Radiators(two)pump out water when the heating system is on?

I have radiators that pump out water when the radiators(in isolation) are switched off but the heating system is on.
They(all the rads)seem to drip a little water at all times,but the leaks are extreme when I have them turned off.

Is it just that the appropriate bits were never tightened and tested when installed?

Help asap please, otherwise my ceilings will cave in and my boys can't wear wellies in their bedrooms all year 'round.

I think it would be more cost effective to tighten something myself rather than have the house convert itself to a bungalow...ooh, and the water seep in to the electrics and the whole thing combust.