soma

Hi, It's well into winter here in Minnesota and I have a very very simple question about how heating and cooling affects the fuel economy of a car.

Does it require gas to run the heat? (temperature dial is set all the way to red)
Does it require gas to not run the heat? (temperature dial is set all the way to blue)

Also, how long should I run my engine before driving in cold weather?

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Daniel_

The heat that comes into the cab in most cars comes from the hot water used to cool the engine block - if dial "RED" some of the water goes through a little radiator in the bulkhead between the engine and the passenger compartment in front of the fan, and if you dial "BLUE" all of the water goes into the radiator in front of the engine block.

Running the FAN takes extra electricity, and running the AC always hurts the gas mileage.

When I used to be poor and have to run old bangers, I would find that if they started to overheat, I could drop the engine temp a few degrees by dialing RED and running the internal fan at max.

Got me home more than a few times.

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Ilow

I don't think that the fan on either the red or blue is a big drag, but running the compressor etc. for the AC is what chews up milage. I have heard from several sources that only about 30-60 seconds of idling to warm up the car is OK as long as it is driven relatively gently until it is fully warm. That sounds a little low to me, but who knows. If it's in the single digits or below I start it awhile before I get in anyway so I don't freeze my ass!

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BadNick

Whether you run the heat in a car or not, the effect on gasoline/energy consumption is negligible. In "almost" all cars, the hot water that cools the engine also runs thru the heater core when you turn the dial to "red" to get some heat, so that is just that much less heat that the engine radiator has to remove ...in any case, insignificant effect on gasoline consumption.

Air conditioning is another matter altogether, since the air conditioner compressor runs from a pulley off the motor and consumes several horsepower when it's on and almost none when it's off; so don't run it unless you need/want airco since that consumes more fuel when running.

As for warming up before driving, once you get the motor started and running smoothly on its own, it will warm up faster if you actually drive rather than just sit there idling. But do not load up the motor until it is warmed up, so just drive moderately and accelerate slowly until the motor warms up.

One strange example of a car that does consume gasoline for heating the inside of the car is the VW 411/412. I had a 412 years ago, I think it was a 1973, and it had a gasoline fired heater for cabin heat. So you can actually hit a button and heat the inside of the car before even starting the motor; this heater burned gasoline from the gas tank and heated the inside air (indirectly in an air-to-air heat exchanger); in that case, heating did consume more gasoline ...weird car that was.

Ilow

I'm going to guess that this might have something to do with the fact that it was probably air cooled and didn't have a liquid cooling system to generate heat. Cool idea though.

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BadNick

Correct, Ilow. That was back when all Vdubs were air cooled. Unfortunately for many owners of the 411/412, including me, the seals in the air-to-air heat exchanger left something to be desired and sometimes leaked, and then the inside of the cabin would smell like gasoline. But it kept my expert VW mechanic busy for a few years.

shakran

on a modern engine, just start it and go. Warming it up actually hurts it more because when you put the engine under load it does a better job of getting the oil to the top of the head. Drive gently until a few minutes after the temp gauge reaches normal.

soma

Thank you for the responses. I posted this question because I make a 3 hour round trip to my favorite ski slope as often as I can. For some reason, I got the idea in my head that running the heat was eating up a lot of gas, so I'd keep the temperature dial to the blue end, hoping to save on gas. So I was confused, and now I'm not!

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Ilow

This makes sense in a way, but intuitively it seems more gentle to let the oil move around when the engine is at idle (900 rpm) versus under driving conditions (2-3000 rpm). I sometimes wait a sec if it's really cold to let the valve "tapping" noise settle down a bit before I drive; I don't know if that's really necessary, but it feels like letting me wake up and get a cup of coffee, versus shrieking out the door when I'm late for work.

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BadNick

As far as running when cold, keep in mind that "modern" multi weight oils are actually thinner when cold then when they get hot so pumping around is not a problem. E.g., the "5W-30" oil is 5 viscosity when "cold" and 30 when hot ...don't ask me how they do it, I'm sure it's magic and if you knew they'd have to kill you.

newtx

Don't be worried about cranking the heating up. I agree that it shouldn't affect your MPG or your engine.

grumpyolddude

One thing that may impact your fuel economy is setting your airflow to "defrost" rather than thru the panel or floor vents. In order to reduce the number of AC compressor failures, selecting the "Defrost" setting engages your AC clutch, keeping your AC system lubricated through the cold weather. Many cars will cycle on and off, many will stay engaged for as long as defrost is engaged. So, to save fuel, only use your defrosters as long as neccessary, then switch to the panel or floor vents.

blizzak

In terms of the whole warming up thing, I am going to have to go with the idea that idling for more than a few seconds is not necessary and just wastes gas. No more than 30 seconds I would say.

In my opinion, the number one mistake that people make when winter driving is letting their car warmup by idling and then driving it too hard right after that. It's not just your engine, but all the drivetrain components and other moving parts that need to get warm as well, and the only way to do that is by light driving for the first 5-10 mins of operation.

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BadNick

^^ I agree; on our '04 Acura TSX turning on defrost turns on the AC; if I have to defrost I turn it on and then manually turn off the AC, that works on this car anyway. On my old beater '89 Acura Legend, turning on defrost does not automatically turn on the AC. As you all probably know, while it costs fuel to run AC, sometimes it's a must if you have to dehumidify the air, AC+heat does that better than just heat.

^just this past week my power steering was starting to feel real tight and hard to turn until the car heated up about 10min or so; it turned out my ps fluid was quite low, I topped it up but I'm sure I have a slow leak someplace that'll need fixin. Also, most manual trannies are obviously "stiffer" when cold and when they're nice and warmed up they are smoother; never noticed that in an auto tranny but I'm sure there is a similar effect going on.

blade02

Just a little point that I could be completely wrong on, but on most of the vehicles that I've worked on, there is no valve that controls whether or not coolant flows through the heater core. In fact the only way to by-pass a leaking heater core is to literally re-route the tubing.

Instead, its the air vents that change/move to determine whether or not you get "hot" air inside your car. So coolant is always flowing through the heater core whether you're on "blue" or "red". Its just that when its on "red" a vent opens that allows the air to be drawn over core and into the cabin.

souzafone

One extreme, hypermilers never use heat or a/c, or lights or anything normal folks do. These people are completely insane, of course, and do other things to improve mileage, like tailgating tankers by 4 feet, taking exit ramps without braking, and driving in circles to scrub off speed to avoid braking. The other extreme is a typical Boston driver, floor it leaving a red light, and locking up the brakes to stop. Unless you're in a competition, just drive normally, keep your car running in top shape, tires inflated, etc and worry about the other guy on the road. If your a/c compressor does not come on in defrost mode, use it once in awhile to keep the components from rusting or seizing.

Deltona Couple

Most all of the modern cars manufactured today no longer have the heater valve that decides weather or not to flow cooland through the heater core or not. MOST systems are a continuous flow, so really there is no affect on mileage, except for the electrical draw of the motor, which in reality is negligable. Also, if you run your fan on a mid level speed, you actually will get better efficiency out of the heater AND the AC. This is due to heat transference time. Physics never lies...lol.

Actually that is not true 100%. AC systems operate durring defrost mode to dry out the air, removing the humidity. Dry air blowing across the windshield helps keep the inside from fogging up. Also, most systems will NOT allow the compressor to engage if temperatures are below 20 degrees F, because the lubricant is not able to lubricate the compressor properly at those temps.

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