Central Heating?

Sounds like 2 of the rads have valves that were partially blocked and have now seized altogether. The reason the pipes going to them are cold is that water is incompressible, so since your rads are blocked the hot water cannot push up the "turnoff" pipe into your rad, and takes the path of least resistance around the main circuit onto the next rad(s) and/or boiler return. If you unscrew the valve caps on the inlet sides of the cold rads and gently pull up the central pin with a pair of firm-grip pliers, it may free them sufficiently to resurrect the flow. Try it.

Mark
November 2008