this pressure, turn it off. Lather, rinse, repeat. A typical R-134 switch is 46 PSI ON, and about 22PSI off.

:smack: :smack: NO NO NO. There are more error in this paragraph than I can count. When you hit the A/C switch the compressor engages and draws low pressure GAS into the compressor. (Don’t forget liquids are not compressible, send liquid to the compressor and you will destroy it) compressing a gas raises it pressure and temperature. (See chart above) You now have a very hot gas at high pressure. The gas then passes into the condenser in front of the radiator. Heat is removed from the hot gas and it condenses into a hot/ warm liquid. (More energy is required to do the change of state than it does to heat or cool the liquid or gas) The high pressure is sent toward the inside of the car where there is a restriction in the line. This restriction is either a fixed orifice, or a variable valve. In either case there is a pressure drop, with the pressure drop comes a temp drop. Warm air from inside the car is blown across the evaporator, and the warm air causes the liquid refrigerant to boil (evaporate) This causes a change in state. Heat is absorbed into the refrigerant and the now gaseous refrigerant is sent back to the compressor to start the trip all over again. The high side switch is ONLY a safety to shut the system down to prevent a burst hose. on the cars I teach on the overpressure cut out (the only high pressure switch/ sensor) is set at 31 BAR. Running the compressor at 30 BAR is very stressful on the compressor doing so on a regular basis, would cause the compressor to die an early death.

This is mostly correct. One nit to pick is that the evaporator is not always the same temp. If you park your car in Phoenix, when you restart it the evaporator will be very warm for quite awhile until the system reaches equilibrium. After that the temp will remain fairly constant assuming you are dealing with an expansion valve system. If we are discussing a cycling clutch system the evaporator is always changing temp within a fixed range.

The last time I checked neither Volvo, Mercedes, or Jaguar were consider either Domestic or POS cars. All of these cars turn the compressor on with the defrost setting as a safety measure. If you can’t see outside the windows due to fog on the inside, you are not safe. A/C is a dehumidification process, A/C will clear all the windows in the car much, much faster than the heater/ defroster alone.