How to open this type of drain valve?

[Scrimps]

Hi folks, I want to partially drain this rad to top up my inhibitor, but can't work out how to open it. The nipple has spanner flats on it (just hidden by the serrated ring in the first picture). Using a spanner on those flats makes the nipple turn around a couple of times within the housing, but no water comes out, even with the entry and exit valves wide open.

Have read in other threads about rubber washers getting stuck etc, but I can't see anything like that in this particular drain valve (see second picture). What am I missing? Do I need to unscrew the serrated ring as well? I don't have anything at hand that will get enough purchase on that.

Thanks I'm advance for any guidance you can give.

 

[Alec_t]

Using a spanner on those flats makes the nipple turn around a couple of times within the housing, but no water comes out

You probably need a lot more turns than 'a couple'; even complete removal of the screw. Apart from the rubber washer there may be sediment blocking the outlet.