When you drained the radiators did they all drain, Just in case some isolator somewhere. If yes forget.
From what you say, boiler heats tank, pump moves water from tank to radiators, ending up back in tank. some rads get hot some stay cold. what type of radiator control do you have, Initial thought is that you need to balance the rads. You have on one side of rad, a valve, thermostatic or other. on the other end is a balance valve, sort of allow more or less water through. so. On the ones that are cold, make sure they are fully open, But allways turn back a touch, so they are not hard against the stop. Then look at the others, the ones that are getting hot, close the valve a bit on the hot ones, does that improve things, if so continue with small adjustments, until all ok. You could use a thermometer to check and get all the same. IF does not work, repost with all details, boiler Make Model etc plus what done.
SD
January 2012
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