Does red and blue make...purple?

July 11, 2010 11:21 AM

Does changing the "temperature" setting on my car's manually controlled AC just waste energy?

I have a Hyundai Elantra without climate control--meaning I can't just set a temperature and it automatically adjust to maintain that temp. All I have is a dial that goes from cold to hot. Or more precisely, from blue to red.

Now, my thinking is that at the max heat setting, it's blowing in hot air from the engine. The max cold setting is pure AC. But if I dial in a setting that's in the middle of those too, what is happening?

Is it making the AC less cold? My guess is that ACs have one setting: ON, so regulating temperature is done by turning it off in intervals, meaning this doesn't happen.

Or do you think that even with the cheaper AC units, it's still doing an on/off cycle to maintain a level of cooling that I dialed in?

Or, is it really just mixing in warm air from the engine (or outside) to temper the cold air? In this case, using any setting other than max cool is basically warming up the cold air to make it something more comfortable--or just wasting energy.

I've always wondered this but don't know much about cheap car temperature controls.

posted by razdrez to travel & transportation (11 answers total)

It depends on the car. Older cars will have ACs that are simply on or off, continuously. Newer cars will cycle the compressor on and off as necessary, but that's usually a feature of automatic climate control systems. Some cars have two AC modes, regular and 'economy' (or some similar naming scheme), but that's not very common.

Since you don't have automatic climate control, I'm guessing that your simply runs the compressor continuously and blends in warm air for anything warmer than the coldest setting.

Sources: Car Talk question from 2003 (compressors are usually on all the time; warm air blended in; but some cars have two modes); Car Talk question from 2006 (some cars with automatic climate control do cycle the compressor on and off)
posted by jedicus at 11:33 AM on July 11, 2010

Oh, to answer your first question, it doesn't really 'waste' energy because the warm air is a free byproduct of running the engine. In a car with a simple AC system, if you're running the A/C at all you might as well dial in the most comfortable temperature for yourself. It doesn't save any energy to freeze yourself compared to blending in some warm air.
posted by jedicus at 11:35 AM on July 11, 2010 [1 favorite]

Jedicus, I'd say the energy wastage comes from the fact that the AC is using exactly as much energy as it would be to be at the maximum setting, even if it's only cooling half as much. That's certainly half as efficient. You probably could save energy by keeping it on the maximum setting and instead of mixing in warm air, setting the fan speeds slower (assuming forcing less air through the AC unit would use less energy — I think it would, but maybe not) or cycling it on and off manually.
posted by floam at 11:44 AM on July 11, 2010

setting the fan speeds slower (assuming forcing less air through the AC unit would use less energy — I think it would, but maybe not)

That would only save energy in that less would be used to turn the fans. I assumed that we were dealing with a situation where even with the fan on the lowest setting it was still too cold for comfort, thus requiring at least some warm air to be blended in.

cycling it on and off manually.

I would think that the attention required to do this would be more dangerous than it's worth. To me it would also be extremely tedious.
posted by jedicus at 11:50 AM on July 11, 2010

I'd say the energy wastage comes from the fact that the AC is using exactly as much energy as it would be to be at the maximum setting, even if it's only cooling half as much.

If you're blending in some outside air then that means you're pulling less airflow across the evaporator coils which means the refrigerant absorbs less heat which means the compressor can cycle off for longer and still maintain the same temperature.

I think it's misleading to say that there are cars where it's "always running." Even on the most unsophisticated models there is a thermostat that turns the AC clutch on and off regularly to maintain a given refrigerant temperature while the AC is in the on position; that's just how they're all designed. It would be wrong to design one without such a duty cycle because AC loads are very much not constant -- you'd either have insufficient cooling on hot days or a frozen evaporator because the refrigerant was too cold on mild days. You don't run compressors open-loop like that.
posted by Rhomboid at 12:10 PM on July 11, 2010

Or, is it really just mixing in warm air from the engine (or outside) to temper the cold air? In this case, using any setting other than max cool is basically warming up the cold air to make it something more comfortable--or just wasting energy.

For a basic system: sort of. The knob that allows you to set from blue to red controls how much heat from the engine is added to the air before it enters the cabin (either by controlling how much air passes through the heater core or how much hot coolant flows through the heater core or both). Wikipedia on the heater core covers this pretty well.

So yes, this is just wasting energy (you are using mechanical energy from the engine to cool the air and then heating it up again, so it would be more efficient to just cool the air less in the first place).

The only situation where this makes sense is where you want to dry the air (The A/C pulls moisture out of the air). Many cars run the A/C and the heater at the same time on maximum defrost to do this.

I assumed that we were dealing with a situation where even with the fan on the lowest setting it was still too cold for comfort, thus requiring at least some warm air to be blended in.

If the lowest fan setting with the A/C is too cold, I'd think that just blowing outside air through the car would be cool enough.

If you're blending in some outside air then that means you're pulling less airflow across the evaporator coils which means the refrigerant absorbs less heat which means the compressor can cycle off for longer and still maintain the same temperature.

This isn't how it works though. The A/C cools the air and then it is heated by passing through the heater core.
posted by ssg at 1:07 PM on July 11, 2010 [1 favorite]

Fan speed has an effect like Rhomboid suggests. Higher fan speed - more incoming air per unit time - more heat transferred to refrigerant. Temp setting has no effect (even on the regulated temperature types, I think but I'm not sure) because the air is still cooled to max and then heat is added back after the A/C has done its job.

Putting the A/C in recirc mode once the car is cool should save some energy, too, since a portion of the air it's having to cool is air that's already been cooled below outside ambient temp. [Recirc should also pull down the inside temp of the car more quickly, since the cooling will be cumulative instead of a more feed/bleed process. This is debatable, but I still think I'm right.] (For cars with a recirc air filter, that's a tradeoff for having to change your filter sooner.)
posted by ctmf at 2:05 PM on July 11, 2010

As far as most cars go, the heater core and evaporator coexist in a plastic housing behind the dashboard. There are a number of ducts and flaps that control the direction of airflow and another flap called the Blend Door that controls the intermix of hot and cold air. The heater core will always be as hot as the engine, since it's nothing more than a tiny radiator. And the AC evaporator will be frosty when the compressor is running and not cold when the compressor is off. So, unless you're blending the hot heater air with fresh uncooled air, you're kind of wasting energy by running the ac compressor and then warming the air back up again.
The compressor cycling in your car has nothing to do with the temperature you have selected. It runs until a pressure switch trips at a maximum psi.
Automatic systems have a variety of controls including variable displacement AC compressors that operate over a range of outputs and thusly a range of loads on the engine and the heater core may have flow control valves that prevent the flow of hot coolant into the cabin when heat is not desired.
If you have a hard time being comfortable in your manual climate control car, just run the AC on max with recirculate on until you're cold and then the AC switch off, leaving the fan running. It'll blow air over the evaporator which will remain cold, if not cool for a while after the compressor has shut off. Then, turn it on again. This will probably waste the least amount of energy. But the efficiency wasted here is kind of beanplating. It's either worth it to be comfortable in your car or it's not.
posted by Jon-o at 2:15 PM on July 11, 2010 [1 favorite]

Thanks all for clearing that up!

I generally just hit the AC button on and off whenever it's too hot or cool enough. It's easy to get to from the wheel, I'd be fiddling around with the blue/red dial anyway to keep adjusting the temp, and I've always suspected that it saved a little bit of energy that way.

But most of the time, I either need full AC or don't need it at all. Also, I've generally gotten into the habit of turning off the AC when I'm close to my destination to let the fan blow out the last of the cold.

I know it's only something like just 1mpg less to run the AC, but I just hate the idea of heating cooled air. It's like cooking family-sized portions and throwing away half of it just because you prefer leftovers.
posted by razdrez at 4:34 PM on July 11, 2010

Not an answer to your question, but an answer to car companies who might ask "what can we do for our customers?" - I have driven an old Toyota Landscruiser that had two dials - one the traditional Blue-Red for the amount of heating, and another A/C dial that actually adjusted the power of the A/C compressor. You could, therefore, have the A/C turned on, but turned down low if you didn't want that much cooling. A brilliant idea that would fix your quite legitimate concerns about "heating cooled air".
posted by Jimbob at 5:07 PM on July 11, 2010

Many vehicles don't cycle the compressor, but have a pressure valve that changes the displacement of the compressor depending on the load.
posted by gjc at 6:41 PM on July 11, 2010