I bought a used car and its the first winter since I had this car. A/C in summer was working just fine. But today I saw the outside temperature was about 55 degrees and I turned the A/C to the blue color (cold side) midway and it starting blowing warm air. I am not sure if this is expected or not when the outside temperature is low. Can someone please clarify this ? I just want to understand how A/C works when I turn it to cold side but outside temperature is already lower.

Note: My car has A/C (not Climate Control).

  • Welcome to Motor Vehicle Maintenance & Repair! Commented Jan 16, 2019 at 17:25
  • Do you perhaps hear any loud hissing noise inside the car when you turn AC on and you set the blower to the minimum speed? Is the compressor's clutch hub (the central armature in front of the compressor's pulley, which is the black part in front of the compressor and it's put into rotation by a belt) rotating when AC is on and engine is on?
    – Al_
    Commented Jan 18, 2019 at 14:46
  • I have not explored that much but no I do not hear any loud hissing noise inside the car when I turn AC on and set the blower to the minimum speed. Commented Jan 18, 2019 at 23:24
  • Provided that the compressor is actually pumping when you check for the hissing, then no loud hissing inside means that you don't have a low refrigerant charge (and a system leak).
    – Al_
    Commented Jan 20, 2019 at 20:06

3 Answers 3


Not to worry. Your car uses refrigerant in a closed loop system to keep things cool when its hot outside.

The refrigerant system works on pressure. There are sensors that keep things sane. At ultra cold temperatures, the pressure is low, and the compressor is disengaged from pumping. At ultra high pressures, same thing, the compressor is disengaged from pumping. (Think 110 deg F, hauling a trailer up a steep mountain. The control computers turn off the A/C to protect the engine from overheating.) Normal operating pressures within the system range from 30 psi to 350 psi and higher.

One thing that's pretty cool (pun intended). The cold evaporator core helps remove water vapor from the air. In fact, when your vehicle is placed in the DEFROST mode, the air conditioning (A/C) is automatically engaged. This removes water vapor and keeps the inside of your windshield clear. Typically this works behind the scenes from around 40 deg F and above. Below that, the A/C compressor should NOT be engaged.

Note that all air in the heating ventilation system is routed through the evaporator core. Thats the part that gets cold when you are driving in the summer.


Now lets talk the red/blue temperature control. Remember in your car there is hot water flowing to the heater core ALL the time. That red/blue control moves a little door (blend door). When it's set to full red, then all the air inside the heating ventilation system is routed thru the heater core. At full blue, no air is routed through the heater core.

Your A/C refrigerant system is totally protected in cold temperatures.
You may well have some other issue related to the red/blue temperature control. I'd expect full blue (full cold) to be just a little bit warmer than outside air temperature (there is a little bit of heat given off from the engine that affects the air you are trapping just in front of the windshield)

Note: I'm assuming you have the recirculation setting (inside/outside) set to Fresh outside air. The recirc/inside/Cabin air setting is bad to be using in cold weather, particularly if its raining or snowing outside. If that setting is set to inside/ Cabin Air (recirculation) you are not getting air from outside, instead you are just recirculating air within the passenger cabin. If there is snow on your boots, that will melt, and pretty quickly fog up the inside windows, affecting visibility. This is unsafe.

Simple thermometer

Easy way to quantify the issue? Use a thermometer. Heres the best one for the money. I used to do a whole lot of work measuring temperature, and I broke about ten different thermometers. That one in aluminum housing, is just awesome. Always works, even if you drop it on cement.

  • Thank you very much @zipzit for your detailed and explanatory answer. So just to confirm again in layman terms, the temperature outside the car affects the temperature of the air being blown from the vents ? Commented Jan 16, 2019 at 22:48
  • When the A/C is off and the red/blue blend door is set to full cool and the recirc door is set to outside air, then yes, the air coming out of the vents should be just a bit warmer than outside air.
    – zipzit
    Commented Jan 16, 2019 at 23:04
  • A/C is on, vents fan is on, recirc door is set to outside air and I feel the air coming out from vent is not that cold. Commented Jan 17, 2019 at 20:05
  • The engineering term for that is "air over ambient" Typically fresh air in a vehicle cabin is 5 to 7 degrees F warmer than outside air. The extra heat comes off the engine. Probably worse if you are stopped at idle. Really bad if you are stopped at idle in front of a brick wall. Airflow thru radiator matters huge. Note: its totally possible that your temp door isn't fully sealing, letting some air pass thru the heater core. That would be bad, particularly in the summer. Easy test, use a thermometer.
    – zipzit
    Commented Jan 18, 2019 at 10:31
  • Yes, it is worse when the car is idle. I will use a thermometer today to check properly and will update you. Thanks a lot for giving detailed answers. Commented Jan 18, 2019 at 15:12

This is completely normal.

Air is heated by the warm coolant liquid from the motor, and since the coolant temperature rises to about 85°C (185°F), the air gets pretty hot, too. The knob with the red/blue scale determines how much air goes through the heater, and how much is bypassed, and so allows to steer the final temperature. If set to fully blue, one gets air of ambient temperature, if set to fully red, one gets hot air, and if set somewhere to the middle, then one gets... warm air.

Air condition is a separate thing. It is placed upstream of the heater with its bypass, so all air passes the air condition. But it only cools the air when the AC is switched on manually!

So, even with A/C on and the knob not set to fully blue, one can get warm air. Since part of the air humidity condensates in the AC, the air is also very dry, which helps to defog the windshield.

By the way, 12°C (55°F) is not that cold. Air condition typically still works at 0°C (32°F), but is switched off somewhere around that temperature automatically.

  • Thank you @sweber for your answer. You are saying that this is completely normal. But can you again state what could be the reason for this behavior ? From your post, I think you are saying that the reason of warm air coming from vents could be the high coolant temperature ? Commented Jan 16, 2019 at 22:50
  • Yes! You request warm air, and you get it. Air is heated by the warm coolant from the motor. This is also why it takes so long until you get warm air, the motor itself has to heat up first.
    – sweber
    Commented Jan 17, 2019 at 5:48
  • I understand now but I still feel that when I turn fan and A/C on and push it to the cold blue side, air coming out from the vents is not that cold as it is used to be in the summer. This worries me. Commented Jan 17, 2019 at 20:01
  • Up till now, our answers were about a flawless system, and you wrote about the knob set to the middle between red and blue... The AC itself looses its refrigerant over time, which is also normal as long as it's not too fast. This will be noticed as a lack of cooling power, later the AC will stop working to protect itself. It's hard to say that at 12°C/55°F. But refrigerant can be replaced by a workshop, and isn't expensive. Yet, it's possible there is a serious defect, but as long as air is just not cooled enough, it's for sure too less refrigerant.
    – sweber
    Commented Jan 18, 2019 at 6:23
  • @VibhuSharma You would have to check the temperature with a thermometer as a subjective value ("I feel...") is not reliable. Commented Jan 18, 2019 at 14:27

I am a commercial HVAC/R technician. What the mechanic hooked up was not 2 wires. He Hooked up a low and high pressure hose to your A/C system and added refrigerant to the system. We call this adding charge to the system. You have a small refrigerant leak that unless it is leaking enough to need re charged more than once a year, I wouldn’t worry too much about it.

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