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I know that you should drive off immediately to warm up your car as your entire car will warm up faster this way. However, the common practice to defrost the inside of your windshield is to blast the heater on for several minutes until it clears up.

I am pretty sure that the heater in a non-electric car uses the heat generated from the engine, which is still cold when you just started up your car. Using that air to defrost your windshield seems counter intuitive as it will bring in cold air to cool off your engine (I think I read you should use the heater to prevent your car from overheating even in the summer).

Using A/C provides air with low-humidity so I know this is better than just the regular heater: does this affect the warm-up period in any meaningful way?

My commute to work is 10 minutes which is barely enough to warm it up and would like to know if keeping the heater or A/C off would help me warm up my car faster. I would like to spend less time driving around freezing while taking care of my car.

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    If you turn on defrost, it will run your A/C and heat at the same time, which will send dry air in which is as warm as the engine will permit. – anonymous2 Dec 16 '16 at 20:38
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    In many cars (well, at least my two cars), setting the airflow selector to "Defrost" will automatically turn on the AC. In one care, the AC button lights up and you can turn it off if you want to, in the other car, the AC is on permanently for as long as the selector is set to defrost. – Johnny Dec 16 '16 at 23:59
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You are right that the heater in internal combustion engine powered vehicles usually uses the waste heat of the engine for heating – that's why in the summer you can use the heater to get a bit of extra cooling if you need it. During the winter the cooling from the heater may slow down the engine warming up, but only by a tiny bit. Until the engine comes up to operating temperature the heater core is the only radiator, and it's not nearly big enough or seeing enough air flow to significantly cool the engine.

Using the A/C probably is close to net zero, since it uses power to chill and dry the air and the power comes from the A/C compressor which is driven off of the engine, so running the A/C requires more power and thus more heat. That power is used to chill the air that is blown past the heater core, so it is might be more effective at pulling heat out of the coolant, but again not by much (I'm not even sure if it is able to cool the air down below ambient in cold weather) and some of that is offset by the load added by the compressor. In many cars, the heating system is designed to turn on the A/C when you put the controls in the defrost position. That also suggests to me that running the A/C is a net win.

The really nice way to speed up heating you car is to get a block heater. They warm the coolant so that when you start the car it is already close to operating temperature and you get heat right away. That should help to reduce start up wear as well.

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Well, this is a question for a Canadian guy. Here's the thing:

  • in extreme cold (-20C and colder), the moisture coming off your breath will condense as ice and fog on the inside of your windows. Air circulation, whether cold or hot, will help with the fog part
  • your engine does not need to be at operating temperature to defrost your windows, even just a little bit of heat from when the needle starts to move is enough to defrost your windows, it will just take more time
  • unless I'm mistaken, the thermostat does not block off coolant circulation to your heater core, it only blocks it to the radiator.
  • You'll get fog at temperatures way above -20° C – it happens here in the balmy northwestern US. – dlu Dec 16 '16 at 23:18
  • @dlu Our fog is nastier because it's metric :) At -20C, breath condensating on cold windows happens within seconds of sitting in the car and closing the door. I think you have more moisture in the air too. – tlhIngan Dec 17 '16 at 0:21
  • Oh, yes our air is so damp we get mold on the clouds… -20° C sounds brisk. I'm amazed it doesn't turn right to frost! – dlu Dec 17 '16 at 22:19
  • @dlu Some of it does turn to frost, some of it turns to fog. Depends on the exact temperature and humidity really. The colder, the frostier. :) – tlhIngan Dec 17 '16 at 22:22
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If I understand things correctly...the thermostat will not open until your car reaches operating temperature. So at first, you're just blowing air over a cold heater core. As long as that thermostat remains closed, and coolant isn't circulating, I suspect running the heater neither affects, nor is affected by, your engine temperature. Once that thermostat opens, though, and warmed coolant begins circulating, that's when you should have an exchange across the heater core, and that is the point where the heater being enabled becomes relevant..

In other words if you leave your car running in the driveway to get it warm, it should begin to heat up just as fast with or without the heat going...you don't need to make a second trip out to the car just to enable the heater. While it might reach the warmest temperature somewhat slower with your heater running from the start, I think the difference will be negligible. (You could always test it.. :)

As to your other observation, yes you can enable your heater in the summer if you are experiencing overheating, and it will provide a secondary/auxiliary heat dissipation surface. But you shouldn't rely on it. I only do this when I see my temperature begin to rise above normal. I'll enable my heater, and I'll give it about 30 seconds to a minute to make a positive effect. Thirty seconds to a minute, tops! Then, if the temperature continues to rise with the heater enabled, I pull over before I truly overheat! By the way...if applying the heater in this situation works and your temperature decreases, it signals an immediate need for some cooling system maintenance. In my case it's usually been a low coolant level, or possibly needing to "burp" the radiator.

Final thought. Contrary to your first statement:

I know that you should drive off immediately to warm up your car..

I believe it is better for your engine to allow it to reach operating temperature under idle conditions, as opposed to reaching operating temperature under load (driving conditions). When you start driving away cold, you're effectively putting very conditional, yet very avoidable, wear on several moving parts. If it were an airplane engine, taking off without a warm engine would be considered downright negligent...I'm just telling you that to underscore the performance relevance of a cold vs. a warmed engine. Obviously, the consequences of a cold automobile engine failing, or not performing as expected, are very different than those of an airplane.

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    Actually if you have a modern car with fuel injection (pretty much anything in the last 30 years), it's generally recommended by manufacturers to not completely warm up the engine before driving. That said, the colder it is, the longer you'll want to let it idle. You're right that driving a cold engine puts more wear on it so you should avoid driving it hard until it has a chance to warm up. There's some pretty good discussion on the topic here mechanics.stackexchange.com/questions/7847/… – atraudes Dec 16 '16 at 20:19
  • washingtonpost.com/news/wonk/wp/2014/12/29/… -- 30 seconds tops on modern cars. RPM you hit has a lot more to do with engine damage than letting it idle warm. – Bryan Boettcher Dec 16 '16 at 22:10
  • The only thing you need to warm up to protect a car engine is the oil, and if the oil grade is correct for the climate you are in, you don't even need to do that. In a piston aircraft engine, the purpose of warming it up on the ground is to check for any problems (overheating, low oil pressure, etc) before you need to make an engine-out emergency landing. Jet engines are different, in that the whole engine structure needs to get close to its working temperature before the clearances over the rotor blades etc are correct and the engine is "safe" to respond to fast throttle movements, etc. – alephzero Dec 17 '16 at 1:26
  • The advice to let the car warm at idle is contrary to most recommendations I have seen. Typically, the recommendations are to start driving after few seconds (after the oil is everywhere in the engine) and avoid high RPMs and high throttle openings until the oil is warm. This means only about 25% of the engine's power should be used, which may be challenging for small cars. Note that most cars have coolant temperature sensor, but no oil temperature sensor. The oil takes about twice as long to reach operating temperature when compared with the coolant. – juhist Dec 17 '16 at 13:25
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    Every car I've worked with has the heater core lines unregulated by the thermostat. It really doesn't make sense any other way. – Lathejockey81 Dec 17 '16 at 14:06
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First, find out yourself what the real temperature of your engine is and how fast is it warming up while you’re there idling. Keep in mind that dashboard gauge is pretty useless, inaccurate and often rounds up the temperature data. Google if your car has the ability to see your coolant temperature digitally.

For example, in my audi 2001 1.9tdi, there’s key configurations I can do to see digital accurate engine coolant temperature.

One guy here said blowing warm air to heat up interior will affect your engine temperature just a ‘tiny bit’

WRONG

Blowing hot air inside dramatically reduces engine’s ability to warm itself up.

For example, when there’s -10C degrees outside, and I’m blowing heat inside interior, my engine wont heat up to operating temperature in the city no matter how long I drive. However, if I turn off the heating to the interior, my engine gets to operating temperature in like 15 minutes of driving.

Also, if it’s really cold outside consider inserting a large piece of cardboard before your engine to block off incoming cold air. (This in my experience helped a lot to heat up my engine quicker but be sure if doing this to not block the whole air intake and you must be able to see accurate engine’s coolant temperature so as to not overheat)

TL;DR Don’t blow heat inside your interior if you want to heat up your engine quickly because all the heat you get inside is heat stolen from the engine itself.

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