help boiler not starting

[whorlicks]

help .thought id be clever and replace a leaking radiator valve ..no prob
but on starting the heating up my boiler dosent fire up.pilot lights up ok
and the lights on the programmer come on as if calling for heat but it just dosent fire up ....any ideas or a way of checking it
gas heating.
ideal w2000.
s plan
towerchron programmer
sunvic valves

 

[oilman]

what is the water pressure?

 

[whorlicks]

how do i check the water pressure..? are you thinking it might be air locked ? there was a lot of sludge from the radiator that was drained.
if there isn't movement of water will this stop the boiler from firing ?