radiator valves..which is which?

[Cozza1969]

The valves on my rads are old and many of them have lost their tops. I'd like to replace them (I have done this before) but as I'm only a DIY'er and not a heating engineer, I'm not sure which side the lockshield and which side the control valves should go on?
Any advice appreciated

Cheers

 

[seco services]

locate the flow side what one gets hot first to each rad.
then fit the wheelhead valve on the flow and the lockshield on return.
use the lockshied to balance the system

 

[superdupergasman]

Locksheild on the return and the control valve on the flow, if its TRV's make sure you put the flow of water through the TRV the correct way.

 

[Cozza1969]

Thanks all... that's just what I needed to know. Also the pressure is going down on my boiler but there are no visible leaks. Presumably it must be on ground floor level. Could the leak be anywhere around the boiler..access to boiler is difficult at the moment so unable to check. Also boiler/system has been rumbling..could this be sludge or limescale? I have bled all the rads. I've been reading about leakex, scalex, protex etc. Should I use some/all of these? I have a drain valve on the kitchen rad... would I add them through that valve? Boiler is a Baxi Potterton combi. I know I should get engineer in really but I'm rather hard up at present.
Any more advice greatly appreciated.