In a typical house, this is usually somewhere outside (at my house in wheaton its next to the garage on the side of the house).
In an apartment building thats on the 17th floor ours was outside between our kitchen window and my bedroom window.
So this man comes, ties a string to his waist and goes outside my window where there is no ledge for him to stand on and starts spiderman-ing the wall. He quickly realized he could not somehow manage to hold on for dear life and pour the gas into the AC unit, so he told us he would need to call another person to help him. Since the cost was too high and we will only live in our apartment for 2 weeks we told him not to bother.
What a crazy job! And so dangerous! I would dred going to work each day. Its hard to remember that people who are not educated must do jobs like this since the part of the Chinese population that I work with and interact with on a daily basis are very well off.