Changing a radiator


I want to change a radiator that is working poorly and has rust. It's not leaking, however.
The radiator is downstairs and I am at odds how to drain the system before removing the radiator.
I located the warm in and cold out lines and switched them off. I opened and air valve of another radiator and let it run dry. The amount of water that came out is in my opinion only what the opened radiator have. Pressure in the whole house radiator system system went to zero in a minute or so. Shouldn't more water come out? Why isn't water upstairs radiators gushing down from the opened valve downstairs?
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stop. turn off the system heat so you don't heat a dry boiler. did you instead forget to let the air out of your cold radiator?
Dave wrote:

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Radiator warm water is supplied from an outside source and there is no boiler inside the house.

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