Sealed System - Why does water not leak out?

[pcsparky]

I have a combi boiler so have a sealed system in a bungalow. Without draining down I cut into one of the heating pipes below floor level (intending to make a quick push fit connection) and after the initial pressurised water escaped no further water came out of the pipes and I could work at leisure.

Why is this when I would have expected that gravity would have emptied the water above the pipework in all of the radiators?

 

[WDIK]

look at the gerbil's water supply

 

[swbjackson]

Not necessarily a good comparison. A lot of those water bottles have a ball bearing in the tube. Instead think about the old trick of a hole in the bottle of still water. Nothing leaks out until the recipient unscrews the top.