I have a Marathon 500B boiler.There is no hot water going to the hot water cylinder regardless of whether the switch is on CH/HW or just HW. The radiators are fine. Could it be an electrical fault on the programmer toggle switch?
Trust you can give me some lead. Thank you.

Mike Blake, December 2014

There is often a valve in the airing cupboard - three way valve or mid position valve.It directs the heated water to the hot water cylinder - or to the radiators - or when it's in the mid position to both.Yours may be stuck in the CH position.

This valve is often a common failure point - it has a small electric motor that drives it - they usually have a lever that sticks out to allow the valve to be hand-driven to the CH setting - check that yours has not been latched.

As a diagnostic set the timer/programmer for continual hot water and no c/h - does the boiler fire up? Is the thermostat on the hot water tank set to a reasonable setting?

If somebody operates the timer programmer alternatively between water & heating can you hear the motor operating inside the three port valve?

Use Google
Select Images
Enter Mid position valve

Good luck..., December 2014