GAH Electrastream ticking when heating turned on

[MK Marty]

Hi,
Enquiring for a friend who has a GAH Electrastream boiler for heating and hot water, not very old, only a few years. When the thermostat is switched up the boiler makes a ticking sound every 5-6 seconds and doesn't fire up for the heating of the rads.
If he turns on the hot water the boiler starts up and then if he truns up the thermostat the rads just get slightly warm, but the ticking does stop.
I noted though that even with the thermostat set to 30 the rads only just get slightly warm.
Would bleeding all the rads help? They have never been done since the system was installed.

 

[muggles]

Sounds like a slipping zone valve to me. A photo of the airing cupboard pipework would help

 

[MK Marty]

Any good?

 

[MK Marty]

Try again

 

[muggles]

Try standing back a bit, you're too close. Could be the silver box in the blurry picture or another similar one elsewhere in the cupboard