Home / Experts/Tech / Why The ‘REST’ Button On German Cars’ Climate Control Panels Is A Wonderful Feature To Have

Why The ‘REST’ Button On German Cars’ Climate Control Panels Is A Wonderful Feature To Have

The REST button on a 2006 BMW 325i

If you’ve ever been in a well-equipped BMW made sometime after Nintendo released the N64, you may have noticed a button on the climate control panel marked REST. Upon first glance, a button marked REST sounds about as preposterous as a button marked YAWN. It doesn’t automatically configure your out-of-office email message, nor does it turn the vehicle into a chaise lounge. It doesn’t even dispense a sleeping mask from the CD slot. So what does it do? It turns out that REST is short for Restwärme, which is German for “residual heat.” Honestly, that really only raises more questions. What is residual heat, and where does it come from?

As I’m sure you’ve figured out by the previous few sentences, the REST button has to do with your car’s cabin heater, so to start, let’s take a look at how a typical heater in a combustion-powered car works.

Your car’s heater works by leveraging the liquid coolant used to keep your engine from overheating. A combustion reaction heats up your engine’s metals, which conduct heat into coolant flowing through the engine’s cooling passages. The ethylene-glycol-based coolant removes that heat and expels it through your car’s radiator. The temperature of the coolant is regulated to what’s called the engine’s “operating temperature” by a thermostat, which varies how much coolant goes to the radiator.

Anyway, there’s a second radiator in every car called the “heater core.” This heat exchanger is typically located near the firewall, usually hidden under a car’s dashboard. Warm coolant from the engine gets pushed through this heater core, which contains little tubes and fins. Those tubes and fins are there to help the air blown by a blower motor (that’s the heater fan that you control on your dash) pick up the heat from the liquid coolant flowing through the heater core. That now-hot air is then sent through the ducts on your dashboard, warming the cabin.

To best understand what the “REST” button in older BMWs does, we have to understand the concept of “thermal mass” and its “inertia.”

You know how when you kick on the heater when you’ve just turned on the car (and the engine’s still cold), you get blasted in the face with cold air? That’s because it takes time for the engine to heat up cold coolant. On the flip side of that coin, coolant also takes a while to come back down to a cold temperature once it’s gotten hot. This “transient response” time or “thermal inertia” is a function of a number of factors including the mass of the coolant, its specific heat, the difference in temperature between the coolant and either the heat source (when warming up) or surrounding air (when cooling down), heat exchange effectiveness, and more. You can think of this as “thermal mass” in that there’s a certain “inertia” (conceptually speaking) associated with heat.

The details of this aren’t important. What matters is that you understand the concept that hot coolant — especially a lot of it (high mass) — takes a while to cool down.

 

E90 Rest Pump
Photo credit: BMW

Where the glory of the REST button kicks in is when you shut your car off after a drive to go into a store (for example). Press the button and a little electric auxiliary water pump (marked as 4 in the above diagram) will circulate the still-hot coolant through the heater core after the engine is switched off. The pump isn’t exactly whisper quiet, but it’s a damn sight quieter than a desktop PC was some seventeen years ago, so I’m happy to cut it some slack. This pump will run through the residual heat program so that when you’re done popping into a shop, you come back to a warm car.

[Editor’s Note: Many cars use belt-driven mechanical pumps that will not flow coolant through the engine/heater core unless the engine is running. One could still run the electric blower motor while the car is off, but all it would do it blow air through a heater core filled with a few pints of hot coolant. As mentioned above, mass is a big factor in the heating equation, so the coolant in that heat exchanger would quickly pass all of its heat to the air blowing through it, and in short order, you’d have cold coolant in the heater core and thus cold air coming through your vents. BMW’s system lets the pump exchange those few pints of cold coolant in the heater core with a large supply of hotter coolant pulled from the engine. -DT]

If I flip to page 84 of my 2006 BMW 325i’s owner’s manual, I’ll find a subhead marked “Residual heat” which features some handy information. In the words of whoever wrote this manual, “The heat stored in the engine is used to heat the passenger compartment, e.g. while stopped at a school to pick up a child.” Honestly, I’d have loved it if my parents’ car had a similar function for when they had to pick me up when I was a child. Fair play to BMW’s manual writer. Anyway, the owner’s manual then goes on to describe the prerequisite conditions for activating the residual heat function. The temperature outside the car must be below 77°F (25°C), the engine must be up to operating temperature and the battery voltage must be good.

E90 Owners Manual Rest
Screenshot: BMW

Do everything right, and the residual heat function will waft warm air through the cabin for up to 15 minutes despite the engine being off. Why not longer than 15 minutes? Concerns about running down the battery. Sounds fair to me. Interestingly, residual heating isn’t some exclusive feature that’s only on mid-2000s BMWs. Several manufacturers have experimented with residual heating functions, with various different control parameters.

The 2006-2012 W164 Mercedes-Benz ML midsize SUV (it was available with a two-speed transfer case and locking diffs in certain markets, it counts as an SUV) also had residual heating, although it functions a bit oddly according to the owner’s manual. Don’t get me wrong, it’s absolutely brilliant that pressing the REST button will send air out of the vents for a full thirty minutes, or until battery voltage and/or coolant temperature drops. The weird part is that there’s no way to control the air temperature of the REST function – the climate control will always target 72°F (22°C). Granted, 72°F is a perfectly good temperature, it just seems a bit strange how the target temperature isn’t adjustable.

W164 ML
Photo credit: Mercedes-Benz

The current Audi A4 also has a residual heat function, although it’s a bit hidden. To activate it, press the left climate control knob with the engine off, and residual heat will be pumped through the cabin for up to 15 minutes. I’ll never know why Audi made accessing residual heat so obscure, but it’s on-brand given how the typical long-term Audi ownership experience goes. I know that’s a bit rich coming from someone who owns a BMW, but between weird packaging and a penchant for triple-square hardware, long-term Audi ownership just seems a bit too masochistic for me.

Cockpit
Photo credit: Audi

While automatic engine stop-start systems have driven a rise in electric auxiliary water pumps, few manufacturers seem to allow the use of auxiliary pumps while their cars are parked. While a constant stream of warm air through the vents in stop-and-go traffic on cold days is lovely, would it really be that hard to also integrate a REST function? Granted, it’s worth recognizing that residual heating will eventually go the way of the dodo. Electric vehicles primarily rely on electric resistance heaters (some use something called a heat pump, but we won’t get into that here) in cold weather, and those can be run while an EV is parked and locked until the high-voltage battery pack runs out of juice. In fact, systems like the Chevrolet Bolt EV’s remote start function are arguably a step up from residual heat as they can warm the interior up before you even put your shoes on in the morning. How’s that for comfort and convenience?

I know that residual heat may seem like a frivolous creature comfort for the terminally pampered, but I promise that it’s actually quite nice. Picture this: It’s a brisk weekday evening in October and you need to pick up some shopping on the commute home because you’ve procrastinated a touch. Hey, it happens to the best of us. After running into the supermarket and picking up tortilla chips, a roast, some bagels and some milk, you get back to your car to find that the interior’s just as chilly as the evening breeze. More importantly, the office breakroom tea you poured into your tumbler has gone all cold, a bit of a downer to kick off the evening. Now instead of this, imagine having hit the REST button before going into the store. You’d get back to your car, your tea and you would be cozy, and you could enjoy the rest of your drive home.

Considering I live in a place that’s stereotyped as a frozen hellscape, I’d love to see more new cars equipped with residual heating functions. Until then, I’ll keep enjoying the fact that I’m fortunate enough to enjoy Restwärme.

Lead photo credit: Thomas Hundal

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47 Responses

  1. Do some modern cars have electric heaters that warm the air as the engine gets up to temp? In my 96 Cherokee, I don’t even bother turning the fans on until the thermostat has edged off 100, or I’ll just be getting cold air blown at me (At least 5-8 minutes of driving). My 2016 Audi on the other hand will be blowing toasty air by the end of the driveway from cold start. There’s no way it’s getting that much heat off the engine that quickly.

  2. Had this in my 2009 W211 E Class, it’s a great feature and came in really handy when sat in the car waiting for the wife/kids at various pickups.
    A lot of the same cars will also have a tunnel mode, where holding a climate button will close all the windows and switch the car to recirculation; in certain areas of Europe you’ll use this often.
    It would be interesting to see some more information on the auxiliary diesel heaters fitted to Range Rovers.

  3. This is an excellent article about a very useful car feature that I had no idea existed! It should be universal, if you ask me. I think I would have been OK if you had turned up the level of technical language a notch or two (think about that article on suspensions and how much people liked it) and assumed that I could handle concepts like thermal inertia and heat pumps (I bet most Autopians can) but still, I learned something and now I wish my car could do this!

  4. I live where it is very hot, and I’ve had a car overheat before (’84 Vanagon. Yes it was water-cooled, yes I still consider it a real VW). This function seems perfect for cooling down a hot engine. Roll the windows down and let the engine REST for awhile before limping home.

    1. The only problem is that it’s pulling heat from the radiator, instead of the engine. When you turned the car back on it would get a rush of “cold” coolant, which might not be great for it. I think you would be better off doing a high idle for a few minutes.

  5. I absolutely loved this feature on my 2006 E90! It also served as a reminder to replace my battery when it just stopped working one winter – that was my first car that just started randomly shutting features off when the battery condition got bad.

    Side note – that same car also had the best cruise control setup. It was a separate stalk that you could operate by feel without taking your eyes off the road. No stupid on/off button on the steering wheel to press before you set it. Also had a cool mechanical indicator on the outer ring of the speedo that would show you where it was set. Maybe worth a story sometime?

    1. My E39 wrote me a letter about its battery getting tired.
      Problem was, I didn’t read it until the sunny afternoon I turned the key and…. click.
      Never once did it crank slow or fail to light immediately. That would be too easy.

      When I first bought the car, one of the things I noticed was the radio didn’t drop out when starting the car. Cool!

      Sitting there that afternoon waiting for jump from AAA, I reflected on the lazy door locks (blamed it on a 20 year old car) and radio drop outs that had been going on for the last month or so.

    2. 90s Toyotas had a very nice separate stalk for cruise control. ON/OFF on the end button, press down for cruise, press up for acceleration, then towards you to reset. I figured it out immediately without ever once looking at it.

      1. The best part of the Toyota cruise stalk is it’s consistency. It’s in the exact same location in my FRS and it is in my girlfriend’s 2012 Rav, as it was in rental Camrys and Corollas through the 20-teens.

  6. Guessing you’re not in the USA? Here in the states, E90s only had this button before the LCI. There is apparently some option in iDrive to do it also though. I hunted for months (in 2019-20) specifically for a 2006 330i that would have no iDrive, 6-speed, and great service records. Will keep this car for a very long time!

  7. I am actually pretty stoked to know what this button means in my 08 Dodge Sprinter (rebadged Mercedes). Thank you! It seems like someone at some point in time, somewhere in Germany, would have thought, “Maybe we should relabel that button for the Americans.” But oh well, the effect is akin to an Easter egg.

  8. I currently own a 2022 X5 xDrive45e (the PHEV), which can spend a significant amount of time with the engine off. I’m not sure what kind of heater it has, but it clearly isn’t driven off of the engine with a mechanical pump.

  9. My former e38 had this feature. The aux pump was built into the heater control valve, when mine went bad I looked into replacing it. Cost $511. Discovered you can replace it with the heater valve from a lessor BMW without the feature for about $50. Guess what feature (never used) my e38 lost!

  10. Poor fuel efficiency on startup / short trips is mostly due to the engine being cold.

    So BMW made a system to extract all the residual heat from the engine during brief stops, ensuring you will always set off again at worst efficiency…

  11. I’ve thought about trying to add a residual heat function to the cruiser. It has an independent secondary heat system under the passenger seat for rear heat. I was thinking it would be cool to be able to find a way to get coolant back to it when the engine is off, because everything else is already there. In the end it was determined to not be worth the effort. Maybe if I could find one of these electric pumps for cheap in a junkyard, but honestly I still don’t think it would be worth it.

    1. I commented elsewhere that you can do something similar in my ’65 Suburban. If you leave the heater core valve on, and the interior fan on, it just blows air through the heater core which will keep the car warm for quite a while. The coolant isn’t flowing/pumping through the heater core , but it’ll still produce warm air for a decent time.

  12. Our 2009 Audi A6 Avant has this as well. According to the manual “With the ignition turned off, you can activate the residual heat function by pressing the [ECON ON/OFF] button.” We’ve been using it in the winter. It comes in very handy when you just want to quickly have to grab something somewhere.

    I remember that our 2005 Audi A6 and 1999 Mercedes E300 Turbodiesel had this as well.

    This is a very nice feature when you live in colder climates and do not havean auxiliary heater installed in your vehicle.

  13. I have a auxiliary heater in both of my cars, Audi and Volkswagen. It burns diesel to pre-heat the engine and the cabin. It also uses the fan to distribute the heat to the cabin and it can be used for up to 60mins. It also comes with remote control and Audi even has an app to control it. Quite common option in Nordics (Finland) and rightly so, since it’s freezing almost half of the year. It a factory option on both cars.

  14. Love it. This makes two things I learned today. This feature has probably saved considerable fuel and engine wear (plenty of people idling just for a tiny amount of heat for 15 min periods). I am actually pretty excited to see hvac advances and ideas that will be applied ro more and more electric and hybrid drivetrains — heat pumps, exceptional insulation , PCM’s, etc.

    1. I’m not so sure. Sucking all the heat out of the coolant means that when you do restart the engine it’s basically a cold start. I’m not sure whether that’s worse than idling, but neither are particularly good for an engine or fuel economy. TANSTAAFL.

      1. I had the same thought, but, upon pondering, the thermostat will close when the coolant drops below whatever temp it’s specced at and the engine itself will only radiate heat after that. Thus keeping it fairly close to running temp-for awhile, anyway.

        Mind you, my brain has led me astray before…

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