Baxi Combi question

[musicrab]

Hello, before I get somebody in, I would like to understand what the fault is. Anybody familiar with combis?
1. pressure is 1.7bar
2. CH appears to be ok (first time I've tried it since Spring); pressure is a stable 1.5bar
3. DHW is where we have a problem; after (cold) bath boiler "Flame failure" and "Safety thermostat" lights both flashing; after reset , its working ...sort of
3a. as water is heating pressure is 2.25bar; as temp builds the pressure starts to waiver (judders between 1.5bar and 2.5bar) and sounds wrong. Issue with DHW pressure differential value?

 

[Crofton Gas]

Possibly blocked secondary heat exchanger

Regards

 

[musicrab]

Possibly blocked secondary heat exchanger

Regards

And stupid question - how could that be blocked? Gunge/water scale? And unblocking I assume is best done by replacement?

 

[Crofton Gas]

Your know if its blocked, when you run hot tap it will dump heat down flow pipe. Can be removed a clean bit normally replaced.

 

[picasso]

Baxi what ? sounds more like a sticking diverter valve but could be wrong.

 

[musicrab]

Baxi what ? sounds more like a sticking diverter valve but could be wrong.

The complete "D-valve" was replaced in january this year; so best to get the same crew in again I guess. PS. a Baxi 105E