my boiler has given 20 years of pretty good service but recently central heating is working OK but hot water cuts off when running a bath or shower then comes on again after 5-10 mins or so. Live in a high limescale area too.

Colin, December 2008

The secondary heat exchanger's primary water ways are partially bloked with sludge (limescale will only slow water flow) it would be expensive to clear and there are no guarantees it would work. You could try adding sentinel x300 to the boilers primary curcuit and running it for 30mins-1 hour (read x300 instructions on bottle).

Ray, January 2009