Hi Claire
Is there water in the top up tank, on top or above the unit.
The pump maybe air locked, or the unit is not hot
The black boost button on the front panel will not light up the red light if the unit is hot or the off peak supply is on. The pcb on the a class will tell you everything that the unit is doing or any faults, A bit complicated, but its in the manual. Download from. www.gledhill.net if you dont have one. Or go to our site www.segel.co.uk
Chris
May 2008