Oil Central heating Problem

[AaronC]

So in my aunts house, its an old enough system, no motorised valves, yet whenever the timeclock is switched on, it takes the burner some time to get power to it. It may kick in after 20 minutes, it may kick in straight away. I changed the timeclock yet the problem persists. I have also renewed the control box at the burner

Anyone any idea why it may take that long for a burner to start after the timeclock has been switched on?

 

[lame plumber]

sticking relays, give it a tap to check