possibly scale in the heat exchanger, dose the system with fernox descaler.
greg
October 2008
Some sort of blockage stopping the water moving round or the pump not moving the water round. Try puyying a magnet on the cold feed inlet pipe (this is the one that comes from the header tank usually in 15mm) if the magnet sticks, your cold feed is blocked and will need colding out.
LTGF
August 2008
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