Why do we keep the air conditioning on in buses and trains when it’s chilly out?

Photo: Getty Images.

“It’s fifty degrees outside so why is the air conditioning on?” is a question we who monitor Metro’s social media see pretty frequently during the cooler months — and sometimes even in the middle of summer on those few particularly cool evenings. It’s a valid question. When the weather outside is less than balmy, why do we still need to blast the A/C on our buses and trains?

The main reason is because buses and trains are enclosed spaces, so the HVAC (heating, ventilation and air conditioning) system has to be kept on to ensure proper air circulation for everyone on board. Even when it’s chilly out, it can quickly get stuffy and warm inside a bus or train, especially if it’s crowded with many heat-generating bodies breathing at the same time.

The second reason for keeping the air conditioning on is that it keeps surroundings dry. Warm, humid interiors tend to promote bacteria growth and help generate stronger odors. Having the A/C on prevents moisture build up on board and combats the various germs and smells that are part and parcel of highly trafficked spaces.

The temperature for passenger areas is pre-set at Metro’s different bus/rail divisions and is generally set to accommodate average ridership. This means it should feel comfortable when the bus or train is approximately half full. The car temperature is also continuously monitored and regulated based on the actual temperature inside the vehicles. However, during transit as people hop on and off,  it will feel much colder when there are less people on board.

What to do if the temperature is absolutely unbearable? You have the option of notifying the bus or train operator. They can adjust the temperature within a narrow pre-set range, or turn off climate control system completely if the situation warrants it. However, keep in mind that what’s hot or cold for you may be different from what someone else is reporting.

I promise no one at Metro is purposefully trying to make your ride uncomfortable. We’re just doing our best to provide comfort to as many people as we can while maintaining the most optimal environment on board.

4 replies

  1. This is not just a Metro problem but a general cultural problem. Engineers are simply not used to the idea that it is OK – and even a good thing – for indoor temperature to be a little cold in the winter and a little warm in the summer. Somehow, over the past generation, the mentality has emerged that the human body is always happiest in a 70F room, even though that can’t possibly be true: if we’re wearing heavy clothes, we will clearly be more comfortable if the room temperature is lower, and if we’re wearing light clothes, we will clearly be more comfortable if the room temperature is higher.

    I suspect that this mentality will change as we all become more conscious of the environmental costs of being climate control freaks.

  2. A/C is only one part of the HVAC system. To call the system A/C is to miss the broader scope. As this article indicates it is for ventilation too. The system may pass the air over the evaporator section of the A/C to dehumidify the air, -then- it is passed through the heating section. (Dry air heats better than damp.) This is the way it works in automobiles when the defroster is on. Also homes that have ‘central air’ with heating will run like this.

    Further air that is moving will feel colder than it really is (the wind chill effect.) So, that air circulation will feel colder. Also, in order to cool the air in a space, the air coming out of an HVAC system needs to be colder than the room air. A 20 to 30 degree differential is normal. So, if the car is a stuffy 80 degrees, the air coming out of the system will be a chilly 60 or lower.

    If about a third to half of the people in a space are too cold, about a third to half are too hot.

  3. Sorry to disagree with Metro’s typically Panglossian spokesperson. (Remember: “The best of all possible worlds”–Voltaire’s famous satire “Candide.”)

    I was taught that the colder the air, the less water vapor that air can hold before some of the vapor condenses out of the air onto (often, even colder) surfaces. When the HVAC system for a bus (or train car) blasts excessively and unnecessarily cold air on a “not-hot” day, the result tends to be an increase in the interior’s relative humidity, actually causing more condensation inside the vehicle–and tending toward just the growth of mold, etc. and increased smells that Metro wants to avoid.

    As a daily rider of Metro buses/trains, I try always to bring along a wrap (sweater/jacket/overshirt) and a hat to guard against my becoming so cold (especially on a long trip on Metro) that my own body’s natural resistance against the very circulating microbes that are so abundant on public transit is overcome–leading to illness (usually respiratory, such as colds, influenza, etc.).

    I suspect that, on average, regular Metro riders annual acquire many more such illnesses than local residents who never use public transit.

    And contrary to Metro’s resident “Pangloss,” I am not among the habitually cold part of the population. Actually, I always have been among the first in any group to complain of being too warm (even sweaty). Therefore, if even I often become very chilled on Metro transit vehicles, I can only imagine how unpleasant the experience must be for smaller, more sedentary riders with less body fat than I.

    This is not to say that the interiors of Metro trains/buses do not sometimes become overheated–especially in summer–perhaps because the operator himself/herself may be wearing too many clothes (sweaters/jackets). While riders on Metro buses can open windows (which ought to be a hint to a bus driver, but which many just ignore) in an effort to cope with the “sweat-box” atmosphere in those cases, Metro train riders just must suffer, because we riders usually cannot just take off more of our clothes on summer-time rides. I have seen many cases in summertime when bus passengers have opened all or most of the windows and many are fanning themselves, but the bus operator leaves the AC off.

    Poorly regulated interior temperatures may be yet another significant reason for Metro’s continuing loss of passengers, despite its continuing expansion of rail mileage.