Cold weather warm up question

So I was raised in a place where heat was much more of an issue than cold, so I grew up hearing that you can use the heater for the passenger compartment to cool the engine, effectively using the heaterbox as a second radiator. Now that I am living in a place that snows, I am being told that on cold mornings, the fastest way to bring the engine safely to temperature is to blast the heat, because the thermostat will run the engine hotter to bring the passenger compartment warmer air.
I have no doubts that modern cars can do that, but I am curious about how the engines bring it up to higher temps. Do they just idle at a higher rpm? How would being driven affect having more heat requested?

on a side note, I am immensely amused that my car is not listed as an option XD I have a '17 Mazda CX-3. Quite a few websites do not list my model car, which is a shame, it’s a darling little car that I could not be happier with.

All you need to do is just start car , give it a few seconds or the time it takes to clear ice or snow from all the windows and drive on without trying to be a race car driver. No need to mess with heater settings or fan speed . If you auto temp just leave where you normally have it and relax.

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Not true. The thermostat knows nothing about the heater, except that it’s acting as another radiator, reducing the need to open the thermostat. It won’t hurt anything, but it won’t make the engine warm up quicker.


+1 to each of the prior respondents.
As a matter of fact, it could be argued that sine the heating system operates by using a small “radiator” (called a “heat exchanger”) to remove heat from the coolant to use in the car’s interior, running the heater could lengthen the amount of time it takes the engine to heat up. NOT running the heater will allow all of the heat generated by the engine to remain in the coolant until the engine warms up.

However, running the heat while the engine warms (blasting is not necessary) will help warm the interior and windshield, making clearing of the ice/snow easier.

The best approach is to follow Volvo’s advice and not obsess over it. Left to its own devices, the engine will take care of itself.

NOTE: welcome to the world of ice and snow. There are some things you need to know.

  • you need to have snow tires with at least 60% tread at the start of the season.
  • you should drain the windshield wash system and replace the fluid with “winter mix” windshield wash. “Summer mix” used in the south will freeze in the lines and on the wipers and windshield.
  • you should bring all of your maintenance up to date.
  • do any needed repairs.
  • drive slowly and carefully. Practice in an empty parking lot should help greatly.
  • always clear all the snow off of the roof, hood, and all windows. In NH it’s the law, and you can get a citation should you get seen uncleared. You could even be held responsible for an accident.
  • “winter” windshield wipers are essential. Metal frames will ice up very easily, making the wipers useless.

I’m sure others here will have other suggestions.

NOTE: these are safety issues, far more important than how fast the engine heats up.


That information if definitely wrong… if your car has manual heater controls, as mine, I do not turn on the heat or defrost until the temperature gauge starts to move up off the cold peg.
So, yes, I agree with all the above comments.

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The only possible grain of truth in the idea that blasting the heat might make the engine warm up faster is the idea that running accessories might cause the alternator to make the engine to work a bit harder, but I wouldn’t count on it making a noticeable difference. I don’t think it’s worth blasting cold air into the inside of your car before the engine gets warm enough to blow warm air.

If it’s cold enough where you live to make you ponder this, maybe an engine block heater might be a better solution.

It does. Especially on older cars where turning the heater off closed off the heating loop entirely - now running the heater means you have a larger volume of coolant to warm up, so it takes longer. But even on newer cars like OP’s, you’re bleeding heat out of the system whenever you run the heater. Once it’s up to temp, that’s fine, but before that you’re taking heat out when it’s trying to put heat in, and so it will take longer to get to temp.

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There was some truth to that in the past, not applicable to modern vehicles. In the past, one of the heater hoses had a valve in it that controlled the amount of hot water that entered the heater core. When the engine was cold and that valve was closed, and naturally the thermostat was closed as well, when the engine was started, there was virtually no flow of coolant in the engine.

It took a while for enough heat to reach the thermostat to get it to open. That could cause the coolant in engine to heat up at different rates depending on how close it was to the heat sources. You could have areas that were overheated until the thermostat opened up. Sliding the heater control all the way over to hot would allow some coolant to circulate allowing the coolant and the engine to heat up more evenly. Fan position had little effect.

In old cast iron engines, this wasn’t really a problem. Cast Iron is more resistant to warping and blowing gaskets. Todays engines do not have a heat control valve in the heater hoses. Heat is controlled by blend doors. The coolant flows through the heater core all the time. Anything you do is irrelevant.

There was a long discussion similar to this years ago but it had to do with the fastest way to get heat into the passenger cabin. I believe the consensus was that turning the heater fan on right away was the quickest way, but it was also the most uncomfortable because the initial air would be so cold. I think we agreed the most comfortable way was to wait and turn the fan on after the needle on the temp gauge got off the cold so that the air would have some heat in it, but you didn’t have to wait until the engine was all the way up to operating temperature.

At least on my older cars, whether you had the heater bypass engaged or not, coolant didn’t flow. Turning on the heater just opened the heater loop, but didn’t do anything to the thermostat so the coolant still didn’t start moving until the thermostat got warm enough to open. All opening the heater loop did was increase the overall volume of coolant, which meant you needed that much more thermal input in order to heat the system to hot.

Alternately, not turning on the heater until the car warmed up meant you’re briefly cooling the overall temperature down a few degrees, which the car compensated for, and meanwhile you still had heat on you.

The old engines I’m familiar with have bypass hoses to allow some coolant to circulate when the thermostat is closed. That’s needed for the thermostat to ‘know’ when to open.

Side story - my '83 GTI had a casting defect in the thermostat housing that blocked off the bypass. So in the middle of winter (in Anchorage!) the car would overheat unless I turned up the inside heat, which acted as a replacement bypass.

Old thermostats used to have a small hole in them to allow a small amount of coolant to flow.

Having spent many years in cold climates, I use a block heater when it gets below 15 degrees, and never changed the heater settings on cold starts, don’t know what safely up to temperature means, plenty safe to let the coolant circulate through the heater core.

If the vehicle isn’t equipped with automatic climate control, crack the window, turn the defroster on, and turn on the AC.

This will prevent the inside of the windshield from fogging/frosting over when you get into vehicle after scrapping the windows.


#1 Make sure no accessories are ON before starting the vehicule.
#2 Start the vehicule and let it idle for 2-3 minutes
#3 Drive the car slowly until engine reaches operating temperature.


Might as well scrap the wind sheild also? Just loving fun because I have had enough of spelling errors myself, and no windows no fogging! :rofl:

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May or may not work. Sounds similar to the question whether hot water freezes faster than cold water in the freezer. Try the experiment a few times. Then you’ll know for sure. Easy enough to do if you have a coolant temp gauge on the dash. Just time how long it takes on your normal morning drive using each method – running the heater or not running the heater – to reach a certain temperature that’s near to but below normal operating temperature. Use a piece of tape or something to mark on the dash gauge which temperature you are measuring to.

Can’t happen.
100% of the heat in the coolant comes from the combustion in the cylinders. And that is a finite variable amount depending on how warm the engine is. An engine cannot be made to make more heat when it’s cold because the heater is turned on. All that can vary because of the configuration of the heating system is how much of the coolant is retained in the engine as it warms up, possibly affecting the speed with which it warms up.

Theoretically the added load on the alternator will make the engine work a bit harder, making it create a tiny bit more heat, but the amount is unmeasurably inconsequential. A heating system fan can easily be operated on a 6VDC lantern battery (I’ve done it to test fan motors) and draws way too little current to have any meaningful effect.

If you’d like to have a bit of fun, ask the person who told you this (in good humor) where the additional heat comes from when you turn the heater on. The answer should be entertaining.

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If the heater configuration somehow affects the amount of flow through the radiator though …

That would affect how much of the heat generated by the still-warming engine is dissipated as the engine runs, but if I understood the comment correctly it implied that it would make the engine create more heat.

The above may be simply semantics. But I’m unaware of any system in which the flow of coolant through the radiator is in any way affected by the configuration of the heating system. Only the T-stat has control of that, and it is in no way directly affected by the heating system.

My understanding of the comments would be taking the heater core out of the circuit would allow for faster heating of the fluid that is not passed through the heater core if controlled by a shutoff valve. Significant, maybe as stated I had to go through Death Valley at 118 degrees with the heat on to keep the engine cool in my 72 nova, but wintertime stuff never tried it, wanted any heat at all for comfort and defrost, and do not believe it bothered the engine one bit.