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.
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.
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.
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.
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