My wife wanted to put her car in the garage due to the cold (relatively cold I suppose, around 10 degrees). I don’t care one way or another, other than the issue that she can’t back into it and I have to. But I got wondering this:
When she gets home after a 30 minute drive in her smallish sedan, is the car heating, or cooling our garage? The garage is part of our basement under the main portion of the house. And while insulated, it’s about 45 degrees per the thermometer in there so I’m normally heating it to some degree. So I’m wondering, what is the net temperature of the car over all? Is it colder than the 45 degrees or hotter? Does the engine and the cooling system, which is probably between 180 and 220 degrees, make up for the rest of the car that is probably 10 degrees? If the garage is normally 45 degrees, what is the net change due to the car?
I came to say, the water in the cooling system runs 210 +/- degrees, but many parts of the engine are hotter than that, especially the exhaust manifolds. All these parts heat the body/frame around them.
The engine and transmission are hot and likely weigh about 800#, the interior of the car is maybe room temperature and probably weighs about 300#. I would say the net change would be slight warming effect.
This is The Dope, and it’s got very smart people in it, so I take that to mean I shouldn’t have to warn you to turn your car off once you’ve done putting it in the garage, right?
I would say if the engine is up to normal operating temperature (which takes 5-10 minutes of normal driving, not idling), then parking it in the garage would add heat. If however it was parked out front for a few hours and simply moved inside, then it would cool the garage. How much is a good question, and hard to figure out. You could probably determine the effect of the car based on its mass and the volume of air inside the cabin to the volume of air in the garage, but only if the garage itself were a closed system. It’s not though, there’s some heat getting in there too, whether radiating down from the living space above, or even some geothermal heat from the ground below. Also keep in mind that after a half hour drive, the car’s cabin will be warmed to nearly 70º and that’s going to bleed out as well as the heat from the engine and cooling system.
I agree that there would be a slight warming effect per the OP’s scenario due to the car being warmer than the ambient garage temperature. Exactly how much would depend on the relative volumes and temperatures of the car and garage air. The warming effect would be less in a two or three car garage than in a single one.
But that slight warming would only start from the moment the garage door is fully closed after the car is in the garage. If the question is what change would occur from a starting point of the closed garage having a temperature of 45 degrees, then opening the garage door, pulling the car in, then shutting the door, there could possibly be a net loss of heat. The outside temperature is 35 degrees lower than inside. Opening the door will cause some amount of heat loss even if the air is still and appreciably more if it’s windy. The action of the car pulling into the garage would move some air by itself, and a double-wide door would allow more air exchange than a single-car door. All of that would lower the garage temperature from the initial 45 degrees and the heat from the car would have to overcome that loss, which seems unlikely to me.
Back-of-the-envelope calculations: Assume the car is a Honda Civic, for a total weight of ~3000lb. Engine weight is ~300lb, so the rest of the car is ~2700lb. Assume the relevant car parts are made of steel and the engine is cast iron, and write off the car inteior with the possibly-unsupportable assumption that it had warmed up to exactly 45 degrees over the course of the drive.
So, in theory, the car should reduce the temperature of the garage. What to? Let’s say the garage is a one-car garage, 20’ x 12’ with a 12’ ceiling, and the air is dry air at sea level:
I think your masses are wrong. It’s not just the engine that’s going to be hot on a recently driven car. The transmission, the radiator, the exhaust system… There’s a lot more than 300 lbs of hot on a recently driven car.
Cool! I have pondered this exact question many times. It is really tough to get a good idea. In addition to the things already mentioned, the tires and some parts of the drive train like brakes are going to be at least warmer then the ambient temperature. The interior, of course, but that is a small number of cubic feet of air and the seats, etc which don’t have much thermal mass.
When my newly built addition is ready for occupancy (next year) maybe I’ll give it a shot but I would normally keep that part heated. It is fully insulated with R16 walls and R30 ceiling and the concrete foundation is insulated for the upper 2 feet, mostly underground. The main door is R8 I think. I am even planning to come up with some type of insulation for the slab/footer in the main door area. The size is fairly small, 10’ x 21’ x8.5’.
Right now while it is zero outside I have the room at 55 degrees F and the floor is at 45 degrees except right by the main door.
OK, I’ll take a back-of-the-envelope shot at it. The interior of the OP’s house is at comfortable human temperature, so if the insulation is the same between the garage and the house as it is between the outside and the garage, and both have the same area of exposure, then we’d expect the garage’s temperature to be midway between comfortable human temperature and outside temperature. Meanwhile, the interior of the car is also at comfortable human temperature, and so we would expect the body of the car to also, on average, be midway between comfortable human temperature and outside temperature, i.e., the same temperature as the garage. But the car isn’t entirely body: There’s also the engine, which is a significant fraction of the total mass of the car, and which is much hotter than comfortable human temperature. So I’d expect that, on net, the car would be warmer.
The garage portion I agree with, given equal insulation and no ventilation and a steady-state environment.
I don’t think you can use the same assumptions on the car body though - you could hollow out the inside of a 10 ton block of steel, line the cavity with 3 inches of insulation, and you could probably heat the interior to a comfortable temperature with a 60W lightbulb, even if the outside is below freezing. That doesn’t mean the steel is at the mid-point temperature, unless the same insulation layer is on the outside of the block of steel too.
It seems to me that letting out all the warm air and letting in all the cold air when you open the door to park in the garage is going to be more significant than whatever temperature equalization happens between the car and the garage after all the air has been mixed. Are we supposed to ignore that?
I’m going to make a concerted effort to go into the garage once an hour for a 4-6 hours after the car ins in the garage to get a feel for it. I think I’ll see what the car states the temperature is as well.
Not sure what it’s worth, but in the summer, the garage feels a lot hotter after the cars in the garage for a bit, but in that case, but the car, and the engine is hot, so I’d think that would be the case. Here, there’s hot and cold.
Okay, I have a 2 car garage that had the door open most of the day. The temp was 45 F. My wife came home after work and driving approximately 25 miles, pullled into the garage and closed the door. That was at 5:15. Now at 7:22 the temp in the garage is 53 and trending up. The car is a Chevy Equinox with a V6. The garage has 4 closed windows and is insulated with fiberglass.
My answer is that yes the car will heat the garage.
Assumptions : The outside temperature is 32F (0C)and the garage gets to a final temp of 45F (7.2C)
Average 2007 car contains 2700lb (1223kg) of steel, 327lb (148kg) of aluminum and (4144 -2700-327) 1117lb (507kg) of Plastic. Cite
Specific heat of steel = 490 J/kg.C
Specific heat of Aluminum = 897 J/kg.C
Specific heat of Plastic ~ 1300 J/kg.C
(Plastic in cars consist of 50% Polypropylene + Polyurethane and the rest is PVC, ABS … - i picked a middle specific heat) cite
So, if this car was towed into the garage, it will take away
(1123490 + 148897 + 5071300)(7.2-0) = 9663307 J or 9,663kJ or about 10MJ
Although I have not personally lived for long periods in cold climates, I am told that they need to use a engine block heater which is typically 1000W for 2 hours to bring up the engine temperature.
So using that as approximate heat content of the engine block, the hot parts of the car will add 1000x60x60x2 = 7200kJ or about 7MJ
Since these are back of the envelope calcs, I don’t think 10MJ is much different than 7MJ - so overall I would you are neither adding nor removing heat.
The engine block takes about 1,000W to keep warm, which can be compared to a clothes dryer (4,000W) or a central house heaters (15,000W) to keep things in perspective.
I don’t recall ever using a 1000W heater for any vehicle. Several hundred maybe. I am using a 200W magnetic heater on my air compressor this winter in my unheated shed. It is designed for vehicles. I’ve not usedfreeze plug heaters. I grew up using the radiator hose heater in the 4-600W range. The higher wattage you mentioned are in the larger diesel enginges or in the far north. In the Midwest around here, not much call for that type in family sized vehicles.
am77494’s specific heat equation works if all parts of the car are at 0C when they go into the garage, and must be heated to 7.2C. But of course not all parts of the car are at 0C in the OP’s scenario, where the car has been driven for 30 minutes. There, a good chunk of the car is at or higher than 100C. Just how much is that hot is kind of the crux of the question. The OP could clearly close any heating vents to the garage and put a thermometer in there and measure things himself, comparing the temperature over time after the garage door was opened and closed without putting a car in the garage, to the temperature with a warmed-up car, to the temperature with a cold car pushed in or just run for the ten seconds needed to pull it in.
The block heater does not heat the entire engine to full operating temperature. Depending on the type, it heats the engine coolant or (sometimes) the oil, but only by about 20C, and after two hours or so there is little further increase in temperature because the engine loses heat as fast as it is added by the heater. So a warmed engine, and transmission, and brakes, and passenger cabin, will have a lot more heat in them than you’ll get from a block warmer.
I’m more confused by why the car has to be backed into the garage.
