CH not working intermittently in evening?

[bananabank]

I had my entire heating system replaced last year. Chucked out dated single panel radiators, ripped out the pipework (zinc/truwell stuff) and replaced the boiler which had died (PCB board).

I've now got a Viessmann 111-w boiler, column radiators and copper pipework (10mm microbore with 15mm tails up to rads)

Around 2 months ago the usual heating schedule kicked in but the heaters didn't warm up. I bled the radiators, re-pressurised the system and all was well for a short time. A few days later the same thing happened and this intermittent problem has been with me since.

No problems with the hot water and for some reason there isn't an issue with the heating coming on in the morning. Only at night when the schedule kicks in at around 4.30pm. The heating will come on for a couple of hours maybe then just stop.

I've tried resetting everything numerous times to no avail. When I stand next to the boiler once things have come back on and the thermostat calls for heat the boiler just doesn't fire up. I can hear some water circulation in the boiler maybe.

Any ideas?

Plumber came out a couple of weeks ago to service the boiler and he found no issues when I mentioned this to him. I called Viessmann and they helped me check the settings and found no problems. Hive customer support found no issues either.

 

[Jurassicspark]

the micro switch in the diverter valve either being triggered or the contacts being burnt/dirty

 

[Deleted member 267285]

Could be some detritus in the system?

 

[bananabank]

Hi all - an update on this - the issue went away on it's own but now it appears to be back!

The heating has been mostly off all summer. I bled the radiators in preparation for the colder weather (a fair bit of air was in the system) and re-pressurised to 1 bar but the heating just doesn't work at all. The thermostat calls for heat and the boiler appears to come on then just sort of gives up and turns off. The pressure goes up to 1.3 bar when it's on then just comes back down to 1 bar once it has given up.

I called Viessmann and they think it's the diverter valve (as mentioned above by another user) since everything has been off all summer. They are coming out to look on Weds so I'll update then.

Any ideas in the meantime? I read somewhere that air or muck can get trapped in the heat exchanger? In any case I'm not sure I'd be able to address that issue on my own anyway...