rad problem

[gremlin1]

Fitted a couple of new rads about a year ago. I think they branch off from the main run although its difficult to tell as all pipes are in concrete.
Problem is they have been getting less warm over time and now they are stone cold.
one of the rads is a double 1600 x 600 high, the old rad was 1600 x 300 high. Is it possible that the extra height is creating more resistance hence no flow thru.
With rad removed water gushes out of rad valves ( not trvs). Vented system by the way (S plan).
Thanks for any help.

 

[75pete]

Does the water gush from flow and return?

 

[gremlin1]

yes it does gush from flow & return. also it could be a one pipe system, difficult to tell because of no visible pipework.