may be faulty pump or pump overrun thermostst. if the pump overrun stat is faulty it can stop power to the pump, and so the heat won't get out of the boiler.
i'd guess it's a slow pump however if it's running for as long as 15 minutes.
as you've already had the limit stat replaced i'd ask your engineer to replace the hi and lo stats at the same time, should only cost about an extra £10 - £15. from past experience i find it best to replace all the stats when one becomes faulty as usually the others are on the road out.
Ian
March 2010
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