i have a normal hot water supply but the heating comes on for short periods then turns off before reaching full heat in the rads. My rad valves are all on maximum and the time clock is on constant. The pcb is new as a leak from the pressure gauge ( since replaced ) damaged it.
if its since the pcb was fit. who ever fit it put the pointiemeter controls in wrong so when its turned to full its really on just above low. or you ch thermisters faulty.