I discovered the hard way today that these things open at several C, not at near zero as I had assumed. Minor flood (rad is off for painting, and TRV is closed) but nothing serious as the newspaper on the floor mopped most of it up. Fan heater soon warmed it up enough to stop the water coming out.
May go out and score some rad caps tomorrow.
Now, this behaviour - is it designed just to avoid local damage to the rad or pipework, or is the idea that water should move round the system as well? Seems to me the latter would require the boiler to be involved too.