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How does a car's heater work? (specifically a 2013 Kia Sportage)

Hey folks,

I’m trying to settle an argument about car heaters in general, but also specifically in a 2013 Kia Sportage. My husband has this habit of cranking the fan and the temperature control to high as soon as he starts the car. He says it heats the car faster. Specifically, he says setting the temperature control to maximum makes the air coming out of the vents hotter than if he sets it to medium or low.

I say he is wrong on 2 counts:

  1. The engine supplies the heat and it gets warmer if the fan is turned off at first.
  2. The temperature control works like an oven temp. You pick a target temp and it shoots for that temp. So, for example, if you choose 90 degrees, it will not get to 70 degrees any faster.

Can anyone explain how a car heater actually works and perhaps whether either of us is right?

We’d be much obliged.


I’ll try. The car heater is just a small radiator with a fan blowing through it. The water going through it is the same as the coolant for the engine or about 210 degrees. On warm up, coolant bleeds off to the heater and once the coolant hits 210, the engine thermostat operates and keeps it at that temp. So there is a constant supply of 210 water for the heater. To cool the heater down from max, a door opens to introduce colder outside air. Turning the fan on or off before warm up has no effect whatsoever on the speed the engine warms up, but very shortly some warm air will come out which is better than shivering without the fan on. Some cars restrict the operation of the fan for a minute until there is some warming.

In addition to what bing says,the thermostat is the controlling mechanism. The basic function of the thermostat is to restrict flow of coolant, including the flow to the heater core until the engine is up to a temp where the flow of coolant is needed to maintain recommended engine temp.

As you can conclude for the preceding remarks, your husband is dead wrong. Turning the fan on high just blows cold air all over you until the engine warms up. If it’s high humidity in the car, putting a lot of air through the defroster may keep the windshield clear by evaporating all that moisture you breathe on it, but it won’t make the car interior any warmer. In fact, if the heater draws in outside air, with the fan on high it will TAKE LONGER to warm the car interior, since your bodies’ natural heat will all be lost.

If your husband posts his opinion on automobile heaters here I hope he has thick skin.

If its a manual climate control unit it will heat the cabin quicker if you put the temperature control to maximum heat because you are not blending any cold outside air into the cabin. You are completely correct about the oven comparison, except that on a car, unless its on max heat, there is cold air being blended no matter where the engine temp is.

If it has automatic climate control its a different story.

Your husband is wrong. He is just causing a “wind chill” factor in the vehicle. It may not be lowering the temperature at all but he is causing discomfort to his passengers. Heat will not arrive until the engine is heated so his actions are completely unnecessary. It’s a habit that he needs to break.

Will the vent temp be any warmer if u use recirc mode? It was -5f the other day and I tried switching between recirc and normal mode and vent felt no different. Usually I set selector to dash/feet but sometimes I use vent/feet so warm air blows on my body. “warm” is relative at -5F.

Turning on the fan anytime…anytime…causes a cooling effect for any heat exchanger, in this case the heater core. Just like blowing on your hot soup or coffee moves the heat away from the hot object, so does the fan blowing over the heater core. IT gets hot from the coolant flowing through it from the engine and gets its heat from that engine.
— turning on the fan over the heater core always causes heat transfer AWAY from the heater core…that’s the idea for getting heat from it.
turning on the fan BEFORE it has gotten hot actually DELAYS its retaining any heat by constantly moving any hint of heat accumulation immediately away…resulting in a DELAY of getting any heat.
( it is common practice to turn ON the heater fan to assist in cooling a car with an overheating issue )

It is true that turning on SOME fan will allow you to feel anything warmer than outside air as it begins heating…
the reciculating mode will allow the slightly warmed inside to get warmer as the heater core warms ( as opposed to it attempting to warm the colder outside air )

But turning on high fan immediately only delays the core getting warm enough to stay warm.

Your husband is right on both counts. Sorry guys but I think you are all reading her post wrong, or I am.

Turning the heat on high right away does not cause the engine to heat up faster, in fact it delays the warming up of the engine by some, but it does heat the interior of the car faster. The coolant going through the heater core is not restricted by the thermostat so any heat generated by the engine, as little as it might be, is immediately dumped into the cabin of the car, so heating of the interior begins sooner.

The heating will be a little more gradual, but the size of the heater core is small so the delay in engine coming up to temp won’t be all that much, meanwhile there is heat being pumped into the cabin whereas waiting until the engine comes up to full temp before turning up the temp control means you freeze that much longer.

Now for part two. The temp control for your car is more like the old fashioned steam radiators. You opened and closed a valve to control the volume of steam or hot water getting to the radiator, thereby controlling the temperature of the radiators surface. In a modern HVAC system in most houses today, the heat one temp, either on or off and the temperature in your house is controlled by the duty cycle of the heater via the thermostat.

The heater in the car is controlled by either a blend door in the vents or a ball valve in the coolant line, or both. The surface temperature of the heater core changes with the position of the ball valve which can be fully open, fully close or an infinite number of positions in between. The more the valve is open, the hotter the surface of the heater core, and the hotter the surface of the heater core, the hotter the air in the vents.

In cars where the temperature is controlled by the blend doors, the heater core surface is always full hot, but the blend door mixes this hot air coming off the heater core with either interior air from the car or exterior air drawn in from the exterior vent under the cowling to cool it down and control the temp. In either case, full hot heats up the interior faster.

Some new cars have a temperature setting much like you house where you set the temp by degrees. If you have one of these, then your husband is not right because the car sets the blend door or the ball valve to full open until the temp is achieved, then closes the control device down to keep the temperature constant. But he is still right on the first count.

BTW, you say fan for your side of the argument and you use the term temperature controller for his side. These are two different things. If you meant fan for both sides, then he is correct on both counts, even in the newer cars with a thermostat in the HVAC.

In three cars that my Dad owned, a 1939 Chevrolet, a 1947 Dodge and a 1947 Desoto, the heater was a box under the dashboard containing a small radiator and a fan. There was a valve under the hood that he would open in the fall so that the coolant from the engine would flow through the heater core and he would close in the spring which would prevent the coolant from flowing through the heater core when heat was no longer needed.
In the winter with the valve open, the coolant would begin flowing through the heater core as soon as the engine was started. There was a bypass to send some of the coolant through the coil even though the engine thermostat was closed, and within 5 minutes the heater core would begin to warm up. At this point the fan could be turned on to distribute the air that began to get warm.
Today, everything is complicated by bringing in outside air, blend doors that open and close to direct the heat, valves that are electrically, pneumatically or cable controlled from inside the car, but the basics are still there. As soon as the engine starts, the water pump begins to circulate the coolant and some of the flow is directed through the heater core if the valve is opened even though the engine thermostat has not opened up.

Wow! It’s been a busy Christmas day around my house and from all the comments it looks like it’s been busy here in the forums too! Thanks so much for all the responses. I really appreciate it and I learned so much! Now I’m off to do a little holiday gloating . Merry Christmas!

[edit] I may have written this response too soon. I read Keith’s response after I wrote this and now I’m wondering who is right again.

Let’s see if I have this correct.

There are 2 relevant climate controls in our Kia: The Fan and The Temp Control. The Kia gets its heat from the engine which runs at about 210 degrees F. For the sake of discussion, let’s assume it takes the engine 60 seconds to reach 210 degrees. Let’s also assume that once I start the car, the engine temp rises at a steady rate - something like 3 degrees per second. And lastly, let’s assume the Kia has 80 cubic feet of space which is occupied by 30 degree air (that’s what passes for cold here in the South).

Now for simplicity’s sake, we’ll ignore the heat of 2 adult bodies and the air they displace. We’ll also assume that the vents can move 10 cubic feet of air per second (I’m totally making that up).

If I start the car and leave the fan off, it will take 20 seconds for the engine to reach 90 degrees. If I then turn the fan on high with the Temp Control set to High, the car will start blowing 90-degree air into the cabin. Assuming the most efficient air transfer possible (not realistic), in 3 seconds we have displaced more than 30% of the air. In the next 6 seconds we have replaced the remaining air with air that is hotter than 90 degrees.

Now that air obviously cools in the cabin so let’s say we lose 50% of the heat to environmental exchange via windows, etc. So after ~30 seconds, the car is at ~45 degrees. Assuming the same pattern, the next 9 seconds would raise the temperature another 15 degrees, except the engine is hotter now, so I’m going to say it’s 20 degrees. That puts us at 65 degrees in 39 seconds. One more round of this puts us well into the 70s if not over 80 degrees in under a minute. Again, I know this isn’t realistic, I’m just checking my understanding of the factors you all mentioned.

So if I understand Keith correctly, if I do everything the same except I set the Temp Control to a lower setting (e.g. “low”), then the Kia will mix in cool air, reducing the temperature of the air being sent into the cabin. This, in turn, would increase the time it takes to get the car to that 70 degree mark.

One more thing, the longer I wait to turn on the fan, the hotter my engine will be when I turn the fan on.

So, if all of that is true, then waiting 30-60 seconds to turn on the fan and setting the Temp Control to High is the fastest way to heat up the cabin.

So, ignoring the actual numbers I used, do I have the concepts correct?


I must strenuously disagree with most of the advice given, on safety grounds.

The cooler air is, the less water vapor it can absorb before it starts producing fog or condensation. On a cold day (20F as an example), the outside air is capable of containing very little water vapor in ssolution.

Now, two living, breathing humans get in the car. The inside of the lungs are very wet, and exhaled breath has a dew point of close to 98.6 F. This is FAR more humidity than 20 F can handle, which is why you can “see your breath” on a cold day.

Well, the driving position requires that you breathe almost directly on the 20 F windshield. What then happens is rime icing…much like the ice that forms on a frozen Margerita glass in humid weather. This is mildly interesting to a science geek, but rather trivial, as there is no pressing need to see through your drink glass.

In driving, however, seeing outside is widely regarded as “a good thing.” Thus, even if the defrost air does not heat things up, it at least mechanically moves that pocket of wet air away from the windshield, mitigating icing/fogging. (Ever notice that morning fog is worst on windless days? Same basic principle.)

This also means that, in freezing weather, the vehicle should be idled until vent air is at least tepid before driving anywhere, to ensure the glass is above freezing, so no icing my occur. The car (generally) doesn’t need an extended warmup: the driver does!

OK, I thought we were talking theory here, not reality. There is a big difference. In theory, your husband is right, not sure I would want to be in the car with him though if he follows this in practice.

If you want numbers, in my experience it takes a car about 5-7 minutes to reach operating temperature when the outside temperature is around freezing (32F). Operating temperature is when the coolant reaches the temperature when the engine goes into closed loop. Closed loop is the term used when the computer uses only its sensors to control the engine and stops running in a preprogrammed mode to compensate for a cold engine. The coolant temperature on most cars for that is around 180F.

From the moment the engine is started, it begins to heat up and the coolant in the block begins to heat up also. The water pump starts circulating the coolant, but because the thermostat is closed, the only circulation is through the heater core. It may take 15-20 seconds before the first ever so slightly warmed up coolant reaches the heater core and can be transfered to the cabin. If the blend doors or the coolant valve is wide open, then that little bit of heat is available.

While in theory, the say 33 or 34F air is warmer than the 32F air in the cabin at start, and it is providing warmth, I’m personally not fond of blowing 34F air on my legs at the maximum wind speed the fan is capable of. I’d much rather have 32F still air on my legs than fast moving 34F air, something to do with wind chill makes it uncomfortable.

In some cars, the temperature gauge shows cold until the thermostat starts to open. I know on our Honda, the gauge stays at the lowest position for about 4-5 minutes, then rises to normal in the space of about 15 seconds. Not all cars do that but some do. But most of them will not start moving the needle until the engine coolant is up to around 100F + so even though the gauge might be on zero a minute after starting, the coolant may now be warm enough to turn the fan on and start heating the cabin up.

For practical purposes, you are looking at a coolant temp rise of about 30F per minute. You really don’t want moving air across your skin that is less than 70F, so if the outside temp is 32F, you need to wait about a minute and 20 seconds plus teh 15 to 20 second delay to get from the engine to the heater core, so a total of about a minute and 40 seconds before turning on the fan, and then you might just use low until the coolant temp rises another 20F or so before going to a higher speed.

Now about your theory, the sooner you open the valve or blend door and turn on the fan, the sooner you start heating the cabin, but the rate of rise in the temperature of the cabin will be very slow at first, but rise to the maximum rate of rise as the engine heats up. If you wait until the engine is fully warmed up, then turn everything on high, you get the maximum rate of rise right away, but you sat in a cold cabin for 5 to 7 minutes when you could have been getting some warmth 3 to 5 minutes earlier.

In other words, you could sit in a cold cabin for 5-7 minutes, then turn every thing on and be warm 3 minutes later, or you could have suffered for 2 minutes, then gradually got relief and could be in a comfortable cabin before the engine reaches operating temperature.

meanjoe75fan brings up a good point that I never considered - which is the added air circulation, even if it’s cold, will help to keep your moisture-concentrated breath from forming fog/frost on the windshield.

So experiment with that and if it helps to keep the windshield clear, it’s worth it.

Hvac said outside temp was 17f last night. Coolant temp said 34f at startup. Drove 2 min and coolant temp was at 75f. 3 more minutes and it was 120f. Got home 2-3 min later and it was 150f. Short trip

Keith is right…in that the vehicle will get warmer quicker if you turn the fan on immediately. Although not by much. I usually wait about 2-3 minutes…and then turn the heat on. Even tough the needle on my temp gauge hasn’t moved…the air coming out the vents is warmer then the air inside the truck.

Years ago - on many vehicles the flow to the heater core was restricted until you turned the heat switch to hot. So there was no heat going to the core when the heat was off. Today heat inside the vehicle is controlled through blending doors - so there’s always hot water in the heater core.

@keith is right- if you want to warm up the car interior as fast as possible turn the heat to high at the highest fan speed. Hubby’s wrong that the air is warmer that way, but it is the way to get the maximum heat into the car interior.

Car’s HVAC systems vary in complexity. Can’t say with any certainty about the OP’er car, but on my early 90’s Corolla I know from experience the quickest way to heat the cabin is to turn the fan on max and the temp to max straight away. There is no climate control feedback system on my car, so with everything at max, it blows as much heat as it possibly can. I’ve never actually done a temperature measurement, but I’m fairly certain if the goal is to to get the cabin to the highest temp as quickly as possible, turn everything to max as soon as you start the car.

In practice I don’t do this though. Why? Because if I turn the fan to max when the coolant hasn’t heated up at all, it “feels” like cold wind. Uncomfortable. Instead I usually wait about 2 or 3 minutes, then I turn everything to max. But I think this method results in it taking longer to reach the highest possible cabin temperature.

If the car has a feedback system for the temperature, it probably doesn’t matter. In that case it is like an oven.

@texases You are technically right, but blasting yourself with very cold air the first few minutes is very unpleasant due to the extreme chill factor. Those of us who face long cold winters put the heat valve on high and the fan on low initially for that reason. After a few minutes the heat starts coming out and we speed up the fan by one notch. Then another notch as more heat comes in.

The purpose of the heater is to provide comfort, and initially blasting yourself with subzero air defeats that purpose. Especially if you are bringing in outside air. We were at a Christmas party recently and it was bitter cold. The standard procedure is for the husband to go out and start up the car (or use the remote starter) and put the heater on medium speed. After saying goodbye to everyone and getting our winter gear on, the car will be sufficiently warmed up to drive away without the windshield fogging up.

So, OP’s husband is also theoretically correct if we completely ignore the comfort factor and put the heater on “recirc”. If we bring in cold outside air at full blast any quick warmup benefits will be lost. OP’s wife speaks with common sense and rates comfort as she feels it.