Why does “recirculating” have more punch than “fresh”?
Fresh brings is some amount of outside air into the car. This outside air is usually warmer and/or higher humidity than the conditioned air in the car, and it takes some of the cooling capacity of the AC to cool and dry this outside air. Recirculating, well, recirculates the air inside of the car without bringing in outside air, and the full capacity of the AC is used for cooling the already conditioned air.
The problem with that is it creates a negative air pressure Wich draws in warm air from outside. It also loses efficiency by cooling air that’s already cool.
Unless you just got into a car that had been parked in the sun, in which case “recirculate” will be harder on the AC.
Two points:
- Huh?
and
- Huh?
The “fresh” setting definitely is drawing a lot of air into the car. Recirculate generally won’t draw air in unless there’s something not right (in design or maintenance).
Sure it will eventually lose efficiency. But if you are getting that cold that it’s wasting energy then you should SHUT OFF THE AC! The car is cold enough.
In no reasonable situation will it be less efficient than warming hot, outside air.
Humidity. AC removes the moisture in the air. You can often see this moisture being discharged by the AC by some water on the ground under the car. In the closed loop of recirculation the air is already dried to a point that it is harder to feel the air getting colder. It is getting cooler, you just don’t feel it as much.
When fresh air comes in with a higher humidity it gets cooled and you feel it more before the AC eliminates the moisture. It is like the difference between 100 degree F low humidity air in Arizona, which can feel just fine, and 100 degree F high humidity in DC which is unbearable.
Unless you have an air tight car, it’s going to draw air in.
The “fresh air” setting is pulling the heat energy out before sending it to the cabin while leaving the air that’s already in there alone.
This creates a positive pressure meaning ALL the air in the cabin has been conditioned vs only some of the air being conditioned with recirc.
Well, yeah, but the right thing to do in that situation is to roll your windows down, run the A/C on full blast non-recirculate, and drive for a little while to get all the much higher than ambient air out of the car.
Then, roll up the windows, and flip the A/C to recirculate, and it’ll cool down the fastest.
Look at it this way- your car is going to spend the same amount of energy taking that 100 degree air and cooling it down on the first pass as if it drew it from outside. That first pass might get you 75-80 degree air that’s been substantially dehumidfied.
On the second circulation of the air, your A/C is going to take probably 80-ish degree air without much humidity and cool it further. That’s a LOT easier than drawing in 100 degree humid air and trying to cool it. So on the second and subsequent recirculations, you’ll get cooler air for the same amount of energy expended by the A/C.
Of course the car’s not hermetically sealed, so you do get some exchange with the outside- I’d guess that’s by design so that the driver and passengers don’t suffocate, if for no other reason.
For the most part, the recirculate setting is probably drawing air in slightly, but not nearly so much as “normal”, which draws from the outside.
Cars and trucks have special “air extractor” vents somewhere in the body, partly to help regulate cabin air flow/pressure. So no need to worry about that part.
How is the negative air pressure being created? The AC isn’t pumping air out of the car, just in a closed circuit, and any volume change due to the change in temperature is negligible.
Cars are relatively air tight, so the air flow due to leakage is going to be minuscule.
I’m confused (with your other post as well). On recirculating, it’s going to use less energy to bring the cabin air down to the temp you want it and keep it there. With less heat in the car, the refrigerant doesn’t absorb as much heat and the compressor doesn’t have to put as much work into compressing it and dumping the heat to the outside.
If you are using the fresh air setting, as long as it’s warmer and more humid outside than inside, it’s going to take more energy to absorb that heat and condense the humidity.
As for the negative/positive pressure thing, I’m not sure what that has to do with anything. However, it should be noted that your car has exhaust ports in the body, usually one on each side hidden behind the rear bumper. While those there to make it easier to close the doors, they’re also going to let air out as your hvac system draws fresh air in. Your car will, more or less, always be at 1atm of pressure, just like outside.
I realized that refrigerating already cold air will make it even colder…I have noticed that in many cars the fan blows even harder during recirculation.
I was wondering is there some electromechanical protocol involved in making the fan motor work harder, or are the air channels in the “fresh” circuit more convoluted, sapping the strength of the air blowing through the vents?
I bought a portable AC and did a lot of research before I did. This guy does a pretty good job of explaining the science. I know there aree differences between a car’s AC and a portable one, but I think the science still tracks.
the vast majority of automotive HVAC fans are “squirrel cage” fans like this. this kind of fan is very sensitive to inlet restriction and backpressure on the outlet. reducing the size of the inlet duct even a small amount can reduce the load on the fan and make it speed up. so I’m wagering when you set it to recirc, the inlet pulling air from the cabin is smaller.
Portable ACs, specifically single hose portable ACs (as opposed to two hose systems), are different. They have the condenser coil (and compressor for that matter) inside the space to be conditioned. That means air, from the room, is drawn into the machine, absorbs the heat from condenser and compressor and then carries it outside. If it blew it back inside the room, it would be the same as a dehumidifier.
Your car’s AC system, similar to a window AC, central AC or mini split AC, has the condenser and compressor already outside. The place they draw the air from and the place they exhaust it to are the same place.
yep. air conditioning unit work pretty much the same regardless of where they’re installed. There’s really no differences other than where the specific components of the refrigeration circuit are located with respect to one another. the main difference with a car system is that the pipes which carry the refrigerant around have to be able to move and flex because the compressor is rigidly mounted to the engine, so automotive systems are far more prone to developing leaks over time.
as an aside, cascade refrigeration is pretty neat. our environmental chambers we had at a place I worked some years ago used that. pretty much the only way to get down to -60C.
Regular refrigeration systems are like that as well. The compressors are typically mounted on very small springs so they don’t rattle everything apart. The connection from the compressor to the next rigid item has to be able to take a bit of flex as well. In my store, we have a few coolers and freezers that have the compressor underneath. Off the compressor, there’s 3 or 4 big loops of copper, not just for vibration, but also so you can slide the compressor out the front to work on it and not have to disconnect everything.
A car’s A/C is nothing like a single hose portable. There is no negative air pressure in your car when running on recirculate. A single hose portable is a terribly inefficient machine that deliberately blows out some air that is already cooled, and as you pointed out, draws in hot air from anywhere it can.
Twin hose portables are better but still not as efficient a a window A/C. A car A/C is more akin to a mini split with a completely separate evaporator and condenser connected with hoses.
The most efficient way to cool the car is to use the recirculating feature which helps to lower the air temperature going through the evaporator. When the evaporator temp is low enough the compressor will un-clutch and turn off.
I have all my auto A/c tech permit books but haven’t done any A/C work in a long time.
Dennis
a car’s engine moves around a lot more (both in amplitude and frequency) than the compressor in a stationary application.
Interesting. I never put the re-circ option on, particularly in winter, because I find it increases internal humidity, to the point where condensation and frost start building up on the windows. I assume it’s because of the dry air we have around here, and the humidity from our breath, which makes the recirc air more humid than the external air.