Why Do I need Bi-Directional TRVs?

[pennypincher]

Guys went to the local heat merchants today looking for TRVs for my radiators and they asked did I want Bi-Directional.
1.Do I need this...I thought it was only if you had vertical radiators I would need bi directional...
2.Do I get the locking shield with the valve or do I need to ask for it?What is it...They're quoting between 13-16 Euro per valve depending on make but will do a deal if I get a few of them.
3.They said it's important that the valve isn't covered by curtains etc,can you fit the valve on either side of radiator as I do have furniture near some of the rads I'm looking at changing,and don't want it not working correctly because of this.
Thanks

 

[beldin]

Bidirectional means it can be put on the flow or the return so either side of the radiator. Usually you get the lockshield valve with them although I left the old valve on mine as there didn't seem any point in changing them.
Make sure you get enough for the whole job, I got 3 to start with but when I went back for more they were using a different brand.