knocking in rad

[Roundy#]

Having a little problem with our rad in the bathroom. TRV is set to 3 (med setting). When the heat is on and after a while for rad to heat up, the pipe inlet to rad starts "knocking"; which is loud enough. Turning the valve up gets rid of the knocking.

Why is this happening? It's as if it can't handle the hot water pressure at the inlet when the TRV is turned down. How do I stop this?

 

[JJ1310]

Try bleeding Rad first, that might help.

 

[theplumber]

TRV is probably flow direction sensitive in other words on the wrong end of the rad. Flow passing wrong way through it pressure builds up behind jumper just before it closes and pushes it in causing knocking

and then again I may be wrong check little arrow on it is it like this ----> I am right
or like <---> If like this I am wrong That will tell you Use mirror to see it if neccessary

 

[Roundy#]

thanks plumber, I'll see if thats it later