Fault on CM Britony 80 Combi Boiler - No CH

Problem started yesterday when CH suddenly went off. I have a C&M Britony 80. The hot water works fine with the burner firing up when hot water tap is on. However when hot tap is off it should go back into CH mode according to instructions but it does not. The burner is lighting for about 5 seconds then goes out, it then waits for 30-60seconds lights again, burns for around 5 seconds then goes out. Just keeps doing that....
Looking at the diagram of the system i suspect the 3 way valve (diverter valve) as the pipe is hot going into it, the heat exchanger (for hot water) is hot and the pump and pipe from that is hot. The outgoing pipe (to the radiators) from the diverter valve stays cold. The pump seems to be runing.

I dont know alot about boilers, but im technically minded

Could it be this or am i barking up the wrong tree????

Has anyone heard of this before and is there anything else i can check?

sounds about right.

from your description id check the dhw flow switch 1st! quite common for them to break calling for heat causing the boiler only to warm the primary circuit

just disconect the lead to the dhw flow switch with the ch on and see if the rads then start to heat up

Hi, I'm wondering if you can help me with a similar problem please?

I have replaced the flow switch to DHW and that hasn't fixed the problem, I also tried replacing the CH flow switch with the one I took off DHW to try and I'm still having a problem.

It's firing up for a few seconds, pump is going, nothing is coming out of the radiator out pipe though (but the rad return pipe gets warm so far along, same as when DHW is called for).

Can you help please?

Edit - I've read another post that talks about the diverter valve and a test, so I have disconnected the DHW flow switch, set it to CH and ran the hot tap - sure enough when the boiler fires up in CH mode I am getting hot tap water with the DHW flow switch disconnected.

Does this mean a new diverter valve, and how easy are these to fit? I can see clips on all the pipes rather than bolts, how do these come off?

Also on the front of the diverter valve there is a connector with two wires which has a clip going through it, is this the actuator motor? There's green stuff coming off it which looks like it is probably furred up and needs a clean, could this be the problem and if so how easy is it to get this part out (and back - will I need new washers?) to replace?

Or am I out of my depth? Basic plumbing and reasonable DIY skills here.

There are three possibilities, pcb failed, div motor failed or wet part stuck.

This is one of the least easy boiler types to work on the div.

Tony

Thanks, so when the DHW flow switch is disconnected and it is in heating mode, I can turn on my hot tap and get hot water when it fires up. Does that rule out PCB?