The reverse of a heat pump is a steam engine.
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yes this is true the sensors will shut off the Ac if it is too cold outside, 
Here is an alternate way to defrost your windshield.
But there was no mention of a ticket being issued.
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Well, thatās not the dumbest or most dangerous thing Iāve seen someone do. Aside from the possibility of cracking the windshield from a rapid increase in temperature, my main concern would be catching the headliner on fire. Knowing the headliner has been removed would make me feel a lot better about this idea.
Maybe the defroster would work if the driver removed the stuff that 's blocking the vents on the dashboard in the picture, but probably not.
My 2008 Honda Accord, 2012 Mazda3, 2014 Mazda6, and 2012 Ford Fusion all engage the AC if you turn on the front defroster and all four of them let you push the AC button to disengage the AC while continuing to blow air through the defroster vents. This is a VERY common configuration and many people do not realize they can disengage the AC, should they choose to do so. Personally I find that using the AC clears the window faster, keeps it clear longer, and makes the air in the cabin of the car nice and dry. This helps melt and evaporate snow and slush that gets inside the vehicle so I leave the AC on even when I switch the heat to Auto and it blows down to the floor.
Are you sure? It is a passive system. Even my 72 Buick did that.
Some of the people that donāt believe the compressor runs when defrost is selected have never opened the hood to look.
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The compressor runs on my Yaris whenever the defrost is on, no matter what I do with the ac button. The ac button light goes on or off but the compressor is still on.
I really donāt think the ac compressor demands a lot of power to drive in cold temperatures. Itās essentially pumping heat sideways instead of up a big thermal hill. Theoretically, if the heat pump is pumping heat downhill, from a high temperature to a low temperature, the compressor could even return energy to the engine, it is now operating as a steam engine. (closed Rankine cycle heat engine) I said ācouldā, not necessarily ādoesā, for a compressor to operate as a decompressor, it would need timed valves like a steam engine. Not spring loaded valves like a compressor.
You would also need a āfeedwater pumpā, to pump the liquid working fluid from the low pressure condenser to the high pressure evaporator (boiler).
Which vehicles? I can assure you every vehicle Iāve owned the compressor NEVER EVER kicked on when defrost is selected.
My Kia has a single serpentine belt so the A/C compressor pulley is turning when the engine is running. Past A/C equipped vehicles made an audible āclackā when the compressor clutch engaged. The Kia does not. The only thing I know for sure is when I push the A/C on button the windshield fogging clears much faster. Perhaps the button only operates blend doors.
I was curious and check today. Both my 05 Tundra and 06 RAV4 will turn on the A/C compressor when defrost is selected. Makes sense to me. That dry air will prevent fog from forming up on the inside of the windshield.
In cold weather there is a large difference in relative humidity between cold and warm air. The released moisture in warm air causes the condensation/frost on the interiors of the glass. When the air conditioner turns on, the compressorās evaporator collects the excess moisture and exhausts it outside the cabin. This keeps the windshield clear on the inside, a definite safety feature, and the reduced humidity helps th keep the other windows clear.
My Camry has the same system, and it works like a charm. With the temperature set at warm temps, cold air is not produced for the cabin; the function is solely to reduce humidity.
Our Camrys, all had the AC on to help remove moisture from the air. In Defrost mode, AC could NOT be disengaged. In other airflow modes, the AC could be selected on or off.
MAX Defrost probably directs the entire airflow to the w/s vents, meant for quick clearing inside and out (and unfortunately encouraging some motorists to neglect clearing frost from other glass).
I have never owned a car with a/c that didnāt run the compressor when defrost was selected. On my PT Cruiser with manual rotary controls, when you selected defrost, the cable moved the recirculate/outside knob to outside as well as engaging the a/c. I do not know if there was a way to defeat the system because in our damp climate, I canāt imagine wanting to.
I am not saying your car works that way, just every one I have driven. Of course, I never drove one with a/c until 1990.
An AC is a heatpump, but only operating in one direction: hot refrigerant flows to the condenser (radiator at front of engine compartment) and gives up heat to outside. āTrueā heat pumps have a reversing valve which lets the system reverse operation; in a car this would mean high-pressure refrigerant flowing through what is ordinarily the evaporator, giving up heat to the cabin. This is not how a gasoline-powered car AC works.
For one, they use the engine as a heat source. For another, the evaporator in a heat pump has to always be colder than the air, so it can absorb heat. In heating mode this would mean the radiator under the hood is literally ice cold (colder than ambient) ā on a rainy day it would rapidly accumulate ice. Icing up is a real problem for household heatpumps, too. The systemās need to routinely shut down or switch to cooling mode long enough to defrost, or incorporate resistive heating elements similar to a refrigerator.
Some very few electric cars, such as 2013+ Nissan Leaf, do have true heat pumps, as it is more energy efficient to pump heat rather than generate it electrically. However not even the fancy Teslaās do this, Nissan is an outlier. Itās a setup that only makes economic and engineering sense on electric cars, since all gas/diesel vehicles will always produce plenty of waste heat. Their thermodynamic max theoretical efficiency is capped at around 45% thanks to our pesky universe.
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My car is a 2003 kia rio and the ac comes on when I turn on defrost. But I can hit the ac button and it turns off. I can feel and hear it turn off.
My 2003 kia rio is exactly like that. Ac comes on when I turn on defrost. But I can hit the button and feel compressor turn off.
Robert , not sure why you bother to turn the AC off. That is what helps keep your windshield from fogging up. That is how most vehicles work now and have for some time.
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Well until recently I didnāt know that it was normal. And I didnāt want to have an unnecessary compressor running that was making my motor work harder. But now that I know, Iāve been leaving it on. I just wanted ti point out that there are vehicles that for whatever reason give you the option.