Stupid Geophysics Question of the Week: Why are underground parking garages hot?

Here’s a simple-minded geophysics question, appropriate for the 95-degree heat that we’re experiencing in Boston right now: Why are underground parking garages hot?

At a friend’s house and they don’t have air conditioning? Go to the basement where it will be cool.

Have a collection of fine wine from Costco that you want to keep cool? Dig a cave and park the bottles there so that they will stay at a constant temperature somewhere near the average temperature at the surface of the Earth (pilots are taught about the standard atmosphere that is 15C at the surface, about 59 Fahrenheit).

Go into a parking garage underneath an office tower or apartment block and it will be hot and stuffy, oftentimes even hotter than the surface shade temperature. How is this possible?

Here are some possible explanations, but I can’t figure out which, if any, is correct.

  • The parking garage is mostly air, which has low thermal mass, with relatively small patches of contact with the cool adjacent (high thermal mass) ground. The air-to-ground-contact ratio is much higher in a parking garage than in a natural cave or a purpose-built wine cave.
  • Much of the thermal mass in the garage consists of cars, which have recently driven in from the hot surface and are therefore hot.
  • The parking garage has high capacity exhaust fans so that people don’t die from CO poisoning. Therefore hot air is being sucked into the cave to replace the dirty air that is blown out.
  • The parking garage has some doors that are typically open to the surface (this one does not seem significant to me since hot air rises).
  • The cars generate a lot of waste heat as they drive around within the garage.

What do folks think? Why isn’t going down into an urban parking garage a pleasantly cool experience, like going into a natural cave?

17 thoughts on “Stupid Geophysics Question of the Week: Why are underground parking garages hot?

  1. C and E, with a bit of B. Might help if the fans were run at night, I’m guessing they only run when cars are moving around in the garage.

  2. C. If you could measure the temperature of the concrete, it would be quite a bit lower, as expected. Your skin perceives the psychometric temperature, ie temperature and humidity combined.

  3. It’s very definitely waste heat from the cars.

    Typical car engine runs just under boiling – 80 C differential from ‘room temperature’.

    The hot sections of most cars will weigh about 250kg, specific heat of steel is 0.49 j/g per degree K

    Meaning each car brings about 6000kj of waste heat into the garage, plus another 100kj+ for each second they run in the confines of the garage.

    Air has a specific heat of roughly 1 j/g per degree K, and a kilogram of air takes up nearly a cubic meter. So a car can heat 200 cubic meters (a cube roughly 6m) of air quite uncomfortably warm, which is much larger than the area the car occupies.

    This is just back-of-the-envelope math, but air has extremely low specific heat by volume, so it’s very easy for a hot engine to heat up a nonsensically large space.

  4. I’m thinking the concrete that makes up the garage and exposed to the heat at it’s top is sinking the heat into the garage where it builds up. With maybe some other mentioned factors thrown in.

  5. The garage under the Cambridge Galleria mall has a bunch of air-conditioner compressor/radiator units relocating the heat from whatever office and retail space they are cooling.

  6. A number of buildings seem to have the outdoor (hot) side of the A/C in the garage. This makes the A/C expensive, but keeps the garage warm (or at least above freezing) in winter, which avoids salting/ other de-icing, which presumably is a lot more expensive. Maybe especially so here (on the west coast) where summers are relatively cool, and freezing temperatures skulk obnoxiously around the point where ice forms but stays wet and therefore slick.

  7. Post-script … maybe not A/C, but refrigeration (in commercial / mixed use buildings with grocery stores)… so maybe the summer premium isn’t as high, since the temperature differential is higher to begin with. Also, maybe, there’s a rebate to the grocery tenant for providing heat to the underground in winter…?

  8. >>Much of the thermal mass in the garage consists of cars, which have recently driven in from the hot surface and are therefore hot.

    A car engine weighs hundreds of pounds and is hot even in the middle of the winter in Saskatchewan. It has been burning gasoline to get to its destination – by definition it is a heat engine. Add in the exhaust system, hundreds of pounds of metal panels and you have a lot of heat with nowhere to go.

  9. When I’ve parked in an [“open air”, i.e. no-door] underground parking garage that sports few cars, I’ve noticed that is has been cooler in general. Before reading this post, I had not considered that it may be because of the cars, or lack of cars, responsible.

    However, I park there knowing that my car shall be cooler than had I parked on the surface, receiving the sun’s rays.

    So. Receiving a “greenhouse effect” via parking on the surface vs receiving a possible “nearby factory” influence from other cars. Interesting question, and I appreciate. One factor would be garage ventilation, and of course air-flow; cure Carbon Monoxide poisoning, and by the wayside cure the heat//cooling problem.

    A sideways factor would be garage capacity, turnover, and traffic. Which can mean location.

    That’s all I can think of, sadly; I’m not a civil engineer.

  10. Quite simply, surface meaning soil temperature might be able to dissipate heat easily. soil, usually damp, is also cooler.

    It really has to do with how fast can the heat escape upwards. This is extremely limited to the high capacity exhaust fans you have mentioned. They may be high capacity but they don’t run quite fast.

    Have you ever been near an exhaust fan? It is just like your regular electric fan at speed.

    I suspect as well that a lot of under ground parking lots are built with the central conditioning of the building nearby. Aircon generates cool air inside the building… with the by product – hotter air usually pointing out of the building. Where do they converge? the exhaust fans underneath the building.

    In a big mall here in the Philippinnes, there is a new underground parking lot that is only one or two levels deep. It is quite cool and just about room temperature. The building on top of it hasn’t yet finished construction and is not yet operational. If it becomes much hotter once that building is operational, I’ll let you know.

    (I’m talking about the new parking structure in front of the SM Megamall – for the curious filipinos reading this.)

  11. I suspect it’s mostly due to vehicle residual waste heat.

    A lot of it is probably due to the catalytic converters on the cars. Those operate at 400 to 800 degrees C and since the vehicles are likely to have been running awhile they’re fully heated up when the cars pull into the garage.

    Plus most cars nowadays are SUVs with V6 or V8 engines so they have 2 to 4 converters so that’s a lot of heat soaking into the environment.

    Also, I’ve noticed some buildings have vents for their AC systems in their parking garages. I’m not sure how wide-spread that is, but if they’re dumping building heat into the garage that could really heat things up.

  12. Phil, this is the perfect excuse for you to expand your camera collection to include a $10,000 FLIR system. Once you get the thermal images of the parking garage, don’t forget to share them online.

  13. The fill above and surrounding the garage is more porous post excavation rather than solid rock of a natural cave. Less insulation. Like filling a glass with rocks.

  14. I think 99% of the answer has already been given above, but for cooling it always helps to think of surface to volume ratio. Elephants overheat easily, and packing cars (or people) into a confined space creates much the same problem. Everything else is just details…

  15. I think 3G nailed it.

    An interesting point of comparison could yield some more insight. Could we compare under harbor vehicular tunnels, like the Callahan, with the same under harbor tunnels for electric trains, like the Blue Line? If waste heat from heat engines is the big contributor, we should find that the Callahan tunnel has a huge stack of air conditioners, while the Blue Line tunnel does not.

    Anyone know?

  16. I think it is the ventilation fans.If it were the cars heating the space wouldn’t the garage be quite warm in wintertime as well? Isn’t the thermal mass of the structure orders of magnitude larger than the cars? My garage is very hot in the summer cold in the winter. The temperature inside the garage seems to rise and fall with the temperature outside.

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