Lets say you're in apartment building with a lot of units. You're in the first apartment. You want the room colder, so you open the window. The room now has colder air, so there is greater delta T from the room to the radiator. The radiator loses heat faster, and cools to a lower temp (lower room temp) so the next guy gets colder water. And, the water going back to the heating plant is colder, also. So they use more fuel. See? Opening windows does at least two bad things. One of which is to cool the water that the later apartments need.
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So let's say you and I live in the same building. Just my family and yours for the sake of the discussion. My radiators are before yours in the system. I decide to cover 1/2 of my radiators to keep the heat down.
Wouldn't your apartment be hotter than before I covered my radiators? Based on your explanation - which I am not necessarily doubting - the water reaching your radiators would be hotter, therefore your radiators would radiate more heat, which would mean that you would have to cover more of your radiators than I did to maintain the same heat, right?
Then the outside temperature goes down and I want more heat, so I uncover 1/2 of the 1/2 I had covered. Now the water reaching your radiators is cooler, so you need more heat for 2 reasons: 1, it's colder outside and 2, the water reaching your radiator is cooler because more of the heat is radiating into my apartment. Therefore you have to uncover more of your radiator.
Now, extrapolate that out to 10 apartments or 50 or more. That seems like a lot of constant covering and uncovering to maintain a comfortable temperature in each apartment. Everytime someone makes a change, the whole building is affected.
I think I'd stick with opening and closing windows which only impact single apartments, or even single rooms within each apartment.