Why does the air conditioning in the underground trains never work?
Let’s clear something up to start: No, our underground trains do not have air conditioning! They use ventilation systems, but why is that?
1. Air conditioning systems require a lot of space
Air conditioning units for trains take up an enormous amount of space. On the underground, it is not possible to install air conditioning units on the roof as is usually done elsewhere. The underground trains are built to fit the tunnels exactly, leaving only a few centimetres of space on all sides.
2. Air conditioning systems consume a lot of energy
Air conditioning would use about 3 0% more energy. In the underground, all doors are opened at every station every minute. This is far from ideal for air conditioning systems. Moreover, it is neither economical nor environmentally friendly.
3. Air conditioning generates heat – where does it go in the tunnel?
Air conditioning systems work like fridges – they remove heat from inside and release it outside. But where does the heat go? Unlike buses, trams, and trains, the underground rarely travels above ground. To avoid overheating in the tunnels, a system for venting the warm air would be needed first.
However, there is good news: you don’t have to sweat for your entire journey. All our trains, regardless of type or year built, have windows that can be tilted to provide individual ventilation. The windows that can be opened are always only on one side to prevent draughts. In addition, our newer underground train models have built-in ventilation systems which support the supply of fresh air, even if that doesn’t replace air conditioning.
Too cold on the bus, too hot on the tram. Why won’t anyone open a window?
An air conditioning system, as installed in all our buses and the majority of our trams, can only cool efficiently in an enclosed environment. If warm air continuously enters, the air conditioning cannot work properly. In our buses and trams, the doors open and close at every stop. This is extremely counterproductive for the cooling effect of our air conditioning units. As a result, it is not possible to keep the vehicle cool during very high temperatures.
Conversely, on particularly cold days, our heating systems cannot keep up. Opening windows would only increase energy consumption further and have no effect. If an air conditioning system fails completely, the drivers can open the windows on one side to at least ensure some ventilation. Unfortunately, this won’t particularly make things any cooler.
Is it true that ventilation and air conditioning systems are spreaders of germs and viruses?
No! All our ventilation and air conditioning systems draw in fresh air from outside. Contrary to popular belief, “used” air is not simply reprocessed inside the vehicle; instead, fresh air is drawn in, cleaned by filters, and only then supplied to the interior of the vehicle.
Fun fact!
The Association of German Transport Companies recommends supplying the passenger compartment with around 20 – 30 m³ of fresh air per person per hour using motorised fans. Assuming a carriage capacity of 100 passengers, that would be up to 3,000 m³ per hour, or 50 m³ per minute, or 800 litres per second. For 15 years now, these standards have been fully met in the H and HK train series (> 30 m³ per passenger per hour).
Despite this air exchange rate, it is physically impossible to prevent the interior temperature from rising above the outside temperature when high heat loads are present. These are mainly the passengers themselves (a person emits about 100 – 120 W of heat output). In a carriage with 100 people, that’s up to 12 kW of “heating power” alone. On overground routes, solar heat radiation adds to this.
And it’s important to note that medical professionals also recommend to not lower the temperature in a vehicle too much. No fellow passenger should experience a cold shock when boarding or feel uncomfortable or fall ill due to excessive temperature fluctuations.