I have a combi boiler about 3 years old used in conjunction with rads that had been in the house for about 15 years running on a sealed system. A couple of weeks ago I turned off two rads, one upstairs one downstairs, when I came to turn them on neither would work. I Bled all the rads and the upstairs one now works but the downstairs one is freezing cold as are the pipes leading to it. I have changed the lockshield valve and drained and flushed this rad but nothing. When I drain water from the system drain valve one of the pipes leading to the rad gets hot but this goes no further than the lockshield valve. I've tried running the CH with all working rads off and the non working one on (for 2 hours) with no luck. Can anyone help?