6 out of 12 Radiators are Cold

[doug910]

I moved into a house with a Greenstar 15i boiler (open vented) it has 12 radiators on two separate switchable circuits 7 upstairs and 5 downstairs all have old Danfoss TRVs fitted.
When I first tried it a month or so ago it had 3 cold radiators upstairs and none downstairs, I then turned some of the TRVs off to do a test then turned them back to max again.
Now I have 4 cold radiators upstairs and 2 downstairs, any suggestions on how I should diagnose this problem?
One of the cold upstairs radiators has a hot inlet pipe, so that is likely a stuck TRV, but the others have no sign of warmth on the pipes or the radiator.

Doug

 

[doug910]

I got all these radiators working, my mate used a hammer drill with a rubber tip against the TRV pipes with a block of wood behind them and it cleared each one within a few minutes of off and on hammer drill action.

Doug